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EM Fields Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 12, 2011

Problem 1

What is the frequency of a red laser beam whose waves measure 7400 Å?

Solution 1

Use the formula for frequency in terms of wavelength. Note that 7400 Å = 7400 × 10 −10 m = 7.400 × 10 −7 m. Then the frequency in hertz is found as follows:

f = 2.99792 × 10 8

= 2.99792 × 10 8 /(7.400 × 10 −7 )

= 4.051 × 10 14 Hz

This is 405.1 THz. To give you an idea of how high this frequency is, compare it with a typical frequency-modulated (FM) broadcast signal at 100 MHz. The frequency of the red light beam is more than 4 million times as high.

Problem 2

What is the wavelength of the EM field produced in free space by the ac in a common utility line? Take the frequency as 60.0000 Hz (accurate to six significant figures).

Solution 2

Use the formula for wavelength in terms of frequency:

λ = 2.99792 × 10 8 /f

= 2.99792 × 10 8 /60.0000

= 4.99653 × 10 6 m

This is about 5,000 km, or half the distance from the Earth’s equator to the north geographic pole as measured over the surface of the globe.

Practice problems of these concepts can be found at: Forms of Radiation Practice Test

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