**Lens Formula: **The lens formulagives the relation between image distance, object distance and focal length. The lens formula is given by:

Where

v = image distance

u = object distance

f = focal length

The lens formula is true for all situations with appropriate sign conventions.

**Similar to the magnification produced by mirrors, magnification produced by lenses is the ratio of height of image to height of object. It can be represented as:**

or = m =

$\frac{h2}{h1}$

Where

m = magnification

h_{2} = height of image

h_{1} = height of object

The linear magnification produced by a lens is equal to the ratio of image distance to the object distance. That is:

$Magnification=\frac{imagedis\mathrm{tan}ce}{objectdis\mathrm{tan}ce}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}m=\frac{v}{u}$

Where

m = magnification

v = image distance

u = object distance

Important notes on magnification:

(i)The positive sign shows that the image is erect and virtual

(ii)The negative sign shows that the image is real and inverted

For a spherical lens, remember for the linear magnification (m):

**Virtual positive, real negative.**

- The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it.
- The power of a lens is defined as the reciprocal of its focal length. It is represented as:

$PowerofLens=\frac{1}{focallengthoflens(inmeters)}$Or

$P=\frac{1}{f}$Where P = Power of the lens

And f = focal length of the lens (in metres) **The unit of the power of a lens is dioptre. It is denoted by ‘D’**.- A concave lens has a negative focal length, so
**the power of a concave lens is negative**. **Power of convex lens is always positive**

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