# NCERT Chapter Summary: Light

Light seems to travel in straight lines. 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. New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

Mirror formula gives the relationship between the object-distance (u), image-distance (v), and focal length (f) of a spherical mirror.

1/v + 1/u = 1/f

The focal length of a spherical mirror is equal to half its radius of curvature. The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object.

A light ray travelling obliquely from a denser medium to a rarer medium bends away from the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a denser medium.

Light travels in vacuum with an enormous speed of 3 × 108 m s-1. The speed of light is different in different media.

The refractive index of a transparent medium is the ratio of the speed of light in vacuum to that in the medium. In case of a rectangular glass slab, the refraction takes place at both air-glass interface and glass-air interface. The emergent ray is parallel to the direction of incident ray.

Lens formula gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.

1/v - 1/u = 1/f

Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.