# A beam of light of wavelength 471 nm passes through two closely spaced glass plates, as shown in...

## Question:

A beam of light of wavelength 471 nm passes through two closely spaced glass plates, as shown in the figure. For what minimum nonzero value of the plate separation d will the transmitted light be bright? (This arrangement is often used to measure the wavelength of light and is called a Fabry-Perot interferometer.)

## Interference of Light:

When two light wave are added, these light waves will interfere with each other. If the waves are in phase, there will be constructive interference. If the waves are out of phase, there will be destructive interference. A tool that makes use of these is called an Interferometer.

Using the equation for the Fabry -Perot geometry, we write:

{eq}2d\cos{A} = mL \\ {/eq}

where:

L is the wavelength

A is the angle

d is the separation

for the smallest value of d, we write:

{eq}2d\cos{0} = L \\ d = \frac{L}{2} \\ d = \frac{471\times10^{-9}}{2} \\ d = 235\times10^{-9} {/eq}