Partial Reflections of Photons

Light as a Photon: How Could that Work?

  • A: All right, Sophie, come into our world.
  • B: Imagine that light is made of photons and let's revisit what Marie showed you earlier: partial reflection, thin films, and the 2-slit experiment.
  • S: Ok.

Partial Reflection

Imagine the setup shown below (Fig 15). We have a light source of a given color (say, red). We emit 100 photons toward a block of glass. Four are reflected and detected by Alice, while 96 are transmitted and detected by Bob inside the glass.

A light source sends 100 photons down toward a block of glass, 4 are reflected and detected by Alice while 96 are transmitted and detected by bob.

  • B: Already we have a problem, Sophie. How does a photon make up its mind about whether to be reflected or to go through? In the case above, 4% of the photons are reflected while 96% go through. Why? All the photons are identical.
  • S: Maybe the glass is made of holes that let most of the photons pass through but not all.
  • A: Yes, you can try theories like that (and Newton, who believed that light was made of particles, tried), but ultimately they fail.
  • S: But aren't you both saying that light is made of particles?
  • A: Yes, but the problem is that these "particles" must behave very differently from billiard balls.
  • B: Here Sophie. Partial reflection is pretty mysterious if the photon moves like normal particles, but maybe we could come up with an explanation. Partial reflection by two surfaces, on the other hand, is completely mind boggling.

Thin Film From Photons

We now have the same setup as before, but the piece of glass is thin and Bob is detecting on the other side. Now by changing slightly the thickness of the glass, Alice detects anywhere between 0 and 16 photons! (see Fig 16).

same setup as before but not the glass is thin and bob is detected after the photon goes through the whole piece. By varying slightly the thickness of the glass, the number of photon detected by Alice can change from 0 to 16. (100 to 84 for Bob)

Q. How could that be? If the photon is a normally behaved particle and we know that 4 % of incoming photons gets reflected off the glass, what is the expectation for the number of photons detected by Alice?

  1. 4


  2. 0


  3. 8 or a little less.


    Let's see. At the first surface, we have 100 photons, 96 go through and 4 are reflected. On the second surface, 4% of 96 should be reflected; that's about 3.84 photons, or about 4 photons. So Alice should detect 8, maybe 7, photons, while Bob will detect 92.

    But sometimes Alice detects no photon or 16, twice as much! How can that be?

    It's like if the photons know about the second surface. If the second surface is a certain distance away, the photons do not reflect from surface 1. If surface number 2 is a little farther away, then we double the reflection rate. Isn't that strange?

  4. 16 or a little less.

    No. This is what Alice sometimes sees, but this is not the expectation if the photon is normal.