A continuous spectrum contains a continuous range of frequencies.
Solid, liquid, or dense gaseous objects radiate a continuous spectrum, which may extend into or beyond the visible region. The process is known as incandescence. The frequency distribution, and hence the dominant colour, depends on the temperature of the object.
- Describe the changes in the spectrum of a filament globe as the temperature of the filament increases.
Filament bulb: Magnified | Temperature | Graph
Simulation: Blackbody Spectrum
Videos: How it works - The Incandescent Lightbulb | How Modern Light Bulbs Work
Videos: Temperature and spectrum | Black body radiation (stop at 5:05)
Atoms can be raised to excited states by heating or by bombardment with light or particles such as electrons.
An atom is in an excited state when an electron has been raised to a higher energy level.
The heated vapour of a pure element emits light of discrete frequencies, resulting in a line emission spectrum when the light is viewed with a spectrometer.
- Describe the general characteristics of the line emission spectra of elements.
- Explain how the uniqueness of the spectra of elements can be used to identify the presence of an element.
- Explain the production of characteristic X-rays in an X-ray tube.
- Solve problems that require comparing spectra of different elements.
Website: Visible Spectra of the Elements
Video: Atomic spectra (Flippin' Science)
The presence of discrete frequencies in the spectra of atoms is evidence for the existence of discrete electron energy-levels atoms.
The different electron energy-levels can be represented on an energy-level diagram.
When an electron makes a transition from a higher-electron energy level to a lower-electron energy level in an atom, the energy of the atom decreases and can be released as a photon.
The energy of the emitted photon is given by the difference in the electron-energy levels of the atom. An atom is in its ground state when its electrons are in their lowest possible electron energy-level in atoms.
- Explain how the presence of discrete frequencies in line emission spectra provides evidence for the existence of states with discrete electron energy-levels in atoms.
- Solve problems involving emitted photons and electron energy-levels.
- Draw electron energy-level diagrams to represent the energies of different states in an atom.
- Draw arrows on an electron energy-level diagram showing transitions between electron energy-levels in atoms.
- Given an electron energy-level diagram, calculate the frequencies and wavelengths of lines corresponding to specified transitions.
Atomic Energy Levels (Khan Academy)
Hydrogen Atom models simulation
Mini Spectroscopy (requires Java, run in IE)
The line emission spectrum of atomic hydrogen consists of several series of lines.
- Draw, on an electron energy-level diagram of hydrogen, transitions corresponding to each of the series terminating at the three lowest-energy levels.
- Relate the magnitude of the transitions on an electron energy-level diagram to the region in the electromagnetic spectrum of the emitted photons (ultraviolet, visible, or infrared).
Video: Emission and Absorption Line Spectra (Lego analogy)
The ionisation energy of an atom is the minimum energy required to remove the electron from the atom in its ground state.
- Determine the ionisation energy (in either joules or electronvolts) of atoms using an electron energy-level diagram.
Energy levels and absorption spectra (SlideShare)
When light with a continuous spectrum is incident on a gas of an element, discrete frequencies of light are absorbed, resulting in a line absorption spectrum.
The frequencies of the absorption lines are a subset of those in the line emission spectrum of the same element.
- Describe the line absorption spectrum of atomic hydrogen.
- On an energy-level diagram, draw transitions corresponding to the line absorption spectrum of hydrogen.
- Explain why there are no absorption lines in the visible region for hydrogen at room temperature.
- Account for the presence of absorption lines (Fraunhofer lines) in the Sun’s spectrum.
Sun's spectrum: Graph | Strips
Video: Fraunhofer lines
One type of fluorescence is when an electron in an atom absorbs a photon to reach a higher electron energy-level, but then reverts to its previous state by emitting two or more photons with lower energy and longer wavelength.
- Explain, using an electron energy-level diagram, the production of multiple photons via fluorescence.
Video: How Fluorescence Works
Fluorescent rocks: under white light | under UV light
When an electron in an atom absorbs a photon and reaches a higher electron energy-level the atom is said to be in an excited state. Excited states are generally short-lived and the electron returns spontaneously to its previous electron energy-level often by emitting a series of lower-energy photons. This is known as ‘spontaneous emission’.
When a photon is incident on an electron that has been raised to a higher electron energy-level, and the energy of the photon corresponds to a transition to a lower electron energy-level, then the photon can stimulate a transition to the lower electron energy-level. This results in two identical photons; the original photon and a second photon that results from the transition. This is known as ‘stimulated emission’.
- Compare the process of stimulated emission with that of spontaneous emission.
Video: Stimulated Emission (Lego analogy)
Stimulated emission in gas lasers produces laser light.
The photon emitted in stimulated emission is identical (in energy, direction, and phase) to the incident photon.
- Explain how stimulated emission can produce coherent light in a laser.
A population inversion is produced in a set of atoms whenever there are more atoms in a higher-energy state than in a lower-energy state. For practical systems, the higher-energy state must be metastable if a population inversion is to be produced.
- Explain the conditions required for stimulated emission to predominate over absorption when light is incident on a set of atoms.
The energy carried by a laser beam is concentrated in a small area and can travel efficiently over large distances, giving laser radiation a far greater potential to cause injury than light from other sources.
- Describe the useful properties of laser light (i.e. it is coherent and monochromatic, and may be of high intensity).
- State the requirements for the safe handling of lasers.
Simulations: Stimulated emission (PhET) | Laser (please excuse the poor graphics)
Videos: Population Inversion | Helium-Neon Laser Demonstration