Science: Light Energy: Magnification, Refraction of Light and Laws of Refraction (For CBSE, ICSE, IAS, NET, NRA 2022)

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The ratio of the size of the image to the size of the object is called linear magnification.

i.e.. ,

Refraction of Light

When light passes from denser medium to rarer medium it bends away from the normal. When it passes from rarer medium to denser medium it bends towards the normal. This phenomenon of bending of light is called refraction of light.

Refraction of Light

Refractive Index of the Medium

  • It is defined as the ratio of the speed of light in vacuum to that in the material medium.
  • When light travels from one medium to another its speed changes.
  • A ray of light from a rarer medium to a denser medium slows down and bends towards the normal.
  • On the other hand, the ray of light going from a denser medium to a rarer medium is speeded up and bends away from the normal.

Laws of Refraction

  • First law of refraction: The incident ray, refracted ray and the normal at the point of incidence, all lie in the same plane.
  • Second law of refraction: How much ray of light refracted depends on that medium. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant and equal to the refractive index of that medium. This law is also called Snell՚s law.


A lens is a portion of a transparent refracting medium bounded by two surfaces. Depending upon the nature of surfaces lens may be of following types.

Convex Lens

Convex lens has its two surfaces bulging outward. It makes the parallel rays of light to converge to a point. Hence, it is called converging lens. The point of convergence is called focus.

Convex Lens

Concave Lens

A concave lens has its two surfaces caving inward as shown in Fig. 15.24. It makes parallel rays of light to spread from a point. Hence it called diverging lens. The point where from light rays appear to diverge is called focus.

Concave Lens

Image Formation in Lenses

  • A ray parallel to the principal axis of the lens converges after refraction at the principal focus of convex lens. It appears to diverge off in the case of concave lens.
  • A ray towards the optical center falls on the lens symmetrically and after refraction passes through it undedicated.

The image formations in convex and concave lenses are shown.

Image Formation in Lenses
  • Object is between focus and lens
  • Object at the first focus
Image Formation in Lenses

Object is between F2 and 2F2

Image Formation in Lenses

Object is at 2F2

Image Formation in Lenses

Image by concave lens.

Sign Convention

  • All distances in a lens are to be measured from optical center of the lens.
  • Distances measured in the direction of incident ray are taken to be positive
  • Distance opposite to the direction of incident ray are taken to be negative
  • The height of the object or image measured above the principal are taken positive whereas below it, are taken negative.

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