Video Transcript
The lines shown in the following graph represent the spectra of light emitted by some objects. Which of the following is correct? (A) The more the range of wavelengths of light emitted by an object decreases, the more the spectrum of light from the object approximates a vertical line. (B) The more the range of wavelengths of light emitted by an object decreases, the more the spectrum of light from the object approximates a horizontal line.
Okay, so our two answer options are different only in one word. Both of them talk about the range of wavelengths of light emitted by an object decreasing. On our diagram, that will correspond, say, to the red spectrum, shrinking down to the blue spectrum, shrinking down to the black spectrum, and so on. That’s what it means for the range of wavelengths emitted by an object to decrease. When that happens, we say that the spectral width of the light emitted by an object decreases. So then, as spectral width gets smaller and smaller and smaller, do the shapes of our resulting spectra more represent a vertical line or do they more represent a horizontal line?
Of all the spectra in our diagram, the one with the narrowest spectral width is the orange spectrum here. We see that this spectrum is actually very similar to a vertical line. And the narrower that spectral width becomes, that is, the more the range of wavelengths of light emitted decreases, the more like a vertical line the resulting spectrum will become. We choose answer option (A). But note that on a graph like this, where intensity is plotted against wavelength, a near-horizontal line is achievable if the intensity of our source was vanishingly small. In that case, we might get an intensity wavelength curve that looks something like this, almost a horizontal line.
In this case though, we’re considering a different scenario. And here, the more the range of wavelengths of light emitted by an object decreases, the more the spectrum of light from the object approximates a vertical line.