CBSE Notes Class 10 Science Chapter 9 - Light: Reflection and Refraction

Light is integral to our everyday experiences, influencing both how we perceive our surroundings and the technology we depend on. Two key phenomena that describe how light interacts with different surfaces and materials are reflection and refraction. These phenomena are fundamental to understanding various optical effects and form the basis of optics. 

1.0Reflection Of Light

Reflection is the process where light rays encounter the boundary between two different media and bounce back into the original medium. Specifically, it refers to the phenomenon where light rays that strike the surface of an object are returned into the same medium rather than passing through or being absorbed.

2.0Laws of Reflection

1. The path of the incoming ray, the path of the reflected ray, and the perpendicular line (normal) to the surface at the point of incidence are all situated in the same plane.
2. The angle at which the incoming light ray impacts a surface (the angle of incidence) is equal to the angle at which it bounces off (the angle of reflection) ∠i=∠r

3.0Some basic Terms Related To Spherical Mirrors

4.0Sign Conventions For Spherical Mirrors

5.0Image Formation By Convex And Concave Mirror

Convex Mirror

Concave Mirror

Uses of Concave and Convex Mirror

1. Concave Mirror-Reflectors in the headlights, search light in torches
2. Convex Mirror-Rear View in an Automobile.

6.0Mirror Formula

The relationship between the object distance(u), the image distance(v) and the focal length(f) is given by Mirror Formula.

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

7.0Magnification

• The ratio of height image $(h_2)$ to the height of object $(h_1)$ is called magnification or linear magnification. $m=\frac{h_2}{h_1}$
• Magnification is also related to object distance and image distance. $m=\frac{h_2}{h_1}=-\frac{v}{u}$
• Magnification also expressed as $m=\frac{f}{f-u}=\frac{f-v}{f}$

8.0Refraction Of Light

The phenomenon of change in path of light when it passes from one medium to another is called Refraction.

Cause of Refraction

The bending of light takes place when it passes from one medium to another because speed of light changes from one medium to another.Speed of light is different in different media.

Laws of Refraction

1. The incident ray, the refracted ray, and the normal at the point of incidence are all contained within a single plane.
2. $\frac{Sini}{Sinr}=Constant(\mu )$

Refraction From Rarer to Denser Medium and Denser to Rarer Medium Ray Diagram

Refraction Through A Rectangular Glass Slab

When light passes through a rectangular glass slab, it undergoes refraction at both the entry and exit faces:

1. Entry Refraction: As light enters the glass from air, it bends towards the normal due to the slower speed of light in the denser glass.
2. Inside the Slab: The light travels in a straight line within the slab, still refracted but at an angle to the normal.
3. Exit Refraction: Upon leaving the slab, the light bends away from the normal as it moves back into the less dense air.

Key Points

• Parallel Faces: The entry and exit rays are parallel because the angle of incidence equals the angle of emergence.
• Lateral Displacement: Although parallel, the light is displaced sideways from its original path, depending on the slab's thickness, the angle of incidence, and the glass's refractive index.

In essence, while the light rays remain parallel, they shift from their initial path due to refraction.

9.0Refractive Index

Refractive index of a medium$(n_{21})$

The relative refractive index of a medium 2 w.r.t medium 1 is the ratio of speed of light in medium 1 to the speed of light in medium 2.

$n_{21}=\frac{\text{speed of light in medium 1}}{\text{speed of light in medium 2}}$

$n_{21}=\frac{v_1}{v_2}$

Absolute Refractive Index of a Medium

The proportion of speed of light in vacuum to the speed of light in a material is called absolute refractive index of medium.

$n=\frac{c}{v}=\frac{\text{speed of light in vacuum}}{\text{speed of light in medium}}$

• Optically denser-Greater the value of n less will be the speed of light.
• Optically rarer-Lesser the value of n greater will be the speed of light

10.0Image Formation In Spherical Lenses

(A). Rules to obtain images in Convex lens

• A ray parallel to the principal axis moves through the focal point after refraction.
•  A ray passing through the focal point emerges parallel to the principal axis.
•  A ray through the optical center continues straight without deviation.

(B). Rules to obtain images in Concave lens

•  A ray passes  parallel to the principal axis diverges as if it were coming from the focal point on the same side of the lens.
•  A ray directed towards the focal point diverges parallel to the principal axis.
•  A ray through the optical center continues straight without deviation.

For Convex Lens

For Concave Lens

1. Image Formation By Convex Lens

2. Image Formation By Concave Lens

11.0Sign Convention For Spherical Lenses

12.0Lens Formula And Magnification

• The relationship between object distance(u) ,image distance(v) and the focal length(f) is given by $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
• Magnification-The ratio of height of image $(h_2)$ to the height of object $(h_1)$

$m=\frac{h_2}{h_1}=+\frac{v}{u}$

$m=\frac{f}{f+u}=\frac{f-v}{f}$

13.0Power Of Lens

• It is the extent of convergence or divergence of light rays. Falling on it.A lens of short focal length bends the light rays more through large angles.
• The power of a lens is defined as the inverse of its focal length. $P=\frac{1}{F}$
• Units is Dioptre (D)
• For convex lens power is positive and for concave lens power is negative.