Doctor Horrible is almost finished with the design of his new freeze ray. The de

Question

Doctor Horrible is almost finished with the design of his new freeze ray. The device uses a blue laser with a wavelength of   440 nm   which is to be focused using a glass lens having index of refraction n=1.55. Dr. Horrible plans to cover the lens with a film of material having   n=1.40to form an antireflective coating.   

Part A

Calculate the minimum thickness needed for the coating to eliminate reflections for   440 nm    light at normal incidence.   

Part B

   After the freeze ray fails to affect Captain Hammer, Dr. Horrible redesigns it to use a red laser instead. Using the same materials, what is the minimum film thickness needed for an antireflective coating for 730 nm light at normal incidence?

Explanation / Answer

Part A:

You need an optical thickness of 1/4 wavelength. So if the thickness is t, you can say that
1.4 x t = (1/4) x 440 nm.

t=78.57 nm

Part B:

You need an optical thickness of 1/4 wavelength. So if the thickness is t, you can say that
1.4 x t = (1/4) x 730 nm.

t=130.36 nm

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