RBSE Class 10 Science Notes Chapter 10 Light Reflection and Refraction

These comprehensive RBSE Class 10 Science Notes Chapter 10 Light Reflection and Refraction will give a brief overview of all the concepts.

RBSE Class 10 Science Chapter 10 Notes Light Reflection and Refraction

→ Light seems to travel in straight line. This straight line path of light is called ray of light.

→ Diffraction of light-If the size of the opaque obstacle on the path of light is very small, light has the tendency to bend around the corners of the obstacle. This phenomenon is called as diffraction of light.

→ To explain phenomena such as diffraction, light is thought of as a wave, then light is treated as a particle in the interpretation of interaction with matter of light.

→ A modern quantum theory of light energed in which light is neither a ‘wave’ nor a ‘particle’-the new theory reconciles the particle properties of light with the wave nature.

→ Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.

→ The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.

→ Reflection of light-When light ray travelling from one medium bounces back in the same medium after falling on any surface, such a phenomenon is termed as reflection of light.

→ Laws of Reflection

(i) Incident ray, reflected ray and the normal at the point of incidence, all lie in the same plane.
(ii) The angle of incidence is equal to the angle of reflection, i.e. ∠i = ∠r
(iii) The speed, frequency and wavelength of the reflected ray remains unaltered.

→ When a glass strip is polished from one side it is called a mirror. Mirrors are mainly of two types
(i) Plane mirror-The image formed by a plane mirror is always virtual and erect and equal in size to that of a object. The image formed is as far behind the mirror as the object in front of it. Spherical mirror-A spherical mirror is a part of hollow sphere whose inner or outer surface is polished. Their reflecting surface is curved. They are of two types
(a) Concave mirror
(b) Convex mirror.
(a) Concave mirror-A concave mirror is one whose reflecting surface is curved inwards towards the centre of the sphere.
(b) Convex mirror-A convex mirror is one whose reflecting surface is curved outwards away from the centre of sphere.

→ Pole of mirror-The central point of the reflecting surface of a mirror is called the ‘pole’ of the mirror. It is represented by ‘P’.

→ Centre of curvature and radius of curvature Centre of curvature is the centre of hollow sphere of which the spherical mirror is a part. It is represented by ‘C’. The distance between the pole and the centre of curvature of a mirror is called the radius of curvature. It is represented by ‘R’.

→ Principal axis-A straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis.

→ Principal Focus-The point on the principal axis at which light rays parallel to the principal axis, after reflection from the mirror, actually meet or appear to come from is called the ‘principal focus of the mirror. It is represented by ‘F’.

→ Focal Length-The distance between the principal focus and the pole of the mirror is called the focal length of the mirror. It is represented by ‘f’. For spherical mirrors of small aparture the radius of curvature is equal to twice the focal length i.e. R = 2f.

→ The diameter of the reflecting surface of spherical mirror is called its aperture.

→ Image formation by a concave mirror for different positions of the object is given in the following table

→ Uses of concave mirror-They are commonly used in torches, search lights and vehicles headlights to get powerful parallel beams of light.

→ Nature, position and relative size of the image formed by a convex mirror –

→ Uses of convex mirror-Convex mirrors are commonly used as rear view (wing)
mirrors in vehicles.

→ Sign convention for reflection by spherical mirror-While dealing with the reflection of light by spherical mirrors, we shall follow a set of sign conventions called the New Cartesian Sign Convention. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system.

→ Mirror formula
Object distance (u) = In a spherical mirror, the distance of the object from its pole. Image distance (ν) = Distance of the image from the pole of the mirror.

Focal length (f) = Distance of the principal focus from the pole.

\(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\)

→ Magnification-Magnification produced by a spherical mirror is given by –

A negative sign in the value of the magnification indicates that the image is real. A positive sign in the value of the magnification indicates that the image is virtual. The magnification m is also related to the object distance (u) and image distance (ν). It can be expressed as :

Magnification (m) = \(\frac{h^{\prime}}{h}=-\frac{v}{u}\)

→ The bending of the light ray from its path in passing from one medium to another medium is called ‘refraction of light’.

→ Laws of refraction-Refraction of light is based on certain laws,

(i) The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as snells law of refraction. If the angle of incidence be i and angle of refraction be r, then,

\(\frac{\sin i}{\sin r}\) = constant.

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

→ Absolute refractive index – It is defined as the ratio of the speed of light in vacuum to that in the material medium.

→ The speed of light is higher in a rarer medium than a denser medium. Thus, a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

→ Refractive index of medium-It is defined as the ratio of the speed of light in air to that of the speed in the material medium. It has no unit. If the speed of light in air is ν₁ and in glass is ν₂ then the refractive index of water with respect to air is given by :

N₂₁ = \(\frac{v_{1}}{v_{2}}\)

Refractive index depends upon the nature of materials, density of medium and wavelength of the colour of light. Refractive index of some material media are

• Benzene = 1.50
• Water = 1.33
• Turpentine oil = 1.47
• Diamond = 2:42
• Carbon disulphide = 2.64.

→ Refraction through spherical lens

• Lens – A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical or one of the surface is plane.
• Biconvex lens – A lens having its two surfaces bulging outwards is called biconvex lens.
• A double concave lens is bounded by two spherical surfaces, curved inwards.
• Convex lens are called converging and concave lens are called diverging lens.

→ Centre of curvature-A lens has two spherical surfaces and each surface is a part of sphere. The centre of the sphere of which the surface forms a part is called the ‘centre of curvature’.

→ Principal Axis-The imaginary straight line joining the centres of curvature of the surfaces is called the ‘principal axis’ of the lens. The centre point of a lens is known as its ‘optical centre’

→ Image formation by lens –

(i) Nature, position and size of image formed by convex lens –

(ii) Nature, position and relative size of the image formed by a concave lens for various position of objects –

→ Lens formula and magnification-The relationship between the object distance (u), image distance (ν) and the focal length (f) is known as lens formula and it is given by

\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)

→ Sign convention for spherical lenses For lenses, we follow sign conventions, similar to the one used for spherical mirrors. All measurements are taken from the optical centre of the lens. According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative.

→ Magnification-If h is the height of the object and h is the height of the image then the magnification m produced by a spherical lens is given by

→ Power of lens-Power of lens is its ability to converge or diverge the light rays falling on it. It is denoted by P. The power P of a lens of focal length fis given by

P = \(\frac{1}{f}\)

The unit of power of lens is dioptre represented by the letter D.

→ The net power (P) of the lenses placed in contact is given by the algebraic sum of
the individual powers P₁, P₂, P₃ ……. as
P = P₁ + P₂ + P₃ + …..

→ 1 Dioptre of power of lens – It is the power of the lens when its focal length is 1 m. 1D = 1m^-1. Hence power of a convex lens is positive and that of a concave lens is negative.