Young's double slit

Problem Statement: Young's double-slit experiment is just one example of two source interference. Which of the following is necessary to see a spatial interference pattern on a screen opposite two slits?

(1) The source needs to be a single frequency wave.

(2) The source needs to be a coherent wave.

(3) The experimental apparatus needs to be in air.

(4) The source needs to be a light wave.

(5) The source needs to be a wave.

ANSWER: (5)

Reasoning: Interference throughout space is possible with any two waves. If they are not coherent sources, which means their relative phase remains constant, then the pattern will change over time, often rapidly. So, for science and making measurements, you want two (or more) coherent sources. We will learn later how one wave can also produce interference but for now, it's easier to consider two or more.