Video Transcript
The following figure shows light waves diffracting. Which of the angles is a diffraction angle?
In our figure, we see a series of wavefronts indicated by these orange lines. We can see that as the wavefront passes around this corner here, it starts to bend around that corner so that part of the wavefront now extends beyond where the wave was able to reach before. This is what it means for a wave to diffract. The dashed line in our figure that shows just how much the wave has diffracted is this one here. That line is defined by this angle, called angle 𝐵. We see that this name defines the angle between a horizontal line and the line showing how far the waves are diffracting.
The other angle in our figure is angle 𝐴. Notice that this label defines an angle between the horizontal and this dashed line right here. If we follow this dashed line though, we see that it intersects with the line along which our wave was already traveling. In other words, the fact that the wavefront exists along this dashed line doesn’t tell us that the wave has diffracted. We can see it would already pass along this line even if it hadn’t bent or diffracted at all.
Since angle 𝐵 does show us how much the wave has diffracted as it moves around this corner, for our answer, we’ll say that angle 𝐵 is the diffraction angle of the light waves.
