A thin film of a specific meterial can be used to decrease the intensity of reflected light. There is destrucive inteference of wave reflected from upper and lower surface of the film. These films are called non-reflecting or anti-reflecting coatings. The process of coating the lens or surface with non-reflecting film is called blooming as shown in figure The refracting index of coating `(n_(1))` is less than that of the glass `(n_(2))`.

5. magnesium fluoride `(MgF_(2))` is generally use as anti-reflection coating. If refractive index of `MgF_(2)` is 1.25, then minimum thickness of film required is (Take `lambda = 500 nm`)

Options

A. 125 nm
B. 75 nm
C. 100 nm
D. 225 nm

Answer

c

Explanation

For destructive interference,
`2n_(1) t_(min) = (lambda)/(2) (because n_(1) lt n_(2))`
`:. T_(min) = (500)/(4 xx 1.25) = 100 nm`