An object of height 4.0 em is placed at a distance of 30 cm from the optical centre ‘O’ of a convex lens of focal length 20 cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘O’ and principal focus ‘F’ on the diagram. Also find the approximate ratio of the size of the image to the size of the object.

Ray diagram:

Here, focal length, f = 20 cm,
u = - 30 cm,
h₀ = 4 cm

Using, lens formula,

therefore, image distance, v = 60 cm
The image formed is real and inverted.

The ratio of the size of the image to the size of the object

(1) The incident ray, the refracted ray and the normal to the interface of two media at the point of incidence all lie in the same plane.

(2) For the light of a given colour and for a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

This is also known as Snell's Law.

Mathematically it can be written as:

$\frac{\sin i }{\sin r} = cons\tan t ={ }^A{\mu }_B$

Here, μ is the refractive index of medium B with respect to medium A.

Refractive index of a medium with respect to the vacuum is known as the absolute refractive index.

The power of lens is defined as the reciprocal of its length. It is denoted by using the letter P.

$P\propto \frac{1}{F}$

Unit of Power is Dioptre (D)

Focal length = 40 cm

Focal length = -20 cm

Convex lens P = 1/f = 100/40 = 2.5 D

Concave lens = P = 1/f = -100/20 = -5D