PROPERTIES OF LIGHT

WAVES

Examples: ocean waves; sound, light. A wave is usually a disturbance in a medium (water, air, etc.)

Characteristics of a wave:

frequency is how frequently a crest washes over you. Wavelength times frequency = velocity of wave ( f = c), so you can specify either

ELECTROMAGNETIC WAVES or EM RADIATION

• medium = electric and magnetic field
• all EM waves travel in vacuum at speed of light, c=3x10⁵ km/s
• From long wavelength to short:

  radio, infrared, visible light, ultraviolet, X-ray, gamma ray

  Only visible light, most radio waves and some infrared get through Earth's atmosphere
• Visible light:
  • range of wavelengths from 4x10^-8 m to 7x10^-8 m is visible
  • The color of visible light determined by wavelength. In order of wavelength, from long to short:

    red, orange, yellow, green, blue, indigo, violet

BLACK BODY RADIATION

Key features:

• at all wavelengths, the radiation increases with temperature
• the peak of the distribution shifts so that the most radiation is emitted at a wavelength that decreases with temperature (inversely: Wien's law)
• Stefan's law: total power radiated is proportional to T⁴

Temperature scales:

Applications: measure temperature of distant bodies; 3 K background radiation

SPECTRAL LINES

Some bodies instead of emitting at all frequencies emit only at certain discrete frequencies called bright or emission lines

can be used to identify elements, or chemical compounds

Applications: discovery of Helium in sun (1868; found on earth in 1895); identification of composition of bodies at a distance (spectroscopy)

DOPPLER EFFECT

radiation from receding bodies is shifted to longer wavelengths (red shift); radiation from approaching bodies is shifted to shorter wavelengths (blue shift)

velocity towards or away is radial; across the field of view, transverse. Only radial velocity contributes to Doppler shift.

Doppler shift can be used to measure radial velocity.

KIRCHHOFF'S LAWS

1. A luminous solid or liquid (or in fact a sufficiently dense gas) emits light of all wavelengths and so produces a continuous spectrum of radiation: black body

2. A low density hot gas emits light whose spectrum consists of a series of discrete bright emission lines. These lines are characteristic of the chemical composition of the gas.

3. A cool, thin gas absorbs certain wavelengths from a continuous spectrum, leaving discrete dark absorption (or Fraunhofer) lines in their place, superposed on the continuous spectrum. Once again, these lines are characteristic of the composition of the intervening gas and occur at precisely the same wavelengths as the emission lines produced by the gas at higher temperatures.

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