• Continuous emission spectra
• Atomic emission spectra
• Atomic absorption spectra

Summary

• When a light ray passes from one optical medium to another, the ray is refracted and its speed and direction change.
• The pattern which forms when a ray of light is broken up into its component frequencies is called a spectrum. The light is broken up by the individual rays of different frequencies being refracted or having their path “bent” as they go through optical (transparent) media of different optical density (different degrees of transparency/light conductivity).

Spectra can be observed with a diffraction grating, a spectroscope or a prism, or in a rainbow after a storm.

10.1 Continuous Emission Spectra

• The spectrum produced when white light passes through a prism is called a continuous spectrum.
• The spectrum emitted by the sun is a continuous emission spectrum.
• The colours in the spectrum follow on each other without any gaps between them. A familiar example of a spectrum is a rainbow that one sees after a thunderstorm.

10.2 Atomic Emission Spectra

• Atomic emission spectra are produced when a gas is heated or by passing an electric current through it in a gas discharge tube.
• The electrons in the atoms of the gas absorb the energy and become excited and move to a higher (excited) energy level. This high energy state is unstable.
• When excited electrons return to the ground state or a lower energy level, the energy is released in specific energy packets called “photons”, or light particles.
• The gas becomes incandescent (glowing).
• The energy of the emitted photon equals the energy difference between the two energy levels. The energy of light is directly proportional to its frequency and the frequency of light determines its colour.
• Only the frequencies (colours) of light that are in the visible range, that are emitted by the atoms, are seen by the eye. Colours out of the visible range, such as ultraviolet and infrared are not seen. The range of frequencies emitted by a particular substance are called a line emission spectrum as most substances do not emit the full spectrum; instead, they emit a particular pattern of frequencies.
• The atoms of each element have a unique set of energy levels, so the line emission spectrum is a set of discrete coloured lines with dark spaces in between where those frequencies are not being emitted.
• The line emission spectrum for each element is unique to that element, and can be used to identify that element. For example, amber street lamps have a sodium lamp in them, and thus produce an amber light, as sodium emits primarily in the yellow band.
  Likewise, fireworks’ colours are determined by the chemicals used in them. So, for example, bright red is produced by strontium (Sr), bluegreen by copper (Cu), and so on.
• Scientists are able to tell what elements are present on distant planets and stars by projecting their light through a prism and capturing the line emission spectrum.

10.3 Atomic Absorption Spectra

An atomic absorption spectrum is a continuous spectrum where certain colours or frequencies are missing. These frequencies appear as dark lines in the spectrum. The region A-B in the diagram below is Infrared. The region B-C and part way to D is red. The region C-D is orange. The region D-E is green. The region E-F is cyan/light blue. The region around F is blue.
The region F-G is indigo, and G-H violet. The region KH is ultraviolet.

You must remember:

• Atomic absorption spectra are produced when light passes through a cold gas.
• The electrons in the atoms of the gas absorb energy from the light and become excited and move to a higher (excited) energy level.
• The energy of the absorbed light energy equals the energy difference between the two energy levels.
• The energy of light is directly proportional to its frequency and the frequency of light determines its colour.
• The light that has not been absorbed by the gas, reaches the eye and therefore shows the range of frequencies in the atomic absorption spectrum.
• The atoms of each element have a unique set of energy levels, so the atomic absorption spectrum is a continuous spectrum with a few black lines. These lines represent the colours (and hence frequencies) of the light that were absorbed by the gas atoms’ electrons.
• The atomic absorption spectrum for each element is unique to that element, and can be used to identify that element.
• The dark lines represent the same frequencies of light that are emitted in the same element’s atomic emission spectrum. If an atomic emission spectrum and an atomic absorption spectrum are combined for a specific element, we see a continuous spectrum.

DEFINITIONS
discrete: clear and individual, separate incandescent: glowing ground state: lowest stable energy state excited state: high and unstable energy state

Activity 1

1. What is the approximate wavelength range of visible light? (2)
2. Name five wavelength ranges and their uses. (5)
3. How can a scientist tell what elements are present on a star? (3)
4. What is the approximate wavelength of red light? And violet? (2)
5. What does the wavelength of UV tell you about its energy levels? (2)
6. Does microwave radiation or gamma radiation have more energy per photon? (1)
7. Give one example of a colour in fireworks achieved through emission spectra. (1)
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