Photons and Light


Spectra of Light :

Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an instrument that photographs or maps spectra is a spectrograph.

The typical spectroscope is a combination of a microscope and a prism. The prism breaks the light into its spectra components (by differential refraction) which is then magnified with a microscope.

Spectra may be classified according to the nature of their origin, i.e., emission or absorption.

An emission spectrum consists of all the radiations emitted by atoms or molecules, whereas in an absorption spectrum, portions of a continuous spectrum (light containing all wavelengths) are missing because they have been absorbed by the medium through which the light has passed; the missing wavelengths appear as dark lines or gaps.

The spectrum of incandescent solids is said to be continuous because all wavelengths are present. The spectrum of incandescent gases, on the other hand, is called a line or emission spectrum because only a few wavelengths are emitted. These wavelengths appear to be a series of parallel lines because a slit is used as the light-imaging device.

Line spectra are characteristic of the elements that emit the radiation. Line spectra are also called atomic spectra because the lines represent wavelengths radiated from atoms when electrons change from one energy level to another. Band spectra is the name given to groups of lines so closely spaced that each group appears to be a band, e.g., nitrogen spectrum. Band spectra, or molecular spectra, are produced by molecules radiating their rotational or vibrational energies, or both simultaneously.


  • The invention of the spectroscope, a device that separates white light into component colors called a spectrum.

  • And the discovery that elements emit a unique spectrum, i.e. produce a unique chemical fingerprint in the spectrum.

    Elements Applet

    Spectrum of the Sun:

    Some unfamiliar lines were seen in the 19th century. These lines were called Helium (for the sun, helios).

    A Helium Spectrum:

    Kirchhoff's Laws:

    Kirchhoff showed that there are three types of spectra emitted by objects:

    1) Continuous spectrum - a solid or liquid body radiates an uninterrupted, smooth spectrum (called a Planck curve)

    2) Emission spectrum - a radiating gas produces a spectrum of discrete spectral lines

    3) Absorption spectrum - a continuous spectrum that passes through a cool gas has specific spectral lines removed (inverse of an emission spectrum)

    Planck's curve:

    One of the primary results from the field of spectroscopy was the discovery of how the spectrum of an object changes with temperature. In particular, was the formulation of the two laws of radiation:

  • Stefan-Boltzmann law: the amount of energy emitted from a body increases with higher temperature

  • Wien's law: the peak of emission moves to bluer light as temperature increases

    Wave-Particle Dualism:

    The wave-like nature of light explains most of its properties:

    But, the results from spectroscopy (emission and absorption spectra) can only be explained if light has a particle nature as shown by Bohr's atom and the photon description of light.

    This dualism to the nature of light is best demonstrated by the photoelectric effect, where a weak UV light produces a current flow (releases electrons) but a strong red light does not release electrons no matter how intense the red light.

    Einstein explained that light exists in a particle-like state as packets of energy (quanta) called photons. The photoelectric effect occurs because the packets of energy carried by each individual red photons are too weak to knock the electrons off the atoms no matter how many red photons you beamed onto the cathode. But the individual UV photons were each strong enough to release the electron and cause a current flow.

    It is one of the strange, but fundamental, concepts in modern physics that light has both a wave and particle state (but not at the same time).