Light

Reflection and Refraction

Fermat’s Principle: Out of all possible paths that light might take, to get from one point to another, it takes the path that requires the shortest time.
Law of Reflection: The angle of incidence equals the angle of reflection.
Plane mirrors: The observer sees the image of a candle at a point behind the mirror. The light rays do not actually come from this point, so the image is called a virtual image.
Causes of refraction: the bending is caused by a change in speed.
Dispersion: Violet light travels about one percent slower in ordinary glass than does red light.
Terms:
Reflection: the return of light rays from a surface in such a way that the angle at which a given ray is returned is equal to the angle at which it strikes the surface. When the reflecting surface is irregular, light is returned in irregular directions; this is diffuse reflection.
Refraction: the bending of an oblique ray of light when it passes from one transparent medium to another.
Critical angle: the minimum angle of incidence at which a light ray is totally reflected within a medium.
Total internal reflection: the total reflection of light traveling in a medium when it strides on the surface of a less dense medium at an angle greater than the critical angle.
Converging lens: A lens that is thicker in the middle than at the edges and refracts parallel rays passing through it to a focus.
Diverging lens: A lens that is thinner in the middle than at the edges, causing parallel ray passing through it to diverge.
Virtual image: an image formed by light rays that do not converge at the location of the image. A virtual image is that reflected by a mirror; it cannot be displayed on a screen.
Real image: An image formed by light rays that converge at the location of the image. A real image can be displayed on a screen.
Aberration: a limitation on perfect image formation inherent, to some degree, in all particle systems.

Light Waves

A soap bubble appears iridescent in white light when the thickness of the soap film is about the same as a wavelength of light.
Bright fringes occur when waves (from both slits) arrive in phase; dark areas result from the overlapping of waves that are out of phase.
Diffraction grating disperses white light into colors, and are used in spectrometers.
The amount of diffraction depends on the wavelength of the wave compared to the size of the obstruction that casts the shadow. The longer the wave compared to the obstruction, the more the diffraction occurs.
Terms:
Huygens’ principle: the theory by which light waves spreading out from a point source can be regarded as the superposition of tiny secondary wavelets.
Diffraction: the bending of light around an obstacle or through a narrow slit in such a way that fringes of light and dark or colored bands are produced.
Interference: the superposition of waves producing regions of reinforcement and regions of cancellation. Constructive interference refers to regions of reinforcement; destructive interference refers to regions of cancellation. The interference of selected wavelengths of light produces colors known as interference colors.
Polarization: the alignment of the electric vectors that make up electromagnetic radiation. Such waves of aligned vibrations are said to be polarized.
Hologram: a two dimensional microscopic diffraction pattern that shows three dimensional optical images.

Light Emission

Summary of terms:
Excitation: the process of boosting electrons in an atom or molecule from a lower to a higher energy level.
Emission Spectrum: the distribution of wavelengths in the light from a luminous source.
Spectroscope: an optical instrument, separates light into its constituent frequencies in the form of spectral lines
Incandescence: the state of glowing while at a high temperature, caused by electrons in vibration atoms and molecules that are shaken in and out of their stable energy levels, emitting radiant energy in the process. The peak frequency of radiant energy is proportional to the absolute temperature of a heated substance; f ~ t
Absorption Spectrum: a continuous spectrum, like that of white light, interrupted by dark lines or bands that result from the absorption of certain frequencies of light by a substance through which the radiant energy passes.
Fluorescence: the property of absorbing radiant energy of one frequency and re-emitting radiant energy of lower frequency. Part of the absorbed radiant energy goes into heat and the other part into excitation; hence the emitted radiant energy has a lower energy and therefore a lower frequency than the absorbed radiant energy.
Phosphorescence: a type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation, which provides an afterglow. The delay is caused by atoms being excited to energy levels that do not decay rapidly. The afterglow may last from fractions of a second to hours, or even days, depending on the type of material, temperature, and other factors.
Laser (light amp. by stimulated emission of radiation): an optical instrument that produces a beam of coherent monochromatic light.
Notes:l=c/f
The temperature of incandescent bodies, whether they be stars or blast-furnace interiors, can be determined by measuring the peak frequency of radiant energy they emit.
Incoherent white light contains waves of many frequency (and wavelengths) that are out of phase with one another
Light of a single frequency and wavelength is still out of phase
Coherent light: all the waves are identical and in phase.