Thin Film Interference — Practice Quiz

Question 1

An oil film on water (n_oil = 1.4) is viewed in reflected light. The minimum thickness for bright reflection at λ = 560 nm (normal incidence) is:

Accepted Answer

100 nm

Answer

50 nm

Answer

150 nm

Answer

200 nm

Question 2

A soap film in air (n = 1.33). Minimum thickness for the film to appear BRIGHT in reflected light at λ = 532 nm (normal incidence):

Accepted Answer

100 nm

Answer

50 nm

Answer

150 nm

Answer

200 nm

Question 3

An anti-reflection coating on a lens reduces reflection by destructive interference. For wavelength 550 nm and coating index 1.38, minimum thickness:

Accepted Answer

100 nm

Answer

50 nm

Answer

200 nm

Answer

400 nm

Question 4

In Newton's rings, the radius of the m-th dark ring (in reflection) varies as:

Accepted Answer

√m

Answer

m

Answer

1/m

Answer

m²

Question 5

Why does a soap film appear black just before bursting?

Accepted Answer

Film becomes very thin — only π-shift remains, giving destructive interference for all colours

Answer

Loss of light absorption

Answer

Total internal reflection

Answer

Refractive index drops to 1