Video: Distinguishing between White and Monochromatic Light Sources

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?

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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.

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