1. Two waves (black line and red dots) interfere
Two waves (black line and red dots) interfere. There sum is the blue wave. When the phase difference is 0, 2, …, (which is called being “in phase”) they add up to give the maximum amplitude for the blue wave as shown on this slide -> constructive interference

2. When the phase difference is different from 0, 2, …, they add up to give a lower amplitude for the blue wave as shown on this slide.

3. … and this one.

4. … and this one.

5. … and this one.

6. … and this one.

7. The two waves now have a phase difference of , 3 …, (which is called being “out of phase”) and they add up to give zero amplitude for the blue wave as shown on this slide -> destructive interference

8. Huygen’s wavelets and the double slit experiment

9. Huygen’s wavelets and the double slit experiment

12. Let the black circles be crests and the red circles troughs
Let the black circles be crests and the red circles troughs. If crest meets crest or trough meets trough on the screen then the phase/path difference leads to constructive interference. However, if crest meets trough then the waves interfere destructively, which leads to an interference pattern on the screen.
screen