CBSE – 10th Standard Science

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Class 10 Chapter 10 – Light – Reflection and Refraction

Light – Reflection and Refraction

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Spherical mirror

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Image Formation by Spherical Mirrors

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Images formed by concave mirror

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Magnification produced by 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 the image to the height of the Object. It is usually represented by the letter m.

If h1 is the height of the object and h0 is the height of the image, then The magnification m produced by a spherical mirror is given by

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Refraction through a Rectangular Glass Slab

The following are the laws of refraction of light.

(i) The incident ray, the refracted ray and the normal to the interface of two transparent media 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 Snell’s law of refraction.

If i is the angle of incidence and r is the angle of refraction, then,

Sin i =constant

Sin r

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

The Refractive Index

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Image Formation in Lenses Using Ray Diagrams

Images formed by convex lens

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Images formed by concave lens

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Lens Formula and Magnification

The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The Power P of a lens of focal length f is given by

P=1/f

The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.

If f is expressed in metres, then, power is expressed in dioptres. Thus, 1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m–1.

You may note that the power of a convex lens is positive and that of a Concave lens is negative.

Opticians prescribe corrective lenses indicating their powers.

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