# Optics Formula

Optics describe the light propagation in terms of the light ray. The light ray in geometrical optics is an instrument which is used to approximate models of how the light will propagate. Light rays tend to bend at the interface of two different medium where the refractive index changes. The geometrical optics gives us the rules for light propagating through optical devices.

The geometrical optics could be made used to explain the geometrical imaging and aberrations.

The lens is one of such optical device with axial symmetry which allows and refracts light ray to either converge or diverge the light beam.

The formula for a thin lens is as follows:

$$\begin{array}{l}\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\end{array}$$

Where

f is the focal length of the thin lens

v is the image distance

u is the object distance.

Magnification of lens is the process by which an object is enlarged in appearance without increasing the size physically.

The formula which helps in getting the thin lens magnification is given by Â m =

$$\begin{array}{l}\frac{h_{i}}{h_{o}}\end{array}$$

$$\begin{array}{l}\frac{heightÂ ofÂ image}{heightÂ ofÂ object}\end{array}$$

Power of lens (dioptre) =

$$\begin{array}{l}\frac{1}{f}\end{array}$$
Â (in meters)

## Solved Examples

Some Examples on Optics Formula are:

### Solved Examples

QuestionÂ 1: AÂ short-sighted person can see clearly at a distance of 20cm. Calculate the lens power essential for the person to see as far as 60 cm?
Solution:

The power required for a far point of 60 cm is
1/f = 1/u + 1/v
Â  Â  Â  Â  =1/60 – 1/20
Â  Â  Â  Â  =-2/60
Â  Â  Â  Â  1/f =-1/30
f=-30cm=0.30m
Power D=1/f=1/0.30
Power=3Â dioptres
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