Light Flashcards
Refraction :
Light travels fastest in a vacuum. It moves somewhat more slowly through air, still slower through water, and slower yet through glass. When light passes from one substance into another its change of speed causes it to be bent, or refracted. Thus when a spoon is placed in a glass of water, the handle appears bent at the surface of the water.
Light waves differ from radio waves, X rays, and other electromagnetic waves only in:
their length. Light waves are of the right length to affect the optic nerves to make sight possible. Other electromagnetic waves are either too short or too long to do this.
Reflection
Part of the light that falls on a surface bounces, or is reflected. Regular reflection occurs in mirrors and smooth, polished, metal surfaces. Diffuse reflection occurs on rough surfaces. Use is made of regular reflection in mirrors, reflecting telescopes, and the reflectors used in spotlights, automobile headlights, and other lighting applications.
Interference
occurs when two waves meet. The waves then reinforce each other when their crests meet, and cancel each other when the trough of one meets the crest of the other. Interference in light waves occurs when light is passed through two tiny holes onto a screen. The effect of interference is to produce alternating bands of light and shade. Interference occurs only in waves.
Diffraction
is the bending of waves as they pass about obstacles and their spreading after passing through openings. Light is diffracted when it passes through a tiny hole or through a diffraction grating, a piece of glass in which closely spaced parallel lines are cut. When white light is diffracted, it is separated into a spectrum consisting of the colors that form white light. Diffraction of light is used in chemical analysis by means of spectroscopes.
Light ordinarily travels in straight lines. One proof of this:
is the sharp shadows of objects lighted by small sources of light.
As it travels, light spreads and grows dimmer. The area covered by a beam of light:
increases with the square of the distance from the source.
The intensity of illumination decreases at the same rate (The area covered by a beam of light decreases with the square of the distance from the source). Thus a book held four feet from a light source receives:
only 116 of the light it would receive at one foot.
The measurement of light intensity is called:
photometry. Photometry is employed in determining the proper lighting of rooms, in photography, and for a number of other purposes.
