The lead architect on the design team of a new skyscraper decides that the outer

Question

The lead architect on the design team of a new skyscraper decides that the outer surface of the building’s windows need to be coated with a reflective layer in order to lower airconditioning costs. MgF2 is chosen for the reflective layer, which has a refractive index n=1.38. Assume that the refractive index of the window glass is 1.5 and that the reflective coating should work optimally at the peak of the sunlight spectrum, 500 nm

A) What is the minimum thickness of film that is needed?

B)Suppose the layer needs to be thicker than 200 nm in order to hold up to the weather. What other thicknesses would also work? Give only the three thinnest ones.

Explanation / Answer

Minimum thickness will be measured using formula

2*n* t = m for destructive inter interference because we don't want light to come in.

for minimum m should be 1 so

t = / 2*n = 500nm / 2*1.38   

t = 181.159 nm ............answer

if t > 200 nm then

t = m* (/ 2*n) = m * [500nm / 2*1.38   ] = m *181.159

now put m = 2 , 3 , 4

t2 = 2* 181.159 = 362.318 nm

t3 = 3* 181.159 = 543.478 nm

t4 = 4*181.159 = 724.637 nm

also work

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