Some Light Concepts
1. Two coherent waves are emitted from two sources. They are matching in every way –
same wavelength, same phase, same speed etc. Somewhere during their journey,
something happens such that when the waves meet at a certain point in space, they are
no longer coherent. Give a minimum of three distinct reasons why this might happen.
 path difference: one wave took a longer/shorter path than the other, making it go out
of phase
 reflection: one wave reflected off a surface, causing a 180o phase change
 refraction: one wave passed through a different medium, causing a wavelength
change
2. Can two waves have the same wavelength but not the same phase over any significant
period of time? Explain.
Yes. Two waves can be the same length, but start at different points in their phase.
Eg. one wave begins at crest, other begins at trough etc.
3. Can two waves of different wavelength be in phase over any significant period of time?
Explain.
No. If they are different wavelengths, they won’t be in phase at all except for brief
moments. This is why you get beats when two different wavelengths are played
together.
4. Can red light and blue light cancel for any significant period of time? Explain.
No. Blue and Red are different wavelengths. They can never cancel.
5. Why is a candle not a coherent light source? Give as many reasons as you can.
A candle sends many, many, many waves every instant:
 a mixture of different wavelengths
 many different phases
6. A candle in a room is like a group of children splashing at the centre of a pool. Explain.
each child sends a different wave (different length, different amplitude) at different
times. These waves spread out in the pool, reflecting off walls etc.
7. Suppose three people in a pool jumped up and down at random intervals. The jumping
creates waves which interfere with each other, but it would be difficult to notice any
pattern.
a) Why would there be no pattern? Because the waves are not coordinated
b) What would the people need to do to create a noticeable interference pattern? They
would have to jump at a regular height and at a regular rate. They could also jump
at the same time.
c) How could a toy boat be used to map out the interference pattern in the pool?
Place the boat at random places: where the boat is still = total destructive
interference, where the boat is displaced the most (up/down) = max constructive
interference. Move the boat around, map out the pool.
8. A stereo has two speakers that emit the same tone in phase. A person standing 5m
away from one speaker and 7m away from the other speaker cannot hear the tone.
What is the wavelength of the tone? (Ignore any reflections/refractions/diffractions in
the room).
Path difference = 2m
Cannot hear = destructive interference: Path difference must be ½ λ
½ λ= 2m
λ = 4m
Tone must be 4m in wavelength
NOTE: Could also say:
path difference is ½ λ, 1½ λ, 2½ λ, 3½ λ etc.
(n+½) λ = 2m
where n = 1,2 ,3, 4 etc.
λ = 2/(n+ ½)
λ = 4m , 4/3 m, 4/5 m, 4/7 m etc. (notice that successive wavelengths are smaller)
9. A stereo has two speakers that emit the same tone in phase. A person standing 12m
away from one speaker and 15m away from the other speaker hears the tone loudly. If
a new tone with half the wavelength of the first tone is played, will the person hear the
tone loudly, somewhat or not at all?
path difference = 3m
hears tone = constructive interference = λ
So λ = 3m
new tone λ = 1.5m
path difference = 3m
path difference = 2 λ ---- path difference is two full wavelengths
So it is constructive interference again!
Yes, person will hear the tone loudly.
10. The sun does not produce coherent light. How do we know this?
Coherent light must be monochromatic (single wavelength) . We can disperse sun light
into colours with a prism/diffraction grating. This means sunlight is made of many
different wavelengths (colours). Therefore, sunlight is not coherent.
11. Name three very different examples that we have studied in this course which
demonstrate the interference of light.
 Double slit, Single slit experiments
 thin film (oil on water, Soap bubbles, etc)
 CD, DVD, Blu-Ray disc
 Newton’s Rings
12. My stereo has two speakers. When I play music, the two speakers create two waves
that constantly interfere with each other. Why can we not detect this interference?
Name as many reasons as you can.
It is not noticeable, but it is there.
 You have two ears. If waves cancel in one ear, unlikely they will also cancel in the
other. Even if they did, they would not cancel in between (your skull) so that it
would resonate, allowing you to hear the tone.
 Waves come directly from speakers, but also bounce off walls, floors, furniture. This
produces many paths. It is unlikely they will all cancel.
 Even if one wavelength does cancel, you will not hear that tone, but you will hear
other tones. Music is made of many many tones you would not notice the missing
tone.
 In general, we all notice that music sounds “different” when you listen to it in
different environments. A song in the car, a song with earplugs, a song in a concert
hall all sound different due to interference effects.
13. Why do thin films have to be thin? Why can’t we see an interference pattern through a
thick medium, such as a window?
A thick film will allow many wavelengths of light to reflect inside of it, making the
reflection appear “white”. A thin film filters out all the visible wavelengths except
one, allowing us to see bands of colour.
14. When I am behind a large building, I can hear my radio, but my cellphone doesn’t work.
When I walk around the building, my cellphone gets better reception. What does this
mean?
It means you should get another cellphone provider.
Just kidding. It means that the radio waves are long enough to diffract around the
building, but the cellphone uses shorter waves that cannot. You have to be in line of
sight of your nearest cellphone tower (usually on top of a building or hill) to get
reception.
15. I look out my window to see a red car driving by. Red light has a different wavelength
in glass than it does in air.
a) Is the wavelength of red light in glass shorter or longer than in air? Shorter
b) Since the red light changes wavelength when it enters the glass, why doesn’t the car
change colour when I look through the window? Because it turns back to red when it
enters the air. Even if it didn’t, it would turn back to red when it enters my eyeball.
Red light will always look red regardless of the medium I am in.
16. On a certain street cars are going by at a rate of 3 per second. They are moving with a
speed of 18m/s.
a) How far apart are they? V = fλ so λ = v/f = 18/3 = 6m apart
b) Further down the street, the traffic slows to 12m/s. How far apart are they now?
λ = v/f = 12/3 = 4m apart