Physical Science 20
2-Dimensional Wave Motion. Interference & Diffraction
Interference pattern for two point sources
1. Fill in the 2 point source interference pattern. Label constructive and destructive interference as well
as the nodes and nodel points.
Physical Science 20
2-Dimensional Wave Motion. Interference & Diffraction
2. What happens when:
a. The crests of two waves meet? What kind of interference is this called?
A larger crest is formed. This is constructive interference.
b. The troughs of two waves meet? What kind of interference is this called?
A larger trough is formed. This is constructive interference.
c. The crest of one wave meets the trough of another? What kinds of interference is this called?
If the crest is larger than the trough, then a smaller crest is formed when they meet.
If the crest is smaller than the trough, then a smaller trough is formed when they meet.
If the crest has the same amplitude as the trough, then they will cancel each other out.
This is all known as destructive interference.
3. a) What happens to the nodal lines when the wavelength is decreased?
More nodal lines appear
b) What happens to the nodal lines when the separation distance between the sources is decreased?
Fewer nodal lines appear
4. a) What would a nodal line of sound waves sound like?
Quieter
b) What would a nodal line of light look like?
Dimmer
5. Two loud speakers are placed a few meters apart on a large field. They are connected to a signal generator and a
single continuous note is produced through each of the speakers.
a) A student walks back and forth in front of the speakers and notices that the volume of the sound rises and falls
depending on his position. Explain this observation.
The two sources produce an interference pattern and the student is hearing alternate nodes and antinodes as
he walks across the pattern.
b) When the experiment was repeated in a classroom the fluctuation in volume was noticeably less. Explain why
this happens. (hint: consider the last lesson)
The classroom is a closed space with many reflecting surfaces. The sound waves would be reflected off the walls
and the roof, acting as if many sources were operating and this would upset the simple pattern produced by the
two speakers on their own. At what used to be a pure node, there will be some sound coming from a reflecting
source.
Physical Science 20
Diffraction
2-Dimensional Wave Motion. Interference & Diffraction
6. a) Sketch the appearance of low frequency waves (left) and high frequency waves (right) when they
encounter a one-sided opening.
b) There is zone behind the barrier that no waves penetrate. What common phenomenon associated
with light is similar to this?
Shadows
c) Does diffraction happen more when the wavelength is large compared to when it is small?
Lower frequencies produce larger waves which have more diffraction around corners or barriers than
higher frequency waves.
d) Sound typically has a wavelength of a few metres, but light has a wavelength of about 10-7 metres.
Use this fact to explain why you can hear sounds around corners but cannot see things around a
corner.
Since the wavelength of light is very small, light does not diffract very well around corners or barriers.
Sound has a relatively large wavelength, so it can diffract very easily around a corner. This is why we
cannot see objects from around the corner but we can hear things around a corner.
Physical Science 20
2-Dimensional Wave Motion. Interference & Diffraction
7. a) Draw sketches of waves passing through an opening or slit (left) and a double slit (right).
b) What happens to the appearance of the diffraction pattern when the slit is made smaller?
There would be more diffraction and the appearance of the wave would look more circular
c) For a very small slit, what do the diffracted waves resemble?
the wave would look like a point source
8. a) Draw sketches of waves encountering two objects, one about the same size as the wavelength of the
waves, and the other smaller than their wavelength.
b) What happens when an object is very small compared with the waves? Give an example of an object
that would behave this way with light waves.
The waves would diffract around the object and there would be no evidence that the object was even
there. Atoms are smaller than light waves, this is why we cannot see atoms.