NCERT Notes for Class 10 Physics Chapter 10 Light-Reflection and Refraction

Class 10 Physics Chapter 10 Light-Reflection and Refraction is one of the most important chapters to learn. In this article, we will learn about reflection, types of reflection, refraction, images formed by spherical, spherical lenses, and different types of lenses and mirrors, magnification, Power of lens, and light passing through a prism. This article is designed to provide students with a comprehensive summary of the entire class 10 NCERT Chapter 10 Notes and include all of the essential topics, formulas, and concepts necessary for success on exams.

What is Light?

Light is an electromagnetic wave that can be visible to humans and help humans to see the object. We can also define Light as a form of energy that helps us to see. Light Wave contains a minute packet of energy particles called photons. As sound requires mediums like solid, liquid, and gas to travel through but light does not require any medium to travel. The speed of light is 3×10⁸ m/s.

Propagation of Light

Light always propagates in a straight line. Since light is an electromagnetic wave it can propagate even without a medium i.e. Light can propagate in vacuum also. Light is also a transverse wave, hence, it propagates in the manner of crest and trough. Light being an electromagnetic wave has both electric field and magnetic field components which oscillate perpendicular to each other and they both oscillate together perpendicular to the direction of propagation.

Reflection of Light

The bouncing back of maximum light when falls on a highly polished surface is known as Reflection of Light. We can see any image in the mirror due to the reflection of light only. We can’t see any image in the dark because of the unavailability of light reflection does not take place. Reflection is of two types:

• Regular Reflection: When a parallel beam of light falls on a highly polished surface and the reflected rays are parallel to each other then reflected rays are known as regular reflection.
• Irregular Reflection: When a parallel beam of light falls on an irregular surface and the reflected rays are not parallel to each other then reflected rays are an irregular reflection.

Laws of Reflection

There are two Laws of Reflection, given as follows:

• The angle of incidence is equal to the angle of reflection.
• Incident, normal, and reflected rays lie on the same plane.

What is a Mirror?

Mirror is highly polished surface which bounces back the maximum light when rays of light fall on it. It forms image that is either real or virtual.

Types of Mirror

There are two types of mirror:

• Plane Mirror: It is a type of mirror whose reflective surface is flat which forms only virtual image.
• Spherical Mirror: A spherical mirror is a type of mirror whose reflecting surfaces is the parts of a hollow sphere of glass.

Plane Mirror

A Plane Mirror is a mirror whose reflection surface is flat and plane in nature. When an object is place in front of plane mirror image formed behind the mirror distance between the object and the plane is same as the distance between the image formed at back of the mirror.

Image Formed by Plane Mirror

The characteristics of Image Formed by Plane Mirror is mentioned below:

• Image Location – Behind the mirror at the same distance as object is in front of the mirror
• Image Nature – Virtual, Erect and Laterally Inverted i.e. Right appears left and left appears right
• Image Size – Same as size of the object.

Image formed by plane mirror.

Spherical Mirror

A Spherical Mirror is a type of mirror whose reflecting surfaces are the parts of a hollow sphere of glass.

Terminologies Used in Spherical Mirrors

• Pole: The middle point P of a spherical mirror is called Pole.
• Center of curvature: The center ‘c’ of sphere from which the spherical mirror is formed is called center of curvature.
• Radius of Curvature: The radius of sphere from which the spherical mirror is formed is called radius of curvature. (R= AC or BC)
• Principal Axis: The line passes through the pole and the center of curvature of the mirror is called principal axis. Line PC in the figure below is principal axis.
• Principal focus: When a narrow beam of light parallel to the principal axis either converges or appears to converge from a point F after reflection from the spherical mirror is called principal focus.
• Focal length: It is the distance between the focus and the pole of the mirror(f=PF).
• Linear aperture: The diameter AB of the circular boundary of the spherical mirror is called Linear aperture

Sign Convention for Spherical Mirror

Sign Convention of Spherical Mirror can be understood by the points added below,

• All the incident light travels from left to right
• All the distance is measured from the pole of the mirror.
• All the distance measured in the direction of the incident light (RHS of the mirror) are taken as Positive
• All the distance measured in the opposite direction of incident light (LHS of the mirror) are taken as Negative.
• The height is measured perpendicular to the principal axis.
• The height measure upward to the principal axis is taken as Positive.
• The height measured downward to the principal axis is taken as Negative

Types of Spherical Mirror

There are two types of spherical mirror mentioned below:

• Concave Mirror
• Convex Mirror

Concave Mirror

The type of spherical mirror in which the reflection of light takes place from the side which is curved inward and the outer bulged surface is silvered polished is called a concave mirror. It is also called the converging mirror. It converges the parallel beam of light at focus(F). The shaving mirror and the mirror used by a dentist are examples of concave mirrors.

Concave Mirror Properties

• Concave Mirror is converging in nature
• In general, concave mirror gives enlarged image
• However, Small, Enlarged and same size image is also possible.
• A ray of light parallel to the principal axis will pass through the focus after reflection.
• A ray passing through the principal focus(F) becomes parallel to principal axis.
• A ray passing through the center of curvature will take the same path after reflection.
• For any ray incident at any angle at the pole(P), the reflected ray follows the law of reflection (angle of incidence = angle of reflection).
• The nature of image formed by concave mirror can be real, virtual, erect, inverted.
• The size of the image can be smaller, larger or equal to the object.

Sign Convention for Concave Mirror

• Object Distance (u) is negative
• Image Distance (v) is negative except when object is between focus and Pole
• Focal Length is Negative

Image Formation by Concave Mirror

Object Position	Image Position	Image Size	Image Nature
At Infinity	At Focus(f)	Highly diminished	Real & Inverted
Beyond C	Between F & C	Diminished	Real & Inverted
At C	At C	Same Size	Real & Inverted
Between C & F	Beyond C	Enlarged	Real & Inverted
At F	At Infinity	Highly Enlarged	Real & Inverted
Between P & F	Behind the mirror	Enlarged	Virtual & Erect

Application of Concave Mirror

• Shaving mirror
• Torch light
• Vehicle headlight
• Dental mirror
• Microscope
• Telescope
• Solar furnaces such as solar cooker

Convex Mirror

The type of spherical mirror in which reflection of light takes place from the outer bulged surface and curved inward surface is silvered polished. It is also called the diverging mirror. The side view mirror of vehicle and magnifying are the example of convex mirror.

Convex mirror

Properties of Convex Mirror

• A ray of light parallel to the principal axis will pass through the focus after reflection.
• A ray passing through the principal focus(F) becomes parallel to principal axis.
• A ray passing through the center of curvature will take the same path after reflection.
• For any ray incident at any angle at the pole(P), the reflected ray follows the law of reflection (angle of incidence = angle of reflection).
• The nature of the image formed by convex mirror will be diminished, virtual and erect.
• The size of the image will be smaller than the object.

Sign Convention for Convex Mirror

• Object Distance (u) is always Negative
• Image Distance (v) is always Positive
• Focal Length (f) is always Positive

Image Formation by Convex Mirror

Object Position	Image Position	Image Size	Image Nature
Infinity	At Focus Behind the Mirror	Diminished	Virtual and Erect
Between Infinity and Pole	Behind the Mirror between Focus and Pole	Diminished	Virtual and Erect

Application of Convex Mirror

• Convex Mirror is used in large works place to allow people to look around the corner (like Hospital).
• It is used in rear-view of automobiles.
• Convex Mirror is used in ATM which allows user to see if someone is standing behind them.
• It is used in telescopes.
• As reflector to reflect the sunlight.

Mirror Formula

Mirror Formula expresses the relation between focal length, object-distance and image-distance.

1/f = 1/v + 1/u

where,

• f is the Focal Length
• v is the Distance of Image From Pole
• u is the Distance of Object From Pole

Example: The radius of curvature of the rear view convex mirror of the truck is 4 m. If the car is 6 m from the mirror of the truck. Calculate the distance at which the image is formed.

Solution:

Given,

• Radius of curvature, R = 4 m
• Object distance, u = -6 m

Focal length is half of Radius of curvature,

f = R/2 = 4/2 = 2 m

Using mirror formula

1 / v + 1 / u = 1 / f

1 / v + 1 / -6 = 1 / 2

1 / v = 1 / 2 + 1 / 6

= 4/6

v = 6/ 4 m =3/2 m

The image is formed at distance of 3/2 behind the mirror.

Magnification by Mirror

Magnification is defined as the ratio of the size of the image to the size of the object. Magnification basically tells how enlarge or small the image is with respect to the object size.

m = h_i / h_o

where,

• m is Magnification
• h_i is image height
• h_o is object height

The other form of Magnification formula in terms of image and object distance is given as

m = -v/u

where,

• m is magnification
• v is image distance
• u is object distance

Example: If the image is a distance of 5 cm and the object is at 10 cm in the front of the concave mirror, Calculate the magnification formed.

Solution:

Given,

1. Distance of object, u = – 10 cm
2. Distance of image, v = – 5 cm

Since,

Magnification is given by,

m = – v / u

Therefore,

m = – (-5 / -10) = -0.5

Hence, the image will be diminished by nearly half as size of object.

Refraction of light

The phenomenon of change in the direction/path of light rays when it travels from one transparent medium to another is called refraction of light. The bending of light is caused by the change in the wave’s speed. light has different speeds in different mediums. When light travels in the same medium, it travels along a straight line but when the medium has changed the speed of light changes and it bends.

Terms Related to Refraction of Light

• Incident ray: The ray of light falling to the surface at point P (above fig.) is called incident ray (AP).
• Refracted ray: The ray of light traveling in glass with a change in direction is called refracted ray (PB).
• Angle of incidence (i): The angle formed between the incident ray and the normal is called the angle of incidence.
• Angle of refraction (r): The angle formed between normal and the angle of refraction is called the angle of refraction.
• Emergent ray: When a light ray passes from first medium to second medium it is know as refracted when that refracted ray passes from second to first medium again then that ray is know as emergent ray.

Refraction of Light through Rectangular Glass Slab

The refraction of Light through rectangular glass slab is illustrated through the image below:

Illustrations on Refraction of Light

• Figure 1: Angle of incidence is greater than the angle of refraction (∠i > ∠r). When a ray of light travels from a rarer medium to a denser medium, it bends towards normal. Air rarer medium and glass is denser medium.
• Figure 2: Angle of incidence is smaller than the angle of refraction (∠i <∠r). When a ray of light travels from a denser medium(glass) to a rarer medium(air), it bends away from normal.
• Figure 3: When a ray of light traveling along with normal shows no deflection. ∠i=∠r=0.

Laws of Refraction of Light

There are two Laws of Refraction of Light

First law of Refraction of Light: It states that the incident ray, refracted ray and normal to the interface at the point of incidence all lie in the same plane.

Second law of Refraction of Light: It states that the ratio of the sine of the angle of incidence and the sine of the angle of refraction is constant. It is given as,

sin i / sin r = constant

Snell’s Law

Snell’s Law states that for the light if given colour and for the given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. It gives the degree of refraction and also tells the relation between the angle of incidence, the angle of refraction and the refractive index of media.

sin i / sin r = μ = constant

where,

• i is Angle of Incidence
• r is Angle of Refraction
• μ is the Refractive Index of Medium

Refractive Index

The ratio of the speed of light in a vacuum to the speed of light in a given medium for the given wavelength is called the “Refractive index” of that medium. It is denoted by “μ”.

μ = c/v

where,

• c is the speed of light in a vacuum
• v is the speed of light in the medium

Denser the medium, more will be the refractive index and less will be the speed of light

Refractive Index of medium depends upon:

• Wavelength of light
• Nature of the medium
• Nature of surrounding
• Temperature

Applications for Refraction of Light

• Used for making peepholes in the doors, binoculars, cameras, lenses in projectors, etc.
• Used to correct the refractive errors of human eyes.
• Used for magnifying the image of an object or reducing the size of the image of an object.

Refraction through Glass Prism

Glass Prism is an optical device made with glass and have triangular faces in which the glasses are cut at angles to carry out optical phenomena such as refraction. The splitting of light into band of seven colours when it passes through a glass prism is called Dispersion of Light. Refraction through the Glass Prism is shown below:

Refraction through glass prism

The parameters in the above diagram is mentioned below:

• PQ = Incident rays
• EF = Refracted rays
• RS = Emergent Rays
• NN= Normal
• MM= Normal
• ∠i = Angle of Incidence
• ∠e= Angle of Emergence
• ∠A = Angle of Prism
• ∠D= Angle of Deviation

When an incident ray(PQ) fall on the surface(AB) of the prism the incident bend towards the normal (NN) as light travel from rarer medium(air) to denser medium (glass). When the ray comes out of the prism it bends away from the normal(MM). When a light travels from denser to rarer medium it moves away from the normal.

When Light passes through the glass prism, the light split into band of seven colors in the order of VIBGYOR from bottom to top. While splitting, the red light travels fastest and has highest wavelength and bend least while the violet has least wavelength and least speed hence it bends the most.

Note: After the refracted come outwards the prism it again bend downwards rather than going upwards as the base of the prism is thicker than the upper portion of the prism.

Spherical Lens

Lens is a transparent glass material through which light can refract. In class 10 we have to study about spherical lens.

A piece of transparent glass bounded by two surfaces, at least one of which is a curved surface, which concentrates or disperses the light rays when passes through them by refraction is called the lens.

Terms used in Spherical Lens

• Aperture: The refracting surface of the lens is called aperture
• Optical Center: The center of the lens is called the optical center
• Center of Curvature: Center of Curvature is the center of sphere of the lens is a part.
• Radius of Curvature: The distance between optical center and center of curvature is called Radius of Curvature
• Principal Axis: The imaginary line passing through the center of curvature and optical center is called Principal Axis
• Focus: The point where the rays parallel to principle axis meet or appears to meet after refraction is called focus
• Focal Length: The distance between optical center and focus is focal length.

Sign Convention in Lens

• All the incident light travels from left to right
• All the distance is measured from the optical center of the lens.
• All the distance measured in the direction of the incident light (RHS of the mirror) are taken as Positive
• All the distance measured in the opposite direction of incident light (LHS of the mirror) are taken as Negative.
• The height is measured perpendicular to the principal axis.
• The height measure upward to the principal axis is taken as Positive.
• The height measured downward to the principal axis is taken as Negative

Refraction in Spherical Lens

Any incident ray when it passes through a lens gets refracted. The rules of refraction are mentioned below:

• All the rays parallel to the principal axis meet at focus after refraction
• The incident rays passing through the optical center passes straight from it without getting deviated

Types of Lenses

There are two types of lens mentioned below:

• Convex lens
• Concave lens

Let’s discuss these types of lenses in details below:

Convex Lens

The lens which is thicker at the middle than the end is called a convex lens. It is also called a converging lens as it converges the parallel beam of light into a point.

Sign Convention used in Convex Lens

• Object Distance (u) is negative
• Image Distance (v) is Positive
• Focal Length is Positive

Image Formation in Convex Lens

The different conditions for image formation in convex lens are tabulated in the table below:

Object Position	Image Position	Image size	Image Nature
At Infinity	At Focus(f)	Highly diminished	Real & Inverted
Beyond 2F1	Between F2 and 2F2	Diminished	Real & Inverted
At 2F1	At 2F2	Same Size	Real & Inverted
Between F1 & 2F1	Beyond 2F2	Enlarged	Real & Inverted
At focus F1	At infinity	Highly Enlarged	Real & Inverted
Between F1 & O	Same side	Enlarged	Virtual & Erect

Applications of Convex Lens

The applications of convex lens are mentioned below:

• Convex Lens are used in Microscope and magnifying glass to view larger image
• Convex lens are used in camera to focus light
• Convex lens are present in human eye
• Convex Lens are used in correcting hypermetropia

Concave Lens

The lens which is thicker at the end than the middle is called the concave lens. It is also called diverging lens as it spreads out the light rays that have been refracted through it. It has the ability to diverge the parallel beam of light.

Sign Convention in Concave Lens

• Object Distance (u) is negative
• Image Distance (v) is negative
• Focal Length (f) is negative

Image Formation in Concave Lens

The conditions for image formation in concave lens is tabulated below:

Object Distance	Image Distance	Image Size	Image Nature
Infinity	At Focus F1	Highly Diminished	Virtual and Erect
Between Infinity and Optical Center	Between F1 and Optical Center O	Diminished	Virtual and Erect

Application of Concave Lens

The uses of Concave Lens are mentioned below:

• Concave Lens are used in Peepholes of Doors
• Concave Lens are used to Correct Myopia
• Concave Lens are used in flashlight to diverge light
• Concave Lens are also used in binoculars and telescope along with convex lens to remove the distortion of the magnification

Lens Formula

Lens formula expresses the relation between focal length, object-distance and image-distance. Lens Formula is given as:

1/f = 1/v – 1/u

where,

• f is the Focal Length
• v is the Distance of Image From Pole
• u is the Distance of Object From Pole

Example: A convex lens forms a real and inverted image of an object 20cm from the lens. Where will be the object placed in front of the convex lens, if the image is of the same size as the object?

Solution:

Given,

• Image distance (v) = 20 cm

Lens type is convex and image is real image

Height of image is as same size as of object i.e., h_i = -h_o

Since, magnification (m) = h_i / h_o = v / u

= -h_o / h_o = v / u

v / u = -1

u = -1 × v

u = -20 cm

1/f = 1/v -1/u

= 1/20 – 1/(-20)

= 2/20

1/f = 1/10

f = 10 cm = 0.1 m

The object is placed 20 cm away from the convex lens.

Magnification of Lens

Magnification of Lens is defined as the ratio of height of the image to the height of the object. Magnification is denoted by ‘m’. Magnification of Lens formula is given as follows

m = h’/h

where,

• m = Magnification,
• h’ = height of image,
• h= height of object.

Magnification of Lens formula in terms of image and object distance is given as follows:

m = v/u

where,

• m is magnification
• v is image distance
• u is object distance

Power of Lens

Power of a lens describes the ability of lens to converges or diverge the rays of lights when falls on the lens. It is also defined as the reciprocal of focal length of a lens. Power is denoted by P.

P = 1/f

where

• P is Power of Lens
• f is Focal Length in m

If Focal length is in cm, the Power of Lens Formula is given as

P = 100/f

where

• P is Power of Lens
• f is Focal Length in cm

Total Internal Reflection

The complete reflection of a light ray at the boundary of two media when the beam travels from the medium with a higher refractive index to low refractive index is known as total internal reflection.

Condition for TIR

• Light should be travelling from Denser Medium(High Refractive Index) to Rarer Medium(Low Refractive Index)
• If the incidence angle is larger than the critical angle.

Application of TIR

• TIR is used in Optical fiber.
• It is binocular, telescope, periscope.
• TIR concept is used in Diamond.
• TIR is used in the formation of Mirage

Sample Questions on NCERT Notes Class 10 Physics Chapter 10

Q1: What is the real image?

Answer:

The image formed when rays of light meet at a certain point after reflection/refraction is real image. Real images can be displayed on screen.

Q2: What is a ray diagram?

Answer:

The type of diagram which helps to trace the path that light takes in order for a person to view a point on the image of an object is called a ray diagram.

Q3: What will be the change in focal length of a lens when it is cut along the principal axis?

Answer:

There will be no change in the focal length of a lens when cut into two halves along the principal axis, because the focal length of the lens is half of the radius of curvature and radius of curvature will remain the same.

Q4: A convex mirror is kept underwater. What would be the change in the focal length of the mirror?

Answer:

As the focal length of a spherical is independent of nature of the medium. So, the focal length of the convex mirror doesn’t change.

Q5: Two concave mirrors have the same focal length but the aperture of one is larger than that of the other. Which mirror forms the sharper image and why?

Answer:

As the concave mirror with smaller aperture is free from spherical aberration. So, concave mirror with smaller aperture forms sharper image.

Q6: Riya stands in front of a spherical mirror. She finds that her image has a very fat body. Tell us what was the shape of the mirror?

Answer:

As the image has very fat body means she is standing in front of a concave mirror.

NCERT Notes for Class 10 Physics Chapter 10 Light – FAQs

1. What is Light?

It is an electromagnetic device that can be visible to humans and help humans to see the object.

2. What is Pole of Spherical Mirror?

The middle point P of a spherical mirror is called Pole.

3. What is Principal Axis?

The line passes through the pole and the center of curvature of the mirror is called principal axis. Line PC in the figure below is principal axis.

4. What is Linear Aperture?

The diameter of the circular boundary of the spherical mirror is called Linear aperture.

5. What is Principal Focus?

When a narrow beam of light parallel to the principal axis either converges or appears to converge from a point F after reflection from the spherical mirror is called principal focus.

6. What is Radius of Curvature?

The radius of sphere from which the spherical mirror is formed is called radius of curvature.

7. What is Refraction?

The phenomenon of change in the direction/path of light rays when it travels from one transparent medium to another is called refraction.

8. What is Snell’s Law?

Snell’s Law states that for the light if given colour and for the given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. It gives the degree of refraction and also tells the relation between the angle of incidence, the angle of refraction and the refractive index of media.

9. What is Dispersion of Light?

Splitting of light into band of 7 colours when it passes through a glass prism is called Dispersion of Light

10. What is Power of a lens?

Power of a lens describes the ability of lens to converges or diverge the rays of lights when falls on the lens. It is also defined as the reciprocal of focal length of a lens. Power is denoted by P.