2. Investigating interference
• These two waves have the same
Wavelength, frequency, phase etc.
At the drawn point, the waves
Constructively interfere.
Because crests only occur ONCE
a wavelength, we call define this point as multiples of the two waves’
wavelength from their sources.
Let’s say the horizontal wave took 3 wavelengths to reach this point and the
diagonal one took 5 wavelengths. Difference in this distance will always be
an integer.
3. Contd
• Same conditions as the last slide!
• At the drawn point, the waves
destructively interference
For there to be a destructive
interference, a crest and a trough
Needs to meet. This requires one wave to travel a multiple of its wavelength,
and the other to travel a multiple + ½ of its wavelength. This ½ wavelength
refers to phase difference of pi. If the phase difference was pi to begin with,
then ½ wavelength isn’t required. The difference in distance will always have a
1/2 .
(If still confused about the reasons for measuring the distance as multiples of a wavelength, go to slide 2 and review the reasoning!)
4. C-15-2
• Two sources that are in phase produce an interference pattern like
the pattern in figure 15-14. if the two sources are out of phjase by pi,
we will observe:
a) No interference pattern
b) The same interference pattern
c) An interference pattern similar to figure 15-14 but with the
destructive interference located where the constructive
interference was observed and vice versa
d) None of the above
5. Let’s look at the diagram
• The caption tells us that
• Green=largely negative amplitude
• Red=largely positive amplitude
• Then therefore, the black areas
Must describe destructive interference!
6. Key information: Out of phase by pi
• This means the one wave has traveled more than the other.
• To be exact, traveled T/2 more. T= period
• Meaning one wave has traveled ½ wavelength more than the other.
7. contd
• We previously learned that for a constructive interference, the
distance differences needs to be an integer; however now all the
points that once had constructive will now have a distance difference
of an integer + ½. Which makes it destructive. For points that were
destructive before, the difference of distance will be an integer now,
meaning constructive interference.
• Therefore the answer is C
