Does this have a continuous spectrum, or a emission-line spectrum, or an absorption-line spectrum?

Question

a) a mercury vapor streetlight
b) an incandescent (tungsten filament) streetlight
c) a star like the sun

how do you know which spectrum it is?

Answer 1

A. Primarily emission.
B. Continuous.
C. Continuous with absorption lines superposed (see ref. 1).
To know which spectrum, you need to know how the object produces its light. Ionized ("hot") rarefied gas emits energy concentrated in emission lines (specific wavelengths of light). Cold gas absorbs energy at the same wavelengths that it would emit at if it were ionized. Both these effects are related to the energy jumps of orbital electrons of the gas atoms; the higher the energy change the shorter the wavelength emitted/absorbed. Thermally hot solids and dense gases emit a continuous "black-body" spectrum. The prominence of the spectral features depends heavily on what other (backgound) radiation is present. For a more thorough discussion of spectrum types, see ref. 2 and its "Next" page.