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COMPARISON OF LIGHT WAVES WITH SOUND WAVES
There are two main differences between sound waves and light waves. The first difference is in
velocity. Sound waves travel through air at the speed of approximately 1,100 feet per second; light waves
travel through air and empty space at a speed of approximately 186,000 miles per second. The second
difference is that sound is composed of longitudinal waves (alternate compressions and expansions of
matter) and light is composed of transverse waves in an electromagnetic field.
Although both are forms of wave motion, sound requires a solid, liquid, or gaseous medium; whereas
light travels through empty space. The denser the medium, the greater the speed of sound. The opposite is
true of light. Light travels approximately one-third slower in water than in air. Sound travels through all
substances, but light cannot pass through opaque materials.
Frequency affects both sound and light. A certain range of sound frequencies produces sensations
that you can hear. A slow vibration (low frequency) in sound gives the sensation of a low note. A more
rapid sound vibration (higher frequency) produces a higher note. Likewise, a certain range of light
frequencies produces sensations that you can see. Violet light is produced at the high-frequency end of the
light spectrum, while red light is produced at the low-frequency end of the light spectrum. A change in
frequency of sound waves causes an audible sensation—a difference in pitch. A change in the frequency
of a light wave causes a visual sensation—a difference in color.
For a comparison of light waves with sound waves, see table 1-2.
Table 1-2.—Comparison of Light Waves and Sound Waves
SOUND WAVES
LIGHT WAVES
VELOCITY IN AIR
APPROXIMATELY 1,100 FEET
PER SECOND
APPROXIMATELY 186,000
MILES PER SECOND
FORM
A FORM OF WAVE MOTION
A FORM OF WAVE MOTION
WAVE COMPOSITION
LONGITUDINAL
TRANSVERSE
TRANSMITTING MEDIUM
ALL SUBSTANCES
EMPTY SPACE AND ALL
SUBSTANCES EXCEPT
OPAQUE MATERIALS
RELATION OF
TRANSMITTING MEDIUM
VELOCITY TO VELOCITY
THE DENSER THE MEDIUM,
THE GREATER THE SPEED
THE DENSER THE MEDIUM,
THE SLOWER THE SPEED
SENSATIONS PRODUCED
HEARING
SEEING
VARIATIONS IN
SENSATIONS PRODUCED
A LOW FREQUENCY CAUSES
A LOW NOTE; A HIGH
FREQUENCY, A HIGH NOTE
A LOW FREQUENCY CAUSES
RED LIGHT; A HIGH
FREQUENCY, VIOLET LIGHT
Q42. What three examples of electromagnetic energy are mentioned in the text?
Q43. What is the main difference between the bulk of the electromagnetic spectrum and the visual
spectrum?
