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1-32 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?

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