11.5 CHAPTER 11. 2D AND3D WAVEFRONTS
Activity: Diffraction
Water waves in a rippletank can be used to demonstrate diffraction and
interference.- Turn on the wave generator so that it produces waves with a high fre-
quency (short wavelength).- Place a few obstacles, one at a time, (e.g. a brickor a ruler) in the rip-
ple tank. What happensto the wavefronts as theypropagate near/past
the obstacles? Draw your observations. - How does the diffraction change when you change the size of the
object?
- Place a few obstacles, one at a time, (e.g. a brickor a ruler) in the rip-
- Now turn down the frequency of the wave generator so that it produces
waves with longer wavelengths.- Place the same obstacles in the ripple tank (one at a time). What
happens to the wavefronts as they propagate near/past the obstacles?
Draw your observations. - How does the diffraction change from the higher frequency case?
- Place the same obstacles in the ripple tank (one at a time). What
- Remove all obstacles from the ripple tank and insert a second wave gener-
ator. Turn on both generators so that they start at the same time and have
the same frequency.- What do you notice when the two sets of wavefronts meet each other?
- Can you identify regions of constructive and destructive interference?
- Now turn on the generators so that they are out of phase (i.e. start them
so that they do not makewaves at exactly the same time).- What do you notice when the two sets of wavefronts meet each other?
- Can you identify regions of constructive and destructive interference?
Diffraction through a Slit ESCFE
When a wave strikes a barrier with a hole only part of the wave can move through the
hole. If the hole is similar in size to the wavelength of the wave then diffraction occurs.
The waves that come through the hole no longer looks like a straightwave front. It
bends around the edges of the hole. If the hole is small enough it acts like a point
source of circular waves.
Now if we allow the wavefront to impinge on abarrier with a hole in it,then only the
points on the wavefrontthat move into the holecan continue emitting forward moving
waves - but because a lot of the wavefront has been removed, the pointson the edges
of the hole emit waves that bend round the edges.