Light Refraction Important Points

Light Refraction Important Points


The bending of a ray of light as it passes from one medium to another is called refraction.

(a)  A ray of light travelling from a rarer medium to denser medium (say, water or glass) bends towards the normal.

(b)  A ray of light travelling from a denser to a rarer medium bends (or refractive) away from the normal.

Laws of refraction

(a) The ratio of sine of the angle of incidence to the sine of the angle of refraction for a particular pair of media is constant, i.e.
sin i / sin r = constant is equal to the refractive index of  the medium into which the light is entering.

(b)  The incident ray, the refracted ray and the normal all lie in the same plane.

Lateral displacement

The perpendicular distance  of separation between the emergent ray and the original path of the incident ray is called lateral displacement.


A piece of any transparent material bound by two curved surfaces is called a lens. A lens which is thicker in the middle and thinner at the edges is called a convex lens. A convex lens is also called converging lens.

A lens which is thicker at the edges and thinner at the centre is called a concave lens.

A concave lens is also called a diverging lens.

Optical centre of a lens

The centre point of a lens is called its optical centre. A ray of light passing through the optical centre does not suffer any deviation.

Image formed by the lenses

A convex lens forms real and inverted images for all the positions of an object at and outside the focus (F). However, when the object is placed between F and O, the image formed by a convex lens is virtual and erect.

A concave lens always forms a virtual, erect and a diminished image whatever may be the distance of the object from the lens.

  Lens formula 

The lens formula is  \frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ .

Power of a lens

Reciprocal of the focal length of a lens measured in metres is called its power. Power of a lens is described in dioptre (D) unit.

Refractive indices of various substances relative to vacuum with light of wavelength 589 mm

Solids (at 20ºC)

Liquids (at 20ºC)

 Gases (at 0ºC, 1 atm)