Lenses

Posted in Physics, Optics

Curvature of wavefronts
Lenses
- The power of the lens
The lens equation
Magnification

Curvature of wavefronts

Light from most sources - like the sun - radiates outwards as a series of wavefronts. These wave fronts are curved and as they travel further from their source their curvature decreases. Light from a far away source such as the sun or stars are assumed to have no curvature as the difference between this and the actual wave length is negligible.

Lenses

The purpose a lens is to either increase or decrease the curvature of the waves entering them.

Converging lenses add more curvature to the entering wavefronts focusing them at a point if they are entering parallel. The distance of this point from the lens is referred to as the focal length of the lens. At this point a 'real image' of the light entering the lens is formed.

Diverging lenses remove curvature from the wavefronts causing them to diverge.

The power of the lens

The power of the lens describes how much curvature a lens adds or removes from the wavefronts entering and is measured in dioptres (D). The power of a lens is given by the formula:

$Dynamic image 0$

Where f is the focal length of the lens

The lens equation

The lens equation relates the focal length of a lens and the curvature of the wave fronts entering and leaving:

$Dynamic image 1$

Where $Dynamic image 2$ is the image distance, the distance between the lens and the point where the in-focus image is formed, $Dynamic image 3$ is the object distance, the distance between the object the image is of and the lens and $Dynamic image 4$ is the focal length of the lens.