A lens (n = 1.60) is coated with a thin film of magnesium fluoride (n = 1.38) to…
PHYSICS · Single correct · A Level · Physics · Waves · Waves
Problem
A lens (n = 1.60) is coated with a thin film of magnesium fluoride (n = 1.38) to reduce reflected intensity for light of wavelength 550 nm in air. Which of the following best explains why choosing a film thickness equal to one-quarter of the wavelength inside the film minimizes the reflection?
(a) Because only the ray reflected at the upper surface reverses phase; the λ/4 thickness supplies the extra half-cycle needed so the two waves cancel.
(b) Because a λ/4 film makes the two reflected rays differ in phase by 2π, causing the reflection to disappear.
(c) Because both reflected rays undergo the same π phase reversal, a thickness of t=λ/(4n) makes their optical path difference equal to λ/2, so they return 180∘ out of phase and interfere destructively.
(d) Because a λ/4 coating keeps the transmitted wave in phase as it exits, and by energy conservation the reflected intensity must therefore be minimized.