Question

A transparent film of refractive index, 2.0 is coated on a glass slab of refractive index, 1.45. What is the minimum thickness of transparent film to be coated for the maximum transmission of Green light of wavelength 550 nm. [Assume that the light is incident nearly perpendicular to the glass surface.]

(1) 94.8 nm

(2) 68.7 nm

(3) 137.5 nm

(4) 275 nm

Answer 1

Correct option is : (3) 137.5 nm

\(\Delta \mathrm{P}=2 \mu_{0} \mathrm{t}=\mathrm{n} \lambda \)

\(\Rightarrow \mathrm{t}=\frac{\mathrm{n} \lambda}{2 \mu_{0}} \therefore\ \mathrm{t}_{\text {min }}=\frac{\lambda}{2 \mu_{0}}=\frac{550}{2 \times 2}=137.5\)