Video Transcript
The lines shown in the following graph represent the spectra of light emitted by two objects. Which line represents the object that emits light at a single wavelength? (A) The blue solid line, (B) the red dashed line, (C) neither of the lines.
Our graph shows intensity of light emitted from an object against the wavelength of that light. We have two curves representing light from two different objects. We want to figure out which of the two curves, either the red dashed one or the blue solid one, or neither, represents an object that gives off light at a single wavelength. Since the horizontal axis of our graph indicates wavelength, we can say that any curve, that is, any spectrum, that has some nonzero width on this axis cannot represent light at a single wavelength.
Take, for example, the red dashed line. This line represents a spectrum, where the wavelengths range from at least here up to at least here. The intensity of light emitted is nonzero for all of these wavelengths. We can say then that the red dashed line does not indicate light at a single wavelength. Rather, this line covers many wavelengths. But then take a look at the blue line, which is a vertical line on our graph. This line represents light that exists only at one particular wavelength, this one right here. At this wavelength, the intensity of light is nonzero. But it is zero at every other wavelength.
For our answer, we choose option (A). The blue solid line represents the object that emits light at a single wavelength.
