Page 387 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 30

Light

ELECTROMAGNETIC WAVES

Electromagnetic waves consist of coupled electric and magnetic fields that vary periodically as they move
through space. The electric and magnetic fields are perpendicular to each other and to the direction in which
the waves travel (Fig. 30-1), so the waves are transverse, and the variations in E and B occur simultaneously.
Electromagnetic waves transport energy and require no material medium for their passage. Radio waves, light
waves, X-rays, and gamma rays are examples of electromagnetic waves, which differ only in frequency; they
are listed here in order of increasing frequency. The color sensation produced by light waves depends on their
frequency, with red light having the lowest visible frequencies and violet light the highest. White light contains
visible light waves of all frequencies.
Electromagnetic waves are generated by accelerated electric charges, usually electrons. Electrons oscillating
back and forth in an antenna give off radio waves, for instance, and accelerated electrons in atoms give off light
waves.
In free space all electromagnetic waves have the velocity of light which is
8
Velocity of light = c = 3.00 × 10 m/s = 186,000 mi/s

Electric field

Direction
of wave
Magnetic field
Fig. 30-1. (From Konrad B. Krauskopf and Arthur Beiser, The Physical Universe, 10th Ed., c 2003, The McGraw-
Hill Companies. Reproduced with permission of The McGraw-Hill Companies.)

SOLVED PROBLEM 30.1
Why can light waves travel through a vacuum whereas sound waves cannot?

Light waves consist of coupled fluctuations in electric and magnetic fields and hence require no material medium
for their passage. Sound waves, however, are pressure fluctuations and cannot occur without a material medium to
transmit them.

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