# Class X, LIGHT - REFLECTION AND REFRACTION | Cbse Ncert Physics (Science) Notes, Study Materials, and Important Sample Questions | Lesson 2 of 3

## LIGHT - REFLECTION AND REFRACTION

### Physics (Science) Important Notes and Study Materials

(LESSON 2 of 3)
Mirrors Formula
There is a relationship between the distance of image (v), distance of object (u) and the focal length of a spherical mirror (f) which is given by the Mirror Formula. The mirror formula is,
Magnification
The magnification of a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size. It is expressed as the ratio of the height of image to the height of object.
The equation holds true for both concave and convex mirror. M is –ve for inverted image and +ve for erect image. So, magnification is always positive for a convex mirror, while it depends on the position of the position of the object with respect to concave mirror.
Refraction
The phenomenon of bending of light as it travels from one medium to another medium is called refraction of light.
When light enters from a rarer medium into a denser medium it will bend towards the normal. Similarly when light gets into a rarer medium from a denser medium it will bend away fro the normal.
Refraction or change in the direction in the light ray (bending) takes place on account of a change in the speed of light on entering the second media. (Why does the phenomenon of refraction takes place? or, Why does the ray of light change the direction when entering from one medium to another medium?)
Laws of Refraction
There are two laws of refraction:
(i) The incident ray, the refracted ray and the normal to the interface of the two media at the point of incidence - all lie in the same plane.
(ii) The ratio of the sine of the angle of refraction for a given pair of media is constant. This is known as Snell’s Law. Mathematically this can be represented as:
Refractive Index

It is the ratio of the angle of incidence to the sine of the angle of refraction when light is refracted from one medium to another medium. Refractive index is also linked to an important physical quantity i.e. the relative speed of propagation of light in different media.
Consider a ray o light travelling from medium 1 (air) into medium 2 (glass) as shown in the above figure. Let v1 be the speed of light in the medium 1 and v2 in medium 2. The refractive index of medium 2 with respect to medium 1 can be expressed as
n21 = (Sin i ÷ Sin r) = (v1 ÷ v2)
Spherical Lens
A lens is a curved piece of glass or any other transparent material bound by two surfaces of which one or both surfaces are spherical, through which light can pass. There are two types of lenses: Concave Lens and Convex Lens.
Concave Lens
A concave or bi-concave lens is made by joining two curved surfaces in such a way that it is thinner at the center. The thickness gradually increases as we move towards edge.
Convex Lens
A convex or bi-convex lens is made by joining two curved surfaces in such a way that it is thicker at the center. The thickness gradually reduces as we move towards the edge.
Optical Center

Optical center is a point at the center of the lens. It lies inside the lens and not on the surface. Optical center is usually represented by the letter O (as shown in the figures). a ray of the light through the optical center of a lens always passes without suffering any deviation.
Center of Curvature of a Lens
It is the center point of arcs of the two spheres from which the given spherical lens (concave or convex) is made. Since a lens constitutes two spherical surfaces, it has two centers of curvature.
Radius of Curvature of a Lens
The distance of the optical center from either of the center of curvatures is termed as radius of curvature.
Principal Axis of a Lens
An imaginary straight line passing through the two centers of curvature of a lens is called its principal axis.
Power of a Lens
The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power (P). The power of a lens is defined as the reciprocal of its focal length.
Cbse Ncert Guide Class 10 Physics Notes, Study Materials (Light - Reflection and Refraction):
• Cbse Guide Class 10, Science (Physics) | Light - Reflection And Refraction | Solutions of Ncert Science In-Text Questions  (to be published)

• Class 10, Solutions of Ncert Cbse Science (Physics) | Light - Reflection and Refraction (CCE type Multiple Choice Questions MCQs Solutions) (to be published)

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