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

Light: Light is a form of electromagnetic radiation (radiant energy) which makes things visible. When the light rays falls on objects, it is reflected back and enters our eyes. This produces the sensation of vision and hence, we are able to see the objects around us. 

Important uses of light: 

1. Light enables us to see even through a transparent medium because light is transmitted through it. 

2. Light makes things around us visible. 

2. Light is highly useful in modern communication. We can transmit thousands of telephonic conversations simultaneously via optical fibres over long distances.

To get erect and diminished image mirror used is a convex mirror.

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.