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.
