Light

The presence of light in the universe is what makes astronomy possible, since it is the analysis of this light, from distant celestial objects, which provides us with our knowledge of the cosmos.  Light, however, is but one small part of a much larger whole; the electromagnetic spectrum.

  • Almost all observations are really observations of light, so we need to understand light to understand astronomy.

    • A. What is Light?

               A.1 Light is a wave.

  • Wave = a disturbance traveling through space without any net motion of the medium.

    • e.g.. sound (air), ocean waves (water), seismic waves (ground), light (electric and magnetic fields).

  • Two Forms of Waves:

    • longitudinal: one in which the medium is compressed in the direction of the wave's movement.

    • transverse: the disturbance is at right angles to the direction in which the wave travels.

    • light is a transverse wave.

      • electromagnetic wave
      Electric and magnetic fields travel together through space as electromagnetic radiation (transverse wave).

  • Example of a transverse wave:

    • the moving disturbance in a piece of string caused by the flick of a wrist.  At no time does the string actually move away from you but the "bump" created by the flick of your wrist, moves down the string like a water wave.

  • How do we know light is a wave?

    • Diffraction: waves bend when passing an aperture

    • Interference: two small apertures produce alternating light and dark bands.

  • Properties of waves:

    • lambda = wavelength

    • f = frequency = number of oscillations/second.                  (units: Hz = s-1)

    • c = speed (m/s, cm/s)

  • Relationships: c = lambda f, lambda = c/f, f = c/lambda

  • For a given medium and type of wave, c = constant.

  • For light in a vacuum, c = 3 x 108m/s (= 3 x 1010 cm/s).

  • Different colors (or types) of light are different frequencies:

    gamma ray, X ray, Ultraviolet, Visible, Infrared, microwave, radio.

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    Increasing lambda, Decreasing f

    • e.g.. Yellow light, lambda = 6000Å = 6 x 10-7m.
      f = c/lambda = 3 x 108m/s/6 x 10-7m
      = 5 x 1014Hz

    A.2 Light is a stream of particles (Photons).

  • How do we know light is made up of photons?

    • The photoelectric effect: High ``frequency'' light causes electrons to be emitted by metals. The electron speed depends on the light ``frequency'', not the amount of light. At low frequencies, no electrons are emitted.

  • Each particle has an energy E = hf = hc/lambda

  • h = Planck's constant = 6.63 x 10-34 Joule sec = 6.63 x 10-27 erg sec.

  • Joule : energy unit in the mks (meters, kilograms, seconds) system.

  • erg : energy unit in the cgs (centimeters, grams, seconds) system.

  • Higher frequency light has photons with higher energy.

    • B. What makes Light?

    • To make a wave, disturb the medium. Light is caused by accelerating electrically charged particles.

    • Matter is made up mostly of charged particles which vibrate, rotate, and move.

    • We feel this motion as temperature.

    • The Kelvin (or absolute) scale: at 0K, no particle motions (-273o C); at 273K, ice melts (0o C); at 373K, water boils (100o C).

    • Every object that has a finite temperature (hence moving or vibrating particles) emits light.

    C. Blackbody Radiation

    • Blackbody = an idealized body that absorbs and emits all light perfectly.

    • The rate and frequency of light emitted depends only on the temperature of the blackbody. The total flux of light from a given area of surface is F = sigma T4. T = temperature.
      sigma = Steffan-Boltzman constant = 5.67 x 10-5 erg/s/cm2/K4.

    • The hotter something is, the more light it emits.

    • The frequency range emitted also depends on the temperature. Higher temperature gives light at higher frequencies.

    • The peak of the energy - wavelength curve for a blackbody is given by Wien's Law: lambdapeak = 0.29/T, where T is measured in Kelvin and lambda is measured in cm.

    • Most opaque objects roughly approximate a black body, so measuring their emitted light determines their temperature.

    • Blackbody radiation is a form of continuous spectrum.

    D. The Doppler Effect

    • When an object is moving towards or away from us, the frequency of the light changes (and since the speed stays constant, the wavelength also changes).

    • For an object moving towards us, the frequency increases (and the wavelength decreases): this is called blueshift.

    • For an object moving away from us, the frequency decreases (and the wavelength increases): this is redshift.

    • A similar effect occurs with sound: when an object approaches, the sound it makes seems to have a higher pitch (shorter wavelength) than when it is moving away.

    • For speeds much less than the speed of light, the wavelength shift is given by lambda/lambdao = 1 + (v/c), where lambda is the observed wavelength, lambdao is the original wavelength (at the source), v is the speed of the object (away from the observer) and c is the speed of light.

    • By measuring these shifts in wavelength, we can measure the speed of moving objects.

    • The same process is used to measure automobile speeds using radar.

    E. Atoms and Light

    • Atoms can emit and absorb specific wavelengths of light as well broad ranges of light (like blackbody radiation).

    • Experiments have shown that atoms are made up of electrons (light, negatively charged particles) ``orbiting'' a nucleus made of protons (heavy, positively charged particles) and often neutrons (heavy, neutral particles).

    • The number of protons in the nucleus determines the chemical element. The number and arrangement of electrons determines the chemical properties of the atom.

    • Unlike planets orbiting a star, electrons can only be in certain orbits (orbitals) with set energies.

    • When moving from one orbital to another with a lower energy (closer to the nucleus), the electron emits a photon with an energy equal to the difference between the two orbitals.

    • In order for an electron to move to a higher energy orbital (further from the nucleus) it must absorb a photon with exactly the right energy.

    • Transparent gases produce dark ``absorption lines'' and bright ``emission lines''.

    • Kirchoff's Laws:

      1. An opaque object gives a continuous spectrum.

      2. A cool, transparent gas produces an absorption spectrum.

      3. A hot, transparent gas produces an emission spectrum.

    • Each element or compound has a unique pattern of lines.

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