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find the minimum thickness of a film which will strongly reflect the light of wavelength 589nm.the refractive index of the material of the film is 1.25?

 find the minimum thickness of a film which will strongly reflect the light of wavelength 589nm.the refractive index of the material of the film is 1.25?

Grade:12

1 Answers

Vikas TU
14149 Points
8 years ago

Strong reflection is referring to constructive interference between rays reflected off the top and bottom of the film.

There is a phase shift of ½λ for rays reflected off the top of the film (reflection against a more dense medium)
There is no phase shift with reflection off the bottom of the film (reflection against a less dense medium)

Reflected rays off the top will combine constructively when alongside waves having a path difference of ½ wave (or 1½, 2½ ....) caused by travelling through the film.
The minimum thickness (t) for a path diff. of ½ wave is given by ..
2 t = ½ λ'' .. (λ'' = wavelength in film = λair / n)

t = ¼( λ / n) .. .. 0.25 x 589^-9m / 1.25 .. .. ?t = 1.18^-7 m

 

plz approve!

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