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Home Page of Peggy E. Schweiger

Light Fundamentals

Characteristics of light as electromagnetic radiation:

  1. it travels through a vacuum
  2. its speed in a vacuum is 3 x 108 m/s
  3. visible light ranges from 700 nm for red light to 400 nm for violet (blue) light where
    1 nanometer = 1 x 10-9 m

luminous body
produces light
illuminated body
reflects light
luminous flux (symbol is P, SI unit is lumen, lm)
measures the rate at which light is emitted from a source
illumination (symbol is E, SI unit is lux or lumens/m2)
refers to the amount of light falling on a surface

intensity (symbol is I, non-SI unit is candela, cd)
- measures how bright or intense the light is striking a surface

We will be measuring the intensity of light experimentally using a Joly photometer. The photometer will be moved back and forth on an optic bench until it is equally illuminated by both light bulbs at each end of the optic bench.

Light and Matter

transmits light readily
transmits light with distortion
doesn’t transmit light (it is absorbed or reflected)
band of colors produced when light is broken into its constituent wavelengths
scattering of light

the color of an opaque object is due to the color that it reflects.
A black object absorbs all color and a white object reflects all color.
The color of a transparent object is due to the combining of transmitted light colors.
Primary colors of transmitted light
red, green, and blue combine to form white light
Secondary colors
produced by combining two primary colors
yellow, cyan, and magenta

The colors of a thin film result from the interference of light reflected from the front and the back surfaces of the thin film. When the film thickness equals ¼ l, that color of light will be constructively interfered with as it reflects from the two surfaces of the film. All others will be destructively interfered with.

Polarized light waves are either vertical or horizontal. They are produced by passing light through a polarizer. The polarizer selectively absorbs the light and only light of one direction is transmitted.

Double Slit Diffraction

spreading of light around the edges of a barrier
Monochromatic light
light of one color (or frequency). Red light with a frequency of 4.3 x 14 Hz is an example of monochromatic light.
Coherent light
light of same wavelength (totally in-phase). Light produced by a laser is an example of coherent light. The light that exists the laser is totally in-phase.

When light interferes, the light waves produce alternating bright and dark bands of colors (interference fringes); nodal lines appear as dark bands and antinodal lines appear as bright bands. Violet light (with the shortest wavelength) is the least diffracted and red light (with the longest wavelength) is the most diffracted.

Angstrom: 1 A = 1 x 10-10 m

    Spectrum types:
  1. Continuous
    • produced by white light
    • contains all the colors in the rainbow
    • red light is diffracted the most and blue (violet) light is diffracted the least
  2. Absorption (dark line)
    • consists of dark lines on a continuous spectrum background
    • energy is absorbed at characteristic frequencies
  3. Emission (bright line)
    • energy is emitted at characteristic frequencies

Diffraction is used experimentally to determine the wavelength of light:

n l = d sin q
n represents the order line
l is the wavelength of the light
d represents the width of the slit
q is the angle through which your eye looks

Single Slit Diffraction

Light Sample Problems

Light Homework