(f) continuous spectrum, emission line spectrum and absorption line spectrum

The idea

A continuous spectrum contains all wavelengths of visible light, appearing as an unbroken band of colour. An emission line spectrum shows only bright lines at specific wavelengths on a dark background, produced when excited atoms in a low-pressure gas emit photons as electrons drop to lower energy levels. An absorption line spectrum shows a continuous spectrum crossed by dark lines at the same wavelengths as the emission lines of the same element, produced when white light passes through a cooler gas that absorbs photons matching its energy level gaps. Stars show absorption spectra because their hot dense core emits a continuous spectrum that passes through cooler outer gases.

What earns the marks

• A continuous spectrum contains all wavelengths and appears as an unbroken band of colour.
• An emission line spectrum consists of bright lines on a dark background, produced by excited atoms in a low-pressure gas emitting photons.
• An absorption line spectrum consists of dark lines on a continuous background, produced when white light passes through a cooler gas.
• The dark lines in an absorption spectrum occur at the same wavelengths as the bright lines in the emission spectrum of the same element.
• Stars produce absorption spectra because the hot dense core emits a continuous spectrum that passes through cooler outer layers.

Common mistakes and how to put them right

• Thinking an emission spectrum has dark lines on a bright background.

  Put it right: In fact emission lines are bright on a dark background.

• Confusing which type of spectrum a star produces.

  Put it right: Stars show absorption spectra, not emission spectra, because the continuous radiation from the core passes through cooler outer gases.

• Believing the dark lines in an absorption spectrum are at different wavelengths from the emission lines of the same element.

  Put it right: They occur at the same wavelengths.

• Assuming a continuous spectrum is produced by a low-pressure gas.

  Put it right: It is produced by a hot dense source such as a filament or a star's core.

Exam tips

• Link each spectrum type to its physical source: hot dense object gives continuous, low-pressure excited gas gives emission, continuous light through cooler gas gives absorption.
• Remember that absorption and emission lines of the same element occur at identical wavelengths, which is the basis for identifying elements in stars.
• When a question shows a spectrum diagram, check whether the background is bright or dark and whether the lines are bright or dark before deciding the type.