Department of Physics, Carnegie Mellon, Fall ’09, Deserno/Franklin
Homework, due in lecture on Monday, Oct 19th
Frequencies and Wavelengths
1. What is the speed of sound in air expressed in “meters per second”?
2. What is the speed of light in air expressed in “meters per second”? Note: the value you will find on
the Internet or in a text book will most likely be for the speed of light in vacuum. Since this is very
close to the speed of light in air, it is OK to use it as an answer to this question.
3. Consider the range of sound frequencies heard by a typical human ear.
a) What is the lowest frequency of audible sound?
b) What is the highest frequency of audible sound?
c) Perform a calculation using your answers to the previous questions to determine the longest
and shortest wavelengths of sound detected by the human ear. Write out your calculation and
be sure to treat the units correctly.
4. Consider the range of wavlengths of light which can be detected by the human eye.
a) What is the shortest wavelength of visible light and what is the corresponding color?
b) What is the longest wavelength of visible light and what is the corresponding color?
c) Perform a calculation using your answers to the previous questions to determine the lowest
and highest frequecies of visible light. Write out your calculation and be sure to treat the units
correctly.
5. You can find the Carnegie Mellon radio station, WRCT, at 88.3 on your FM radio dial. This means
that their radio station broadcasts radio waves at a frequency of 88.3 MHz. What is the wavelength
of their radio signal?
6. What is the wavelength of the microwaves used in a microwave oven? Some microwave ovens have
turntables that move the food around because the ovens have “hot spots and cold spots”. Explain
why microwaves of this wavelength bouncing off the oven walls result in “hot spots and cold spots”
in the oven.