Question

Reflections from a thin layer of air between two glass plates cause constructive interference for a particular wavelength of light λ. By how much must the thickness of this layer be increased for the interference to be destructive?
A. λ/8
B. λ/4
C. λ/2
D. λ

Answer 1

Answer:

C

Explanation:

Thin film interference is most constructive or most destructive when the path length difference for the two rays is an integral or half-integral wavelength, respectively. That is, for rays incident perpendicularly, 2t = λn, 2λn, 3λn, . . . or  2t = λn/2, 3λn/2, 5λn/2 , .....

We see a constant phase shift off λn/2. Hence option C