Interference patterns from a single slit

How can a single slit produce an interference pattern?

Wrong Track: It can't be possible to get an interference pattern from a single slit. You need two slits to get the two sets of waves to interfere.

Right Lines: A narrow single slits does produce an interference pattern. You need to think about rays of light coming from different points along the single slit and interfering constructively and destructively.

Think about rays from different points along the slit

Thinking about the learning

Many students find it counter-intuitive that a single narrow slit can produce an interference pattern, especially if interference ideas have been introduced by considering a two-slit case.

Thinking about the teaching

A good starting point for teaching about single-slit diffraction is to show your students the physical effect:

Teacher: So you can see what is produced on the screen when the light is passed through a single slit. What does it look like?

Naomi: It looks a bit like an interference pattern: fringes sort of thing.

Teacher: And where did we see that?

Emmy: With the twin slits…

Teacher: OK! You're right! But this is odd! How on earth is it possible to get an interference pattern with a single slit? What's doing the interfering? Any thoughts?

In this way the problem of how the interference pattern might be produced is raised and tossed back to the students to think about. In our experience some students do edge towards the correct solution of thinking about interference between contributions from different points along the single slit. These contributions interfere constructively and destructively due to the differences in trip times as they travel out to a screen from different points along the slit.