Spherical lenses are the lenses formed by bounding two spherical transparent surfaces together. There are two basic kind of spherical lenses: concave lens and convex lens. Spherical lenses formed by binding two spherical surfaces bulging outward are known as convex lenses while the spherical lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses. Images formed by these lenses can be real or virtual depending on their position from the lens and can have a different shape too. The image distance can be calculated with the knowledge of object distance and focal length with the help of lens formula. Lens formula is an equation which relates the focal length, image distance and object distance for a spherical mirror. It is given as,

\( \frac{1}{v} \)

v= distance of image from the lens

u= distance of object from the lens

f= focal length of the lens

The lens formula is applicable to all situations with appropriate sign conventions.

# Calculating magnification with the help of lens formula:

Magnification of a lens is defined as the ratio of the height of image to the height of object. It is also given in terms of image distance and object distance. It is equal to the ratio of image distance to that of object distance.

\( m = \frac{h_i}{h_o} = \frac{v}{u} \)

Where, m= magnification

h_{i} = height of image

h_{o}= height of object

## Power of lens

Power of a lens is the measure of degree of convergence or divergence of the light rays falling on it. The degree of convergence or divergence depends upon the focal length of the lens. Thus we define power of lens as the reciprocal of the focal length of the lens used. It is given as,

Where f is the focal length of the lens used. SI unit of power is Dioptre (D). The power of concave lens is negative while the power of convex lens can be positive.

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