Question

1.     a. (3 pts) One of the challenges in physics at the end of the 19^th century regarded blackbody radiation. Explain what the difficulty was and how it was resolved.

b.(2 pts) Identify 2 other physics challenges at the end of the 19^th century, besides blackbody radiation.

2.     (5 pts) Anti-reflective coatings on lenses use thin-film interference to eliminate the reflection of a particular color. Suppose a glass lens (n_g = 1.51) is covered with a thin film (n_f = 1.32) to prevent green light (λ = 530 nm) from being reflected. What is the minimum thickness the film can have?

Answer 1

1.

a)

the difficulty was to explain blackbody radiation using wave nature of light. This difficulty was resolved by treating light as particle ( Planck’s radiation law). [article nature of light successfully explained blackbody radiation

b)

in addition to blackbody radiation, there were difficulties to explain photoelectric effect and Compton scattering using wave nature of light.

2.

The condition for destructive interference in the reflected light from a thin film is

2nd = λ for the 1st order minima

Here, n = nf/ng = 1.32 /1.51 = 0.87

The minimum thickness of the thin film , d = λ/2n = 530 x 10-9 /2x0.87 = 0.304 x 10-6 m

The minimum thickness of the thin film, d = 0.304 x 10-6 m