 How
sound
and
light
travel
Q: I understand vaguely the difference between light and sound travel, but not technically.
What makes them so different? ?
A: "Many things in nature wiggle and jiggle," says Paul Hewitt in his lucid book, Conceptual
Physics.
"We call a wiggle in time a vibration and a wiggle in space and time a wave." A wave cannot
exist in one place but must extend from one place to another, like ripples spreading out from a
dropped pebble in a lake. Similarly, a vibration cannot exist in one instant but needs time to
move to and fro, just as a plucked guitar string.
Light and sound are both vibrations that travel through space as waves. They are, however,
different.
Sound is a vibration of some medium that it travels through, like air or water. Sound vibrations
travel through air by compressing the air, then rarefying the air, compressing the air, then
rarefying... "If there is no medium to vibrate," says Hewitt, "then no sound is possible."
Light is a vibration of magnetic and electric fields--a vibration of pure energy.
As you know, light waves can pass through many media: air, water, glass, to name a few. But
they do not need any medium. Sunlight, moonlight, and starlight pass through the vacuum of
outer space to reach us.
The source of all waves is something vibrating. The vibrating prongs of a tuning fork generate a
sound. Vibrating electrons in an atom generate light. The basic difference between sound and
light travel is that sound requires an intervening substance to change and impose a wave pattern
on. Whereas light does not. Sound travel is motion; light travel is radiation.
Further Surfing:
Conceptual Physics on the Web by Paul G. Hewitt
Magnetic Field Lines, Electricity and Magnetism, Molecular Expressions, Interactive Java
Tutorials
How a speaker works, Electricity and Magnetism, Molecular Expressions, Interactive Java
The nature of waves, The physics classroom, Glenbrook South High School, Glenview, IL
Traveling waves, Hyperphysics, by Carl R. (Rod) Nave, Department of Physics and Astronomy,
Georgia State University
Figure Caption: [USGS] One of many shallow volcanic earthquakes at Mount St. Helens
observed several days to two weeks before each dome-building eruption from 1980 through
1982.
|
