When electrons are bound to their atoms in a gas, they can only exist in one of a discrete set of energies referred to as energy levels. Energy levels are negative because energy is required to remove an electron from the atom; an electron with zero energy is free from the atom. The energy level with the most negative value is the ground state. When an electron moves from a lower to an electron moves from a lower to a higher energy level within an atom in a gas, the atom is excited. Equally, when an electron moves from a higher energy level to a lower one it loses energy as a photon of light (de-excitation). The energy, and therefore wavelength, of this photon depends on the difference between the energy levels.
Each element has its own unique set of energy levels.
Three types of spectra exist:
Emission line spectra: atoms in a gas are excited (e.g. by heating). When electrons drop back down into lower energy levels, they emit photons with a set of discrete frequencies specific to the element.
Continuous spectra: all visible frequencies or wavelengths are present. Produced by a heated solid metal.
Absorption line spectra: a series of dark spectral lines are seen against the background of a continuous spectrum. Light from a source that produces a continuous spectrum (e.g. a star) passes through a cool gas. Gas atoms absorb photons so electrons are excited. Photons are re-emitted in all directions so the intensity in the original direction is reduced. Thus, frequencies absorbed show up as dark lines.
Different atoms have different spectral lines which can be used to identify elements within stars. The emission and absorption spectra are almost exact opposites of one another (but the absorption spectra is missing a few lines as the electrons return to the ground state in stages, but electrons that are excited usually start in the ground state).
Diffraction gratings are optical components used to split light into beams of different colours travelling in different directions. These beams can be analysed to determine the wavelengths of spectral lines.
- Monochromatic light is incident normally on diffraction grating
- Light is diffracted at each slit and an interference pattern emerges due to superposition
- Varying path and phase differences lead to a series of maxima and minima, according to the grating equation:
This can be used to determine the wavelength of light: plot a graph of sin() against and multiply the gradient by .
Stellar luminosity:
At any given temperature above absolute 0, an object emits electromagnetic radiation of different wavelengths and intensities. Such a hot object can be modelled as a black body (idealised body that absorbs all EM radiation and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature). Wein’s displacement law relates the absolute temperature of a black body to the peak wavelength emitted (at maximum intensity).
This can be used to determine the wavelength of light: plot a graph of sin() against and multiply the gradient by .
Stellar luminosity:
At any given temperature above absolute 0, an object emits electromagnetic radiation of different wavelengths and intensities. Such a hot object can be modelled as a black body (idealised body that absorbs all EM radiation and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature). Wein’s displacement law relates the absolute temperature of a black body to the peak wavelength emitted (at maximum intensity).
Objects like stars can be modelled as approximate black bodies, permitting estimation of their peak surface temperature.
Stefan’s Law states that the total power radiated (luminosity) per unit area of a black body is directly proportional to the fourth power of its absolute temperature.
