Lesson Video: The Properties of Laser Light | Nagwa Lesson Video: The Properties of Laser Light | Nagwa

# Lesson Video: The Properties of Laser Light Physics • Third Year of Secondary School

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In this video, we will learn how to describe the properties of laser light and use technical terms to refer to these properties.

13:07

### Video Transcript

In this video, our topic is the properties of laser light. As we’ll see, the light that comes from lasers is not like the light from other sources, such as light bulbs or even the Sun. In this lesson, we’re going to name and describe the properties of laser light that make it so unusual.

Now, as a point of reference for light from a laser, let’s begin by thinking of light that comes from a more familiar source, say this incandescent light bulb. If we were able to study all the photons individually given off by this bulb, we would see that they have varying wavelengths. For example, this photon here has a longer wavelength than this one here. Some of the photons, like this one here and this one, don’t even have a discernible wavelength. And on top of this, all these photons are moving in different directions. The light from this bulb then varies in frequency. It varies in phase, and it varies in direction.

If we compare this with a light that comes from a laser, say this is our laser light source and this is our red laser, we would see that unlike the light from our bulb, these photons have the same frequency, the same phase, and are moving in the same direction. Light behaving in this way is said to be coherent, and this is one of the hallmarks of laser light. One way to demonstrate the difference between coherent radiation and incoherent radiation, which is the light that an incandescent bulb gives off, is to create a plot of the intensity of light from a source against the wavelength of that radiation.

If we were to plot out the intensity versus wavelength of the light coming from our bulb, it might look like this curve which, we can see, is fairly broad. This is an indication of all the different wavelengths that are a part of this light. On the other hand, the intensity-versus-wavelength curve of the light from our laser might look this way. Notice that the maximum intensity is much higher and also that this light is confined to a much smaller range of wavelengths. Now, if the light coming from our laser was truly all exactly the same wavelength, then the width of this intensity-versus-wavelength curve would be zero. It would collapse to a vertical line.

Real laser systems, though, aren’t perfectly monochromatic. In other words, they don’t emit light just at one specific wavelength, but rather over a range like we see here. However, that range is so small compared to most other common sources that we typically do say that lasers emit monochromatic radiation. We use this term, understanding that, in this case, it really means radiation over a very narrow range of wavelengths. So, light from a laser is both coherent light and it’s also monochromatic. And this is different from the radiation coming from our light bulb, which is incoherent and, as we saw, exist over a range of wavelengths, and so it’s not monochromatic.

And next, if we consider a beam of light coming from these two sources, we’ll learn a couple more properties of laser light. If we clear away our graph and then imagine the radiation coming from each one of these sources, continuing to travel through space, we can see that the light from our bulb will spread out as it moves away from that source, while the laser light remains in a tightly confined beam. One way to understand this difference is to imagine a series of light rays coming from our light bulb at what we’ve drawn as the beginning to this widening beam of light.

As we look at all these different rays we’ve sketched in, we can see that each one is moving in its own individual direction. So if we extend each of the rays out a bit, we can see that because they’re moving in all these different ways, this beam of light is getting broader and broader as the rays move away from the source. And the reason this happens is that all these rays of light are not originally moving in the same direction.

That’s very different from the light that comes from our laser. This light is all pointed the same way from the very start. This means that if we showed individual rays of light within this broader beam, then those individual rays would effectively be parallel to one another. When a beam of light is this way, we say that it’s collimated. The width of a perfectly collimated beam doesn’t change as the beam moves along. This is why whereas our incandescent bulb is giving off an ever-widening beam of radiation, we can depict the light coming from our laser source as a straight line. And that’s because the individual photons move in the same direction.

Now, there’s one last property of laser light that we can look at. And to demonstrate this property, instead of comparing the light from a bulb to the light from our red laser, let’s say we now compare the light emitted by two different lasers, one that gives off green light and then our original red laser. Now, from our perspective, we’re viewing both of these lasers from the side like this. So, we’re watching these two laser beams side on as they move perpendicularly to us. And notice that for a red laser, we see a steady, continuous red beam. While for the green laser, we just see a few dots of light. The difference in the appearance between these two laser beams comes down to what’s called scattering.

Let’s consider again our red laser beam. We said that because this beam consists of light from a laser, all of that light, at least originally, is traveling in the same direction. Now, if the light stayed that way, if it was always moving left to right as we’ve drawn it, then our eye wouldn’t actually be able to see any of this beam. The only way we can is if some of the photons in the beam scatter, that is, if they bounce off of some object in the way, such as an air molecule. It’s only those scattered photons which, if they’re scattered at the right angle, arrive at our eye and let us see the path that this laser beam follows.

Now, because this entire beam is visible to our eye, that means that quite a lot of scattering is going on. This indicates lots of scattering particles in the path of the beam. And we could create a scenario like this by, for example, taking a spray bottle and spraying droplets of water into the air that the laser beam passes through. By doing this, we put lots of scattering objects, water molecules, in the path of the beam. And so, more of the laser light scatters off of these objects, reaches our eye, and makes the beam visible.

Comparing this to the light we see from our green laser beam, we can say that in comparison to the red beam, this one is scattered much less. It’s only at a few spots along the path of the beam where these green dots occur that laser light in the beam is scattering and reaching our eye. At the other points along this beam path, the light, if it’s being scattered at all, is not reaching our eye, which indicates that, overall, relatively little scattering is going on.

Just as a side note, it is possible to have light from a laser that’s completely invisible when viewed from the side while the laser light is nonetheless undergoing very extreme scattering. It may simply be the case that this laser is emitting light not visible to our eyes. So it’s only for visible wavelengths of radiation like green light and red light that, when viewing a laser beam from the side on, we can really get a sense for how much that beam is scattered. Knowing all this about the properties of laser light, let’s get a bit of practice now through an example.

The diagram represents the resultant waveform of the waves emitted from a laser light source. Which of the following diagrams most correctly represents a group of the waves emitted from the laser light source?

All right, so when our question talks about the diagram, it’s referring to this waveform right here that we see in purple. It tells us that this is the resulting waveform from the combination of a series of waves emitted from a laser source. We’re then told about these other diagrams labeled I and II, which shows two different groups of waves. One of these two groups, we’re told, correctly represents the waves emitted from the laser source.

So, we could put the question this way. When we combine the waves shown in group I and when we combine the waves shown in group II, which combination will yield the resulting wave in purple that we’re shown here? We can figure out the answer to this question by looking closely at this waveform. Notice that it has a very consistent and regular wavelength all throughout the length of the waveform. Along with that, the displacement of the wave from equilibrium goes through very even cycles. The wave always reaches the same maximum value here and here and here and so on, as well as the same minimum values here and here and here and so on and so forth.

If we were to add up a series of waves in order to create this resultant, then those individual waves would need to be very like one another. They would need to have the same wavelength, for example. That’s what leads to this resulting wave having the same wavelength all throughout. And they would also need to line up with one another so all the peaks were in line and all the troughs were in line. In other words, the waves must have a phase difference of zero or be in phase with one another.

Knowing this, as we compare the waves in diagram I with those in diagram II, we can see that the waves in diagram I meet our conditions. They’re in phase, and they have the same wavelength as one another all throughout, while the waves in diagram II have varying phase relationships from left to right and also different wavelengths from one another. It’s hard to predict exactly what the result of adding all these waves together would be. But we can be confident that it wouldn’t be as regular and orderly as this resulting waveform. Such a well-ordered result requires very similar waves being added together.

And so, we see that the waves in diagram I most correctly represent a group of waves emitted from this laser light source.

Let’s now look at a second example exercise.

The graph shows how the output intensity of two light sources varies with the wavelength of the light that they emit. Both light sources emit most strongly at one peak wavelength, with output decreasing as wavelength varies from the peak wavelength. Which color curve represents the light emitted by an incoherent light source? Which color curve represents a more monochromatic light source?

These two questions refer to our graph which, we can see, plots intensity in watts per meter squared versus wavelength in units of nanometers. Our graph is basically showing us how much light from two different sources is emitted per wavelength of that source. So, for example, at this wavelength value right here, whatever wavelength value that is, both of our sources emit their peak intensity. And then, as our problem statement explains, as we move away from that wavelength, the intensity from both sources diminishes.

But we can see that that diminishing happens much more rapidly for this line colored in red, where intensity drops off quite quickly as we move away from this wavelength, as compared to the intensity of the line indicated in blue. This also eventually drops to zero, but not as quickly, we could say, as the red line. This tells us that what we could call the wavelength spread of our blue line is greater than that of the red one. The source giving off light whose intensity versus wavelength is indicated by the blue line emits light at more wavelengths than the source whose light corresponds to this red line.

Now, our first question says, which color curve represents the light emitted by an incoherent light source? Let’s clear some space on screen and recall a bit about what this term means. Incoherent light is, naturally enough, the opposite of coherent radiation. One of the properties of coherent light is that it consists of photons that all have the same wavelength. So, for example, if we had two waves of light like this that do have the same wavelength and, as we’ve drawn them, also are in phase with one another, then we could say that these waves are coherent.

All this to say that when we’re talking about light from an incoherent source, we no longer say that this light will all have the same wavelength like coherent light does. So, light from an incoherent source will exist over a range of wavelengths. And looking at our graph, we see that the line indicated in blue agrees with this. This line also indicates light that, as we’ve seen, exists over a relatively broad range of wavelengths. Because of this, we’ll say it’s the blue curve that represents light emitted by an incoherent source. Incoherent sources are characterized by a wide wavelength or frequency spread.

The next part of our question asks, which color curve represents a more monochromatic light source? Looking again in our graph, right away, we can see the answer. The red curve exists over a narrower span of wavelengths and therefore is more nearly monochromatic than the blue one. So, it’s the red curve that represents a more monochromatic light source.

Let’s now summarize what we’ve learned about the properties of laser light. In this lesson, we studied distinctive properties of laser light. These include that laser light is coherent radiation. In other words, the photons in a laser beam all have the same wavelength and the same phase relationship. Along with this, laser light is nearly monochromatic. That is, the wavelength or frequency spread of laser light is very small.

Laser beams are what we call collimated. This indicates that the photons in these beams are traveling in the same direction, parallel to one another. This accounts for the very small amount of beam divergence or spread as laser lights propagate through space. And lastly, we saw that laser beams can scatter in different ways and that, typically, the more of a laser beam is visible, the more scattering it has undergone. This is a summary of the properties of laser light.

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