Light is a form of energy, which enable us to see the object.

In this chapter we will study the phenomena of reflection and refraction using the property of light i.e. straight line propagation (Light wave travel from one point to another, along a straight line).

Reflection of Light

When the light is allowed to fall on highly polished surface, such as mirror, most of the light gets reflected.

4. The image formed is as for behind the mirror as the object is in front of it.

Reflection of light by spherical Mirrors

Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical mirror.

1. Principal axis : Line joining the pole and centre of curvature of the spherical
2. Pole : The geometrical central point of the reflecting spherical (aperture), denoted by (P).
3. Aperture : The width of reflecting spherical
4. Centre of curvature : The reflecting surface of a spherical mirror form a part of It has a centre, which is known as centre of curvature, denoted by (C)
5. Radius of curvature : The separation between the pole and the centre of ie.        PC = R

6. Focus point : The point on the principal axis, where all parallel rays meet after reflection, denoted by (F)

Note : A ray of light passes through centre of cus-valerie reflecting spherical surface is always act as normal at the point of incidence. If we know the normal we can draw angle of incidence and angle of reflection

Uses of Concave Mirror

1. Used in torches, search light and headlight of
2. Used to see large image of face as shaving mirror
3. Used by dentist to see large images of the teeth
4. Large concave mirror used to focus sunlight (heat) in solar

Uses of Convex Mirror

1. Used as rear-view mirror in vehicles because it gives erect image. It also helps the driver to view large area.

Sign Convention for Reflection by Spherical Mirror

1. The object is always placed to the left side of
2. All distance should be measured from pole (P); parallel to principal
3. Take ‘P’ as Distances measured

Right of the origin (+ x – Axis) are taken positive

Left of the origin (– x-Axis) are taken negative

Perpendicular to and above principal axis (+y-Axis) are taken positive

Perpendicular to and below principal axis (–y-Axis) are taken negative

REFRACTION OF LIGHT

Refraction of Light : Happens in Transparent medium when a light travels from one medium to another, refraction takes place.

A ray of light bends as it moves from one medium to another

Refraction is due to change in the speed of light as it enters from one transparent medium to another.

Speed of light decreases as the beam of light travel from rarer medium to the denser medium.

Some Commonly observed phenomenon due to Refraction

1. The stone at the bottom of water tub appear to be
2. A fish kept in aquarium appear to be bigger than its actual
3. A pencil partially immersed in water appears to be displaced at the interface of air and

Refraction through a Rectangular Glass Slab

When a incident ray of light AO passes from a rarer medium (air) to a denser medium (glass) at point. O on interface AB, it will bends towards the normal. At pt O1, on interface DC the light ray entered from denser medium (glass) to rarer medium (air) here the light ray will bend away from normal OO1is a refracted ray OB is an emergent ray. If the incident ray is extended to C, we will observe that emergent ray O1B is parallel to incident ray. The ray will slightly displaced laterally after refraction.

Note : When a ray of light is incident normally to the interface of two media it will go straight, without any deviation.

1. Centre of curvature – A lens, either a convex lens or a concave lens has two spherical Each of these surfaces form a part of sphere. The centre of these two spheres are called centre of curvature represented by C1 and C2.
2. Principal axis – Imaginary straight line passing through the two centres of curvature
3. Optical Centre – The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated e. it goes straight.
4. Aperture – The effective diameter of the circular outline of a spherical
5. Focus of lens – Beam of light parallel is principal axis, after refraction from

EXERCISE

(Question Bank)

Very Short Answers Type Questions (1 Mark)

1. If the angle of incidence is O°, what is the angle of reflection?
2. What is the nature of image formed by concave mirror if the magnification produced by the mirror is +3?
3. Give two uses of concave mirror?
4. Find the focal length of a convex mirror, whose radius of curvature is 30 cm?
5. What do you understand by magnification of a spherical mirror?

An object is held at the principal focus of a concave lens of focal length Where the image will form?9. Define the SI unit of power of lens

10. When light undergoes refraction at the surface of seperation of two media, what happens to speed of light.

Short Answer Type Questions (2-3 Marks)

1. What do you understand by refraction of Draw the labelled ray diagram, when ray passes through glass slab.
2. The refractive index of glass is 54 and the speed of light in air is 3×108 m/s. Calculate the speed of light in water?
3. A convex mirror used on an automobile has a focal length of If vehicle behind is at a distance of 12m. Find the nature and location of image.

(4m, virtual erect small)

4. A concave lens of focal length 15cm, forms an image 10 cm from the How far is the object placed from the lens? Draw the ray diagram?
5. Two thin lens of power +3.5D and – 5D are placed in contact. Find the power and focal length, if the lens are in combination. (p = + 10, f = 1m)
6. What are the law of Define refractive index of a medium.

Very Long Answer Type Questions (5 Marks)

1. Draw the ray diagram, showing the image formed by concave mirror, when object is placed at
   1. at infinity b)     between F22F
   2. c) At 2F d)     At F
   3. e) between F&P
2. Draw the ray diagram, showing the image formed by convex lens, when object is placed
   1. At infinity b)     between F1 & 2F1
   2. c) At 2F1 d)     Beyond 2F1
   3. e) between F1 & optical centre ‘O’