Light of wavelength in air is normally constructively reflected by a thin film of…
PHYSICS · Single correct · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
Light of wavelength λ in air is normally constructively reflected by a thin film of thickness t. When the angle of incidence in air is gradually increased while t remains fixed, the reflected intensity at the same wavelength decreases and eventually becomes destructive. Which of the following best explains this change?
(a) The light travels a longer distance inside the film at oblique incidence, increasing the optical path difference until it becomes destructive.
(b) Beyond a certain incident angle the internal angle exceeds the critical angle, so the second reflected ray disappears and the previously constructive interference is lost.
(c) The component of the light’s path inside the film perpendicular to the interfaces shortens as the angle increases, reducing the optical path difference between the two reflected rays so that the phase condition becomes destructive.
(d) Snell’s law makes the wavelength of the light inside the film decrease with larger angles, so the fixed thickness no longer satisfies the constructive condition and the reflection becomes destructive.