Physics > Refraction of Light > 5.0 Lens makers formula & Other Functions of lens.
Refraction of Light
1.0 Introduction
2.0 Laws of refraction
3.0 Apparent shift of an object
4.0 Thin lenses
4.1 Sign convention
4.2 Some important terms
4.3 Ray tracing
4.4 Image formed by covex lens
4.5 Image formed by concave lens
5.0 Lens makers formula & Other Functions of lens.
5.1 Thin Lens Formula
5.2 Magnification and Power of lens
5.3 Combination of lenses
5.4 Displacement method to find focal length.
5.5 Silvering of lens
6.0 Total internal reflection
7.0 Refraction through prism
8.0 Scattering of light
9.0 Optical instruments
9.1 Spectrometer
9.2 Simple microscope
9.3 Compound microscope
9.4 Astronomical telescope (Refracting type)
9.5 Terrestrial telescope
9.6 Galileo's terrestrial telescope
9.7 Reflecting type telescope
5.2 Magnification and Power of lens
4.2 Some important terms
4.3 Ray tracing
4.4 Image formed by covex lens
4.5 Image formed by concave lens
5.2 Magnification and Power of lens
5.3 Combination of lenses
5.4 Displacement method to find focal length.
5.5 Silvering of lens
9.2 Simple microscope
9.3 Compound microscope
9.4 Astronomical telescope (Refracting type)
9.5 Terrestrial telescope
9.6 Galileo's terrestrial telescope
9.7 Reflecting type telescope
5.3.0 Magnification Of Lens
The magnification $m$ produced by a lens can be written as,
$$m = \frac{{{\text{height of image}}}}{{{\text{height of object}}}} = \frac{{{h_i}}}{{{h_o}}}$$
As $\Delta OO'P$ and $\Delta II'P$ are similar,
So, $$\frac{{II'}}{{OO'}} = \frac{{PI}}{{PO}}$$ or $$\frac{{ - {h_i}}}{{ + {h_o}}} = \frac{{ + v}}{{ - u}}$$ $$m = \frac{v}{u}$$
Note:
- If $m$ is negative, then the image is inverted with respect to the object.
- If $m$ is positive, then the image is erect (not inverted) with respect to the object.
5.3.1 Power Of Lens
It is defined as the ability of the lens to deviate path of rays passing through it.
Mathematically, power is defined as the inverse of the focal length in metres.
$$P\;(D) = \frac{1}{{f\;(m)}}$$
Note:
- Unit of power is dioptre which is also written as $D$.
- $1\ D=1 \ m^{-1}$
- Unit of focal length should be in metres.
- Nature of the lens depends on the power.
| Type of lens | Focal length | Power | Nature | Ray diagram |
| Convex lens | $+\ ve$ | $+\ ve$ | Converging |   |
| Concave lens | $-\ ve$ | $-\ ve$ | Diverging |   |
