Question

Define principal focus of a convex lens. Draw light ray diagram of an image formed, when the object is placed in between principal focus $F$ and $2F$.
Write lens formula.
Calculate power of a lens if its focal length is $0.5m$.

Answer 1

Principal Focus

When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.

Light ray diagram of an image formed, when the object is placed in between principal focus FF and 2F2F is given in the above figure.

Lens Formula is given by:-

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

where,

f = focal length of the lens.

v = distance of image from the optic center of the lense.

u = distance of object form the optic center of the lens.

The power of any lens is given by the formula:-

$Power=\frac{1}{\text{Focal length of the lens}}\text{(in diopter)}$

therefore, $Power=\frac{1}{0.5}=2\text{diopter}$

Fand 2F is shown in the diagram above.