Refraction of Light Part-1


The travel in the same direction in all media. It appears that when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.

Example: The bottom of a tank or a pond containing water appears to be raised. Similarly, when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.


 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. (This is true for angle 0 < i < 90)

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

 $\frac{Sin'i'}{Sin'r'}$Sin'i'Sin'r'  = constant .

Refractive Index:

  • The refractive index can be linked to an important physical quantity, the relative speed of propagation of light in different media. It turns out that light propagates with different speeds in different media.
  • Light travels fastest in vacuum with speed of 3×10⁸ ms-¹ In air, the speed of light is only marginally less, compared to that in vacuum. It reduces considerably in glass or water. The value of the refractive index for a given pair of media depends upon the speed of light in the two media.





Lens is a trans missive optical device that focuses or disperses light beams using refraction. A simple lens consists of a single piece of transparent material, while compound lenses consist of several simple lenses arranged along with a common axis.

Types of Lens:

The two main types of lenses are:

  • Convex Lens (Converging)
  • Concave Lens (Diverging)


Ray Diagram for formation of image by Convex Lens:



      Ray Diagram for Image formation by Concave Lens: