Optics for Optometry

In everyday life, the interference of light most commonly gives rise to easily observable effects when light impinges on a thin film of some transparent material. For instance, the brilliant colours seen in soap bubbles, in oil films floating on puddles of water, and in the feathers of a peacock's tail, are due to interference of this type. Suppose that a very thin film of air is trapped between two pieces of glass, as shown in Figure. If monochromatic light (e.g., the yellow light from a sodium lamp) is incident almost normally to the film then some of the light is reflected from the interface between the bottom of the upper plate and the air, and some is reflected from the interface between the air and the top of the lower plate. The eye focuses these two parallel light beams at one spot on the retina. The two beams produce either destructive or constructive interference, depending on whether their path difference is equal to an odd or an even number of half-wavelengths, res

Fringe visibility is used for quantifying how good is the interference pattern being formed and it is given by the relation(Also called the Michelson fringe visibility, the fringe visibility is defined in terms of the observed intensity maxima and minima in an interference pattern ) V = I m a x ​ + I m i n ​ I m a x ​ − I m i n ​ ​ With the help of visibility, knowledge about coherence, fringe contrast an interference pattern is obtained. V = I max − I min I max + I min = 2 I 1 I 2 − − − − √ ( I 1 + I 2 )  If  I min = 0 ,  V = 1  (maximum) i.e., fringe visibility will be best. Also if  I max = 0 , V = − 1  and If  I max = I min , V = 0