What is Light?

What can we know about light without knowing what it is? Inverse square law
Finite speed c = 3 x 10⁸ m/s = 300,000 km/s
Reflection and refraction
Dispersion: White light is a mixture of colors But what is light? Newton's particle model
Huygens's wave model
Properties of waves: For all waves v = where v = speed, = frequency, and

(lambda) = wavelength.
Interference: waves do it, particles don't.
Wavelengths of visible light

= 400 - 700 nm But what is waving? Maxwell's sythesis of electricity, magnetism, and light: it's electric and magnetic fields
Hertz's demonstration
The electromagnetic spectrum Light has particle properties, too. Black body radiation: Light is emitted and absorbed in photons.
Each photon carries energy h = hc/ lambda

where h = Planck's constant and c = speed of light.
Spectral lines and quantum jumps: a gas emits and absorbs photons carrying just the right amount of energy to take an atom or molecule from one energy level to another. Each gas has its own unique set of wavelengths. Looking for more? Prefer the web to books? Try Light Tour from the Center for Science Education at the Space Sciences Lab at UC Berkeley, What is Light? by Ken Bigelow, or Optics for Kids by Bruce Irving at Optical Research Associates. It's not just for kids! Another good website is part of Nick Strobel's Astronomy Notes.

Please send comments, additions, corrections, and questions to
joe.tenn@sonoma.edu
JST

2003-02-24