Video Transcript
The diagram represents the beam emitted by a green laser light source. The beam is viewed perpendicularly to its length. Which of the following most correctly explains why the green beam appears to consist of many point-like particles? (A) The beam produces a longitudinal wave. (B) The beam produces an interference pattern of light and dark regions. (C) The point-like particles seen are individual electrons ionized from the air by the laser beam. (D) Most of the light emitted travels parallel to the beam, and only a few rays are scattered enough to travel perpendicularly to the direction of the beam. (E) The point-like particles seen are individual photons in the beam, and the beam has such low intensity that there is significant space between individual photons.
Alright, so we actually have five answer options here. And in order to see them all at the same time, let’s clear space at the top of our screen and then write down shortened versions of options (A), (B), and (C). Answer option (A) essentially said that the beam is a longitudinal wave. Option (B) said the laser beam is an interference pattern of light and dark regions. Option (C) can be summarized as saying that the bright spots we see in our diagram are individual electrons, that they’ve been ionized from the air by the laser beam.
Now that we see all of our answer options, let’s consider our figure. We’re told that we have a green laser light source here that emits a beam traveling to the right. That is, when we look at this image, we’re looking perpendicularly to the beam direction. That part is important. Whenever photons are emitted by a laser, not only are they in phase with one another, but they also travel in the same direction. This means that if we put our eye here, which is how we would look at this beam, looking at it perpendicularly, the photons in the beam would tend to pass in front of our eye but not reach our eye. And if photons from the beam don’t reach our eye, the beam to us is invisible.
If this laser beam were traveling in a vacuum and we looked at it perpendicularly, we would literally see nothing. But in this case, the beam isn’t traveling in a vacuum. It’s traveling through air. Therefore, there are particles that the photons in the beam can run into and then scatter from.
Let’s say that this here is an air particle in the path of the beam. Photons in the beam could scatter in nearly any direction off of this particle. And this is the process by which all of these small green dots are visible to our eye. These are places where photons have scattered off air particles and they’ve scattered in just the right direction so that those scattered photons reach our eye. The more scattering goes on, the more likely it is that photons do reach our eye. But at this density of air, with this much scattering taking place, we see that it’s only a few tens of photons that seem to be reaching our eye. The rest of them are traveling along in the original beam direction or are scattered in some other direction that our eye can’t see.
Thinking now of our answer options, since we know that these green dots are in fact photons from the original beam, that means that they’re not individual electrons. We’ll cross out option (C). Answer choice (B) says that the beam is an interference pattern. However, for interference to occur, there needs to be more than one beam involved. A beam can’t interfere with itself without deliberate splitting and then rejoining of its path. We won’t choose option (B) then either.
Option (A) states that the beam is a longitudinal wave. However, since this is a laser beam, that is, an electromagnetic wave, we can recall that light travels not as a longitudinal wave but as a transverse wave. We won’t select option (A). Considering then answer option (E), this option holds that the point-like particles we see, these small green dots, are individual photons in the beam. Now, that’s true, but this answer option does not claim that these are scattered photons. In other words, option (E) holds that these individual photons are all still moving to the right. However, if that was the case, they would not reach our eye. And therefore, they would be invisible to us.
It is in general possible for laser beams to have lower or higher intensity. This corresponds to a fewer or greater number of photons within some volume of the beam. It could be then that a laser beam is this sparsely populated by photons. However, if we were looking at that sparsely populated beam perpendicularly, as we are, we couldn’t see any of those photons. They would need to reflect off of something to reach our eye. We can’t choose option (E) then either.
It’s answer option (D) that is correct. Most of the light emitted by the laser source travels parallel to the beam. And only a few rays are scattered enough to travel perpendicularly to the direction of the beam so that our eye can see them.
