Homework #1 ANSWERS

#1, What is meant by "wavelength"?

The distance from one wave crest to the next is the wavelength.

#2 What is meant by the "period" of a wave?

The time for one complete wave cycle to pass is the period.

#3. You are sitting on a beach watching the waves roll in. In 12 seconds, 6 waves roll by. What is the period of the waves (T)? What is their frequency? (f)

The period is the time for one wave to pass, which is 12 seconds/6 = 2 seconds

The frequency is 1/period = 1/2seconds = 0.5 Hz

#4 What is the speed of light in a vacuum?

In a vacuum, light moves at 3 x 108 meters/second (or, 186 000 miles / second)

#5 An AM radio station broadcasts at a frequency of 660 kHz. (note that k means 1000) Since radio waves move at the same speed as light waves, what is the wavelength of this radio wave?

Using the equation c = lf and solving for wavelength, l = c/f = 3 x 108 / 660 x 103 = 455 meters

#6 Compare your answer to problem #4 to the example on page 7, which is for an FM station broadcasting at 100 MHz (M = 1 000 000). Which type of broadcast, AM or FM, has longer wavelength waves?

The AM wave has a much longer wavelength (in the text example, the FM wave has a 3 meter wavelength). Commercial AM radio uses kHz frequencies and FM uses MHz frequencies, and the lower frequencies of AM correspond to much longer wavelengths.

The drawing below shows two waves- the top one has a long wavelength and the bottom a short wavelength. Now imagine that both of these waves are passing by you at the same speed. That is, they are printed on the same piece of paper, and the paper is moving past you. Think about the rate at which the peaks of each wave pass by- the bottom wave will have more peaks pass by per second (higher frequency) than the top wave.

If two waves move at the same speed, a lower frequency will correspond to a longer wavelength.

#7 List the electromagnetic spectrum in order of INCREASING wavelength (shortest to longest).

gamma (shortest), x-ray, UV, visible, IR, microwave, radio

#8 List the colors of the visible spectrum in order of INCREASING wavelength (shortest to longest).

violet (shortest), blue, green, yellow, orange, red

#9 What colors correspond to the following wavelengths:

a)450 nm blue/violet

b)550 nm green

c)650 nm red

#10 What is an instrument that allows scientists to study light spectra?

A spectrometer, which uses a prism or a diffraction grating to break light into its constituent colors (spectrum).