Video Transcript
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.
