Question

Light from a small region of a 100 Watt incandescent bulb passes through a yellow filter and then serves as the source for a Young's double-slit interference experiment. Which of the following changes would cause the interference pattern to be more closely spaced?

(a)  Use a blue filter instead of a yellow filter

(b)  Use a 10 Watt bulb

(c)  Use a 500 Watt bulb

(d)  Move the bulb closer to the slits

(e)  Move the slits closer together 

Answer 1

(a) Use a blue filter instead of a yellow filter 

The angular position of the bright fringes is given

sin θ = m λ/d.

A more closely spaced pattern implies that for a given m, θ is smaller. Thus, we want to increase d or decrease λ and from problem 15 we see that λ_blue < λ_yellow.