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 460 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 = 1.37 to form an antireflective coating. Calculate the minimum thickness needed for the coating to eliminate reflections for 460 nm light at normal incidence. 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 750 nm light at normal incidence? Express your answer using two significant figures.

Explanation / Answer

Part A:

You need an optical thickness of 1/4 wavelength. So if the thickness is t, you can say that
1.37 x t = (1/4) x 460 nm

t=83.94 nm

Part B:

You need an optical thickness of 1/4 wavelength. So if the thickness is t, you can say that
1.37 x t = (1/4) x 750 nm

t=136.86 nm

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