TNPSC Physics – Light : Complete Study Material

If you are preparing “Light” for TNPSC Physics, you may come across many concepts such as reflection, refraction, mirrors, lenses and the human eye. Understanding these concepts clearly is more useful than simply memorising them. In this post, we have put together Light Complete Study Material which is “Free Study Notes” in simple and easy-to-understand English. The notes cover the important concepts of Reflection, Refraction and other topics related to Light that are useful for TNPSC Physics preparation. After studying the notes, you can also take our TNPSC Physics test series to practise questions and check how well you have understood the topic. Take your time, learn the concepts step by step, and use the tests to improve your preparation.

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1. Introduction to Light

Light is a form of energy that enables us to see the objects around us. We see an object when light from the object, or light reflected by the object, reaches our eyes. Light → Object → Eye → Vision

Important characteristics of light:

• Light enables us to see objects.
• It travels from one place to another.
• It generally travels in a straight line through a uniform medium.
• It can be reflected from surfaces such as mirrors.
• It can be refracted when it passes from one transparent medium to another.
• It can be absorbed by materials.
• It can be transmitted through suitable materials.
• Under suitable conditions, light can be dispersed and scattered.

Luminous and non-luminous objects:
Objects that produce their own light are called luminous objects. The Sun, stars and an electric bulb are examples. Objects that do not produce their own light are called non-luminous objects. A book, chair and Moon are examples. We see non-luminous objects because light falling on them is reflected towards our eyes.

Example: The Moon is a non-luminous object. It appears bright because it reflects sunlight towards our eyes.

Light and materials:
Different materials behave differently when light falls on them.

Rectilinear propagation of light:
Light generally travels in a straight line when it moves through a uniform medium. This property is called rectilinear propagation of light. It helps us understand the formation of:

• Shadows
• Eclipses
• Images in a pinhole camera

Example: When an opaque object is placed in the path of light, it blocks the light and a shadow is formed behind the object.

Ray and beam of light:
A ray of light represents the direction or path in which light travels. A group of light rays travelling together is called a beam of light.

Reflection, Refraction and Other behaviours:
When light strikes a surface, different things can happen depending on the nature of the material and the surface.

• Reflection: Light returns into the same medium after striking a surface.
• Refraction: Light changes its direction and speed when it passes from one transparent medium to another.
• Absorption: A material takes in some or all of the light energy.
• Transmission: Light passes through a material.
• Dispersion: White light separates into its constituent colours.
• Scattering: Light is redirected in different directions by particles in a medium.

Important terms:

2. Sources of Light

A source of light is an object that emits or produces light. Sources of light are mainly classified as natural sources and artificial sources.

Natural sources of light: Natural sources are found in nature and are not produced by human-made devices.

• Sun – the main natural source of light and energy for Earth.
• Stars – distant natural luminous bodies.
• Lightning – produces a bright flash of light during thunderstorms.
• Fire – can occur naturally and produce light.
• Bioluminescent organisms – some living organisms produce light naturally.

TNPSC Point: The Sun is the primary natural source of light and energy for the Earth.

Artificial sources of light: Artificial sources are sources of light made or developed by humans, Examples include:

• Electric bulb
• LED lamp
• Tube light
• Candle
• Torch
• Oil lamp
• Street lamp

Bioluminescence :

Some living organisms can produce light through a chemical process inside their bodies. This natural production of light is called bioluminescence. Examples:

• Fireflies
• Some marine organisms
• Certain bacteria and fungi

Incandescence

When a substance becomes sufficiently hot, it may emit visible light. This emission of light due to high temperature is called incandescence. Example: The glowing filament of a traditional incandescent electric bulb emits light because it becomes very hot.

Luminescence

Light can be produced by a substance without heating it to a very high temperature. This emission of light is generally called luminescence. Example: The glow produced by phosphorescent materials, such as some glow-in-the-dark materials, is an example of luminescence.

Quick Memory:
Sun → Natural + Luminous
Bulb → Artificial + Luminous
Moon → Non-luminous
Firefly → Bioluminescence
Glow-in-the-dark material → Luminescence
Hot glowing filament → Incandescence

3. Propagation of Light

Propagation of light means the way light travels from one point to another. Light can travel through transparent media and also through vacuum.

Rectilinear propagation of light: When light travels through a uniform and transparent medium, it generally travels in a straight line. This property is called rectilinear propagation of light.

Example: A narrow beam of sunlight entering a dark room through a small opening appears to travel in a straight path.

Ray of light: A ray of light is an imaginary straight line used to represent the direction in which light travels. The direction of a ray is usually shown using an arrow.

Beam of light: A group of light rays travelling together is called a beam of light. Depending on the direction of the rays, beams are classified as follows:

Propagation through different media

Light behaves differently depending on the medium through which it travels.

• Light travels through vacuum; therefore, it does not require a material medium.
• Light can travel through transparent materials such as glass and water.
• The speed of light is maximum in vacuum.
• When light enters a transparent medium such as glass or water, its speed decreases.

The speed of light in vacuum is approximately:

c = 3 × 10⁸ m/s

Memory Point: Light does not require a material medium for its propagation. This is why sunlight can travel from the Sun to the Earth through space.

Formation of shadows

The straight-line propagation of light explains the formation of shadows. When an opaque object comes in the path of light, it blocks the light from reaching the region behind it, producing a shadow.

Example: A tree, building or person placed between a light source and a surface can produce a shadow.

Pinhole camera

A pinhole camera works mainly on the principle of rectilinear propagation of light. Light from different points of an object passes through the small pinhole and forms an image on the opposite side. The image formed by a simple pinhole camera is:

• Real
• Inverted
• Usually smaller than the object when the object is sufficiently far away

Important terms

4. Reflection of Light

When a beam of light falls on a surface and returns into the same medium, the phenomenon is called reflection of light. Reflection allows us to see many objects and is the basic principle behind mirrors and several optical devices.

How does reflection occur?

When light falls on a surface, it may be:

• Reflected back
• Absorbed by the surface
• Transmitted through the material

Note : A smooth, shiny surface reflects light more regularly than a rough surface.

Important terms related to reflection

Example: When sunlight falls on a mirror, the light is reflected from its shiny surface.

Types of reflection

Reflection is mainly classified into regular reflection and diffuse reflection.

Regular reflection : When parallel rays of light fall on a smooth and polished surface, the reflected rays remain orderly. This is called regular reflection. Regular reflection produces a clear image when the surface is suitable for image formation.

Diffuse reflection : When parallel rays fall on a rough surface, the reflected rays travel in different directions because the surface is irregular. This is called diffuse reflection.

Important: Diffuse reflection does not mean that the laws of reflection are violated. The individual rays still undergo reflection according to the laws of reflection.

Reflection in everyday life : Reflection of light is used in many situations:

• Mirrors used for seeing images
• Rear-view mirrors in vehicles
• Periscopes
• Reflectors in torches and headlights
• Solar cookers
• Optical instruments

Why can we see ordinary objects?

Most objects around us are not sources of light. They become visible because light falls on their surfaces and is reflected towards our eyes. The nature of the reflected light depends on the surface of the object.

Example: A book is visible in daylight because sunlight falls on the book and some of the light is reflected towards our eyes.

Quick Memory:
Smooth surface → Regular reflection
Rough surface → Diffuse reflection
Mirror → Clear reflection
Ordinary objects → Mostly diffuse reflection

5. Laws of Reflection

The behaviour of reflected light is explained by two basic laws of reflection. These laws apply whenever light is reflected from a surface, whether the reflection is regular or diffuse.

First Law of Reflection: The incident ray, reflected ray and the normal at the point of incidence all lie in the same plane. In simple words, the incoming ray, the outgoing reflected ray and the normal are all present on the same flat plane.

Second Law of Reflection: The angle of incidence is equal to the angle of reflection.

Formula: i= r, Here:

• i = angle of incidence
• r = angle of reflection

The angles are always measured with respect to the normal, not with respect to the reflecting surface.

Example: If the angle of incidence is 40°, the angle of reflection is also 40°.

Important terms

Important point about the normal

The normal is an imaginary line drawn perpendicular (90°) to the reflecting surface at the point where the incident ray strikes it. Therefore, if a ray falls normally on a plane reflecting surface:

Angle of incidence = 0°
Angle of reflection = 0°

The ray is reflected back along the same path.

Relation between the ray and the surface

Since the angle is measured from the normal: i + angle between incident ray and surface = 90°

Similarly, r + angle between reflected ray and surface = 90°

Note: A light ray makes an angle of 30° with the reflecting surface. Its angle of incidence is 60°, because the normal is perpendicular to the surface.

Reflection from a plane mirror

When light is reflected from a plane mirror, the reflected ray follows both laws of reflection. Changing the direction of the incident ray changes the direction of the reflected ray while maintaining: i = r

Quick Memory:
Same plane → First law
i = r → Second law
Angle is measured from → Normal
Normal → 90° to surface

6. Plane Mirror

A plane mirror is a flat, smooth and highly polished reflecting surface. It reflects light regularly and is commonly used for seeing our own image. When light falls on its surface, it undergoes regular reflection according to the laws of reflection.

Common examples: household mirrors, dressing mirrors and some reflecting surfaces used in optical devices.

Important characteristics :

• The reflecting surface is flat.
• It produces regular reflection.
• It can form a clear image of an object.
• The image formed is generally virtual and cannot be obtained on a screen.
• The image appears behind the mirror.
• The size of the image is equal to the size of the object.
• The image is laterally inverted.

Position of the image : For an object placed in front of a plane mirror, the image appears behind the mirror at the same perpendicular distance as the object is in front of it.

Object distance = Image distance

For example, if an object is placed 2 m in front of a plane mirror, its image appears 2 m behind the mirror.

Nature of the image:

The image formed by a plane mirror has the following characteristics:

Lateral inversion

The apparent left-right reversal of an image in a plane mirror is called lateral inversion. For example, when you raise your right hand in front of a plane mirror, the image appears to raise its left hand.

Example: The word AMBULANCE is written laterally inverted on the front of many ambulances so that it appears correctly in the rear-view mirrors of vehicles in front.

Multiple images

When two or more plane mirrors are placed at an angle to each other, an object may produce multiple images. The number of images depends on the angle between the mirrors. This principle is used in devices such as kaleidoscopes.

Uses of a plane mirror

• Dressing and household mirrors
• Periscopes
• Kaleidoscopes
• Decoration and interior applications
• Some optical arrangements

7. Image Formation by Plane Mirror

When light from an object falls on a plane mirror, the mirror reflects the light according to the laws of reflection. The reflected rays reaching our eyes appear to come from a point behind the mirror. Therefore, we see an image of the object behind the mirror.

Nature of the image : The image formed by a plane mirror has the following characteristics:

• It is virtual.
• It is erect.
• It is the same size as the object.
• It is formed behind the mirror.
• The image distance behind the mirror is equal to the object distance in front of the mirror.
• It is laterally inverted.

How is the image formed? : Consider an object placed in front of a plane mirror. Light rays from different points of the object fall on the mirror and are reflected. When these reflected rays enter our eyes, they appear to come from behind the mirror. The point from which the rays appear to originate is taken as the position of the virtual image.

Important idea: The reflected rays do not actually meet behind the mirror. They only appear to meet there. Therefore, the image is called a virtual image.

Image distance: The distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

Image distance = Object distance

For example, if an object is 3 m in front of a plane mirror, its image appears 3 m behind the mirror.

Size of the image: The height of the image formed by a plane mirror is equal to the height of the object.

Image height = Object height

Therefore, a plane mirror does not produce a magnified or diminished image under normal conditions.

Lateral inversion : The image undergoes lateral inversion. This means that the apparent left and right sides are reversed.

Example: If you raise your right hand in front of a plane mirror, the image appears to raise its left hand.

Relationship between object and image

Image formation when the object moves :

If the object is moved closer to or farther from a plane mirror, the image also moves by the same amount behind the mirror.

For example, if an object is moved 1 m towards the mirror, its image also moves 1 m towards the mirror. Therefore, the relative distance between the object and its image changes by 2 m.

Quick Memory:
Plane mirror → Virtual + Erect image
Size → Same as object
Distance → Object distance = Image distance
Position → Behind the mirror
Left-right reversal → Lateral inversion

8. Multiple Reflection and Its Applications

When light is reflected more than once from one or more reflecting surfaces, the phenomenon is called multiple reflection of light. It occurs when light undergoes successive reflections before reaching our eyes. Multiple reflection is useful in several optical devices and everyday applications.

Multiple reflection between two plane mirrors

When two plane mirrors are placed facing each other, light can be reflected repeatedly between them. As a result, multiple images of an object can be formed. The number of images depends mainly on the angle between the two mirrors.

For two plane mirrors inclined at an angle $\theta$, when the angle is such that the number of images is an integer: N = 360°/θ − 1, where $N$ is the number of images.

Example: If two plane mirrors are placed at an angle of $90^\circ$, the number of images is
$N=\frac{360^\circ}{90^\circ}-1=3$.

Kaleidoscope

A kaleidoscope works on the principle of multiple reflection. It contains two or more plane mirrors arranged at an angle inside a tube. Small coloured pieces placed between the mirrors undergo repeated reflections and produce beautiful symmetrical patterns.

Periscope

A simple periscope uses two plane mirrors placed parallel to each other, usually at $45^\circ$ to the direction of the tube. Light from an object is reflected by the upper mirror and then by the lower mirror before reaching the observer’s eye. It is useful for viewing objects that are not directly in the line of sight.

Example: Traditional periscopes were used in submarines to observe objects above the water surface while the observer remained below the surface.

Other applications

Multiple reflection is also used in:

• Reflecting devices
• Optical instruments
• Decorative arrangements
• Some signalling and lighting systems

9.Spherical Mirrors

A spherical mirror is a curved reflecting surface that forms a part of a hollow sphere. Depending on which part of the spherical surface acts as the reflecting surface, spherical mirrors are of two types: concave mirror and convex mirror.

Types of spherical mirrors

Basic terms

• Pole (P): The centre point of the reflecting surface of a spherical mirror.
• Centre of curvature (C): The centre of the sphere of which the mirror forms a part.
• Radius of curvature (R): The distance between the pole and the centre of curvature.
• Principal axis: The straight line passing through the pole and centre of curvature.
• Principal focus (F): The point on the principal axis where rays parallel to the principal axis converge, or from which they appear to diverge, after reflection.
• Focal length (f): The distance between the pole and the principal focus.

Relationship between focal length and radius of curvature : For a spherical mirror, under the usual paraxial approximation:

Focal length = Radius of curvature ÷ 2 or f = R/2

Example: If the radius of curvature of a spherical mirror is 20 cm, its focal length is 10 cm.

10. Concave Mirror

A concave mirror is a spherical mirror whose reflecting surface is curved inward, towards the centre of curvature. It is also called a converging mirror because parallel rays of light are reflected towards the principal focus.

How does a concave mirror work?

When rays parallel to the principal axis fall on a concave mirror, they are reflected and converge at the principal focus (F). The nature, size and position of the image depend on the position of the object.

Uses of concave mirrors

The converging property of concave mirrors makes them useful in several applications:

• Shaving and makeup mirrors – produce a magnified, erect image when the face is placed between F and P.
• Dentist’s mirrors – help dentists view an enlarged image of teeth.
• Vehicle headlights and torches – help produce a strong beam of light.
• Solar cookers – concentrate sunlight at a point to produce heat.
• Reflecting telescopes – use large concave mirrors to collect and focus light from distant objects.

Example: In a vehicle headlight, a light source placed near the focus of a concave reflector produces reflected rays that travel approximately parallel to the principal axis, forming a strong beam.

11. Convex Mirror

A convex mirror is a spherical mirror whose reflecting surface is curved outward. It is also called a diverging mirror because parallel rays of light are reflected outwards and appear to come from the principal focus behind the mirror.

Image formation by a convex mirror

A convex mirror always forms an image with the following characteristics:

The image remains virtual, erect and diminished irrespective of the position of the object in front of the mirror.

Uses of convex mirrors

The wide field of view provided by convex mirrors makes them useful in many situations:

• Rear-view mirrors in vehicles – allow the driver to see a wider area behind the vehicle.
• Security mirrors – used in shops, parking areas and buildings to observe a large area.
• Road safety mirrors – placed at bends and intersections to improve visibility.

Example: Convex mirrors are used as vehicle rear-view mirrors because they provide a wider field of view and form an erect, diminished image.

12. Applications of Mirrors

Mirrors are used in daily life, scientific instruments and safety devices based on their ability to reflect and control the direction of light.

Example: Convex mirrors are used at road bends and blind corners because their wide field of view helps drivers see vehicles or objects that may otherwise remain outside their direct view.

13. Refraction of Light

When light passes from one transparent medium to another, its speed and direction may change. This bending or change in direction of light is called refraction of light. Refraction occurs because light travels at different speeds in different transparent media.

Examples of refraction

• A pencil partly immersed in water appears bent.
• A coin at the bottom of a container appears raised when viewed through water.
• The bottom of a swimming pool appears shallower than it actually is.
• Lenses form images through refraction.

Direction of bending

The direction in which light bends depends on the optical densities of the two media.

• From a rarer medium to a denser medium, light bends towards the normal.
• From a denser medium to a rarer medium, light bends away from the normal.
• When light falls normally on the surface, it passes without changing its direction, although its speed changes.

Example: When light travels from air into glass, it bends towards the normal because its speed decreases in glass.

Important terms

Refraction and speed of light

The speed of light changes during refraction, but its frequency remains unchanged when light passes from one medium to another. Its wavelength changes because wavelength depends on the speed and frequency of the wave.

14. Laws of Refraction

The refraction of light is governed by two important laws. These laws describe the relationship between the incident ray, refracted ray and the normal at the boundary between two transparent media.

First Law of Refraction : The incident ray, refracted ray and the normal at the point of incidence all lie in the same plane.

Second Law of Refraction (Snell’s Law) : For a given pair of transparent media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

sin i / sin r = constant

This constant is called the refractive index of the second medium with respect to the first medium. Here:

• i = angle of incidence
• r = angle of refraction

Example: If the angle of incidence is increased while light passes between the same two media, the angle of refraction also changes, but the ratio sin i / sin r remains constant.

The angles of incidence and refraction are measured from the normal, not from the surface.

15. Refractive Index

The refractive index of a medium indicates how much light slows down when it travels through that medium compared with vacuum. It is represented by n. Refractive index can be expressed as:

n = Speed of light in vacuum ÷ Speed of light in the medium

n = c/v

where c is the speed of light in vacuum and v is the speed of light in the medium.

For light passing from one medium to another, the refractive index can also be related to the angles of incidence and refraction:

n = sin i / sin r

for the refractive index of the second medium with respect to the first.

A higher refractive index generally means that light travels more slowly in that medium.

Example: Since the refractive index of diamond is much higher than that of water, light travels more slowly in diamond than in water.

16. Refraction through Glass Slab

When a ray of light passes through a rectangular glass slab, it undergoes refraction at both the air-glass and glass-air boundaries. At the first surface, light travels from air to glass and bends towards the normal. Inside the glass, it travels in a straight line. At the second surface, it travels from glass to air and bends away from the normal.

Lateral displacement

Although the emergent ray changes direction at the second surface, it travels parallel to the original incident ray. However, it is shifted sideways from the original path. This sideways shift is called lateral displacement. The amount of lateral displacement depends mainly on:

• Thickness of the glass slab
• Angle of incidence
• Refractive index of the glass

Important observations:

• The incident and emergent rays are parallel.
• The emergent ray is laterally displaced from the incident ray.
• The direction of the emergent ray is parallel to the original incident ray.
• The glass slab does not produce an overall angular deviation between the incident and emergent rays.

Example: A coin or object viewed through a glass slab may appear slightly displaced because of refraction.

Difference between angular deviation and lateral displacement

Angular deviation: Change in the direction of the emergent ray compared with the original direction.

Lateral displacement: Sideways shift of the emergent ray while remaining parallel to the incident ray.

17. Lenses – Basic Concepts and Types

A lens is a transparent optical device bounded by two surfaces, at least one of which is curved. Lenses work mainly by refraction of light and are used to form images.

Types of lenses : Lenses are mainly classified into convex lenses and concave lenses.

Convex lens : A convex lens is thicker at the centre and thinner at the edges. It is called a converging lens because parallel rays passing through it are refracted towards a common point called the principal focus.

Concave lens : A concave lens is thinner at the centre and thicker at the edges. It is called a diverging lens because parallel rays passing through it spread out after refraction and appear to come from the principal focus.

Basic terms

• Optical centre (O): The central point of a thin lens through which a ray of light passes approximately without deviation.
• Principal axis: The straight line passing through the optical centre and the centres of curvature of the lens surfaces.
• Principal focus (F): The point on the principal axis where rays parallel to the principal axis converge, or from which they appear to diverge, after refraction.
• Focal length (f): The distance between the optical centre and the principal focus.

Uses of lenses

• Spectacles
• Magnifying glasses
• Cameras
• Microscopes
• Telescopes
• Projectors

Example: A magnifying glass uses a convex lens to produce a magnified image of a nearby object.

18. Image Formation by Lenses

The image formed by a lens depends on the type of lens and the position of the object. A convex lens can form real or virtual images, whereas a concave lens always forms a virtual, erect and diminished image for a real object.

Image formation by a convex lens

Image formation by a concave lens

A concave lens always produces an image between the optical centre (O) and principal focus (F) on the same side of the lens as the object.

Example: A convex lens can act as a magnifying glass when the object is placed between its optical centre and principal focus. The image formed is virtual, erect and magnified.

Real and virtual images

A real image is formed when refracted rays actually meet at a point. It can generally be obtained on a screen.

A virtual image is formed when the rays only appear to come from a point. It cannot be obtained on a screen.

19. Lens Formula and Magnification

The relationship between the focal length, object distance and image distance of a thin lens is given by the lens formula,

1/f = 1/v − 1/u

where:

• f = focal length of the lens
• u = object distance
• v = image distance

The signs of u, v and f depend on the direction of light and the position of the object and image. For numerical problems, the Cartesian sign convention should be followed consistently.

Magnification

Magnification (m) = Image height / Object height

For a lens: m = v/u

Magnification tells us how large or small the image is compared with the object.

• m > 1 → Magnified image
• m = 1 → Same-size image
• m < 1 → Diminished image
• Positive magnification → Erect image
• Negative magnification → Inverted image

Example: If the image height is 6 cm and the object height is 3 cm, magnification = 6 ÷ 3 = 2. The image is twice the size of the object.

20. Power of a Lens

The power of a lens indicates its ability to converge or diverge light. It depends on the focal length of the lens.

Formula : Power (P) = 1/f

Here, f is the focal length measured in metres.

The SI unit of power is dioptre (D).

• A convex lens has positive power.
• A concave lens has negative power.
• A lens with a shorter focal length has greater power.

Example: A convex lens has a focal length of 0.5 m.
Power = 1 ÷ 0.5 = +2 D

Relation between focal length and power

If the focal length is given in centimetres, convert it into metres before calculating power.

Power (D) = 100 / focal length (cm)

21. Dispersion of Light and Spectrum

When white light passes through a transparent medium such as a glass prism, it separates into its different colours. This phenomenon is called dispersion of light. The band of colours obtained after dispersion is called the spectrum.

Formation of spectrum

White light consists of different colours having different wavelengths. When it passes through a prism, these colours are refracted by different amounts and spread out into a spectrum. The seven colours of the visible spectrum are remembered as: VIBGYOR

Deviation of colours

Different colours of light have different wavelengths and are deviated by different amounts when passing through a prism.

• Violet has the shortest wavelength and undergoes the maximum deviation.
• Red has the longest wavelength and undergoes the minimum deviation.

Thus, in the spectrum formed by a prism, red appears at one end and violet at the other.

Why does dispersion occur?

The refractive index of a transparent material is different for different colours of light. Therefore, each colour travels at a different speed in the material and is refracted by a different amount.

Example: The colours seen when white sunlight passes through a glass prism are an example of dispersion.

22. Scattering of Light and Rainbow

Scattering is the phenomenon in which light is deflected in different directions when it interacts with small particles present in a medium such as air.

Scattering of light

The amount of scattering depends on the wavelength of light. Shorter wavelengths are scattered more strongly than longer wavelengths. This explains several natural phenomena:

• The sky appears blue because blue light is scattered more strongly by atmospheric particles than red light.
• The Sun appears reddish during sunrise and sunset because sunlight travels through a longer path in the atmosphere, causing much of the shorter-wavelength light to be scattered away.
• The sky appears nearly dark to astronauts in space because there is no substantial atmosphere to scatter sunlight.

Rainbow : A rainbow is a natural spectrum of sunlight seen when sunlight interacts with water droplets in the atmosphere. The resulting colours are seen as an arc in the sky.

The formation of a rainbow involves:

1. Refraction of sunlight as it enters a raindrop.
2. Dispersion of white light into different colours.
3. Internal reflection inside the water drop.
4. Refraction again as the light leaves the drop.

Example: A rainbow is commonly seen when sunlight shines through falling rain, with the Sun behind the observer.

Primary rainbow: A primary rainbow is formed mainly due to one internal reflection inside each water droplet. Red appears on the outer side and violet on the inner side of the primary rainbow.

23. Human Eye, Vision and Optical Instruments

The human eye is a natural optical organ that enables us to see objects by receiving light and forming images. The eye works mainly through refraction of light by its transparent parts and the focusing action of the lens.

Parts of the human eye

Accommodation of the eye

The ability of the eye lens to change its focal length to focus clearly on objects at different distances is called accommodation. For a normal eye, the near point is about 25 cm and the far point is at infinity.

Common defects of vision

Optical instruments

Lenses and mirrors are used in optical instruments to obtain magnified, distant or detailed views.

• Magnifying glass: Uses a convex lens to obtain a magnified view of a small nearby object.
• Camera: Uses a convex lens to form a real image on a light-sensitive sensor.
• Microscope: Uses lenses to produce a highly magnified view of very small objects.
• Telescope: Uses lenses or mirrors to observe distant objects such as celestial bodies.

Example: A magnifying glass uses a convex lens with the object placed within its focal length to produce a virtual, erect and magnified image.

24. Important Formulae, TNPSC Facts and Quick Revision

Formulae

Here, f is focal length, R is radius of curvature, u is object distance, v is image distance, c is the speed of light in vacuum and n is refractive index. For lens power, focal length must be expressed in metres.

Quick Revision :

• Light travels in a straight line in a homogeneous transparent medium.
• Reflection is the return of light into the same medium after striking a surface.
• Refraction occurs when light passes from one transparent medium to another and its speed changes.
• Plane mirror: virtual, erect, same-sized and laterally inverted image.
• Concave mirror: converging mirror; can form real or virtual images.
• Convex mirror: diverging mirror; forms a virtual, erect and diminished image.
• Convex lens: converging lens.
• Concave lens: diverging lens.
• Refractive index: indicates how much light slows down in a medium compared with vacuum.
• Violet has the shortest wavelength among the visible colours and undergoes maximum deviation in a prism.
• Red has the longest wavelength among the visible colours and undergoes minimum deviation.
• Blue colour of the sky: mainly due to scattering of shorter-wavelength light.
• Rainbow: involves refraction, dispersion and internal reflection in water droplets.
• Myopia: corrected using a concave lens.
• Hypermetropia: corrected using a convex lens.
• Power of a lens: measured in dioptre (D).
• 1 dioptre is the power of a lens having a focal length of 1 metre.

For TNPSC preparation : Remember the mirror and lens image properties, laws of reflection and refraction, VIBGYOR order, scattering phenomena, optical instruments, and the basic formulas. These provide a compact revision base for the complete Light topic.

Further Reading:

For students who want to explore the topic further, the National Council of Educational Research and Training (NCERT) provides official school-level Science textbooks and learning resources. You can refer to the NCERT Science material Link given below for additional reading and to strengthen your understanding of important concepts related to Light, Reflection and Refraction.

Official NCERT Resource: Light – Reflection and Refraction

Now that you have completed these Free Study Notes, it’s time to check how well you have understood the important concepts of TNPSC Physics. Continue your preparation with Our Free Light Test Series and practise questions based on topics such as Reflection, Refraction, mirrors, lenses and other important areas of Light. Take the test, check your answers, learn from your mistakes and use the results to improve your preparation. Keep practising and move one step closer to your TNPSC goal!