Continuous Spectra and Line Spectra

Splitting light

When light is passed through a prism (or a grating), it splits according to its wavelength: this is a spectrum. Depending on the source, this spectrum takes two radically different forms.

The continuous spectrum

A hot, dense body — the filament of a light bulb, the Sun — emits all wavelengths at once. Its spectrum is an unbroken gradient, a complete rainbow:

   Spectre continu (corps chaud) :

   |violet|bleu|vert|jaune|orange|rouge|
   |###############################...|   <- aucune interruption

The emission line spectrum

A heated gas (or one carrying an electric discharge) does something entirely different: it emits only a few very specific wavelengths. Its spectrum is no longer a gradient, but a set of isolated bright lines on a black background:

   Spectre de raies d'emission (gaz d'hydrogene) :

   |violet|bleu|vert|jaune|orange|rouge|
   |  |         |               |      |   <- seulement quelques raies
      |         |               |
    violette   bleue          rouge

Each chemical element has its own set of lines, always the same. Sodium's pattern is not hydrogen's, which is not helium's. These lines are a true fingerprint of the atom.

The absorption line spectrum

If a cold gas is illuminated with white light (a continuous spectrum), the gas absorbs exactly the wavelengths it would emit if it were hot. One then sees the continuous spectrum crossed by black lines:

   Spectre d'absorption :

   |###|#|########|#####|#|#####...|
       ^          ^     ^          <- raies NOIRES aux memes positions
       (absorbees)                    que les raies d'emission

The positions of the black lines (absorption) coincide exactly with those of the bright lines (emission) of the same element.

What it's really useful for

This phenomenon is astronomy's most powerful tool: we cannot go and take a sample of a star, but by analyzing the lines of its light, we read directly which elements it is made of. This is how helium was detected in the Sun even before it was found on Earth — hence its name, from Helios, the Sun.

The big question

Why does an atom emit only certain colors, always the same ones, and not a continuous gradient? Classical physics has no answer. It takes the quantization of energy — the subject of what follows.

In summary

A hot, dense body gives a continuous spectrum; a gas gives a line spectrum — bright lines if it emits, black lines if it absorbs white light. Each element has a unique set of lines, its signature. These discrete lines, inexplicable in classical physics, reveal that the energy of atoms is quantized.