The Physics
Hypertextbook
Opus in profectus

Electromagnetic Spectrum

Problems

practice

1. Write something.
2. Write something.
3. Write something.
4. Write something completely different.

conceptual

1. Two simple facts
1. What is the source of all magnetism?
2. What is the source of all electromagnetic waves?
2. The door on a microwave oven is basically a double layer of safety glass with a perforated metal foil layer in between. The perforations allow you to see the food inside while at the same time keeping the microwaves trapped inside. How is this possible? Why is it that the light can escape the oven, but the microwaves can't?
3. Why are TV broadcasts in the VHF range more easily received in areas of marginal reception than broadcasts in the UHF range? Remember, UHF frequencies are higher than VHF frequencies. (Assume that both types of frequencies are broadcast under identical conditions.)

numerical

1. Sound is a longitudinal mechanical wave and radio is a transverse electromagnetic wave. Humans and other animals have organs called ears for receiving sound waves. They do not have organs for receiving radio waves. Such a device is called a radio receiver or just a radio. Analog radio relies one of two basic techniques to encode sound information into a radio signal — by varying the amplitude of the radio signal (amplitude modulation or AM) or by varying the frequency of the radio signal (frequency modulation or FM).
1. AM radio broadcasts in the US are allocated frequencies from 530 to 1710 kHz. What range of wavelengths correspond to these frequencies?
2. FM radio broadcasts in the US are allocated frequencies from 88 to 108 MHz. What range of wavelengths correspond to these frequencies?
2. Mobile phone companies use many frequencies, but across the Americas most of them are in the 800 MHz or 1,900 MHz bands. The 800 MHz band is the original "Cellular" band, introduced in 1983. The 1,900 MHz band is the newer "PCS" band, introduced in 1994.
1. The 800 MHz band spans frequencies from 824 to 894 MHz. What range of wavelengths correspond to these frequencies?
2. The 1,900 MHz band spans frequencies from 1,850 to 1,990 MHz. What range of wavelengths correspond to these frequencies?
3. The wireless networking standard known as Wi-Fi encompasses several frequency bands in the microwave part of the electromagnetic spectrum. The two most common are the 2.4 GHz and 5 GHz bands.
1. Wi-Fi channels in the 2.4 GHz band range from 2.401 GHz to 2.473 GHz in the US. What range of wavelengths correspond to these frequencies?
2. Wi-Fi channels in the 5 GHz band range from 5.170 GHz to 5.835 GHz in the US. What range of wavelengths correspond to these frequencies?
4. An analogy.
1. What is the separation between the scratches on a diffraction grating that would be used in a typical high school or undergraduate physics lab?
2. What is the wavelength of the approximate center of the visible spectrum?
3. What is the separation between the pickets of a picket fence?
4. For what wavelength could a picket fence be used like a diffraction grating?
5. What is the frequency of the electromagnetic wave with the wavelength calculated in part d?
6. What kind of electromagnetic radiation is this?
5. A typical household microwave oven operates at a frequency of 2.45 GHz.
1. What is the wavelength of this radiation?
2. Hotspots in a microwave oven are caused by the formation of standing waves within the cooking chamber. What is the separation between hotspots in a 2.45 GHz microwave oven?
3. How is the problem of hotspots dealt with in the design of microwave ovens or in the cooking techniques employed by the people that use them?
6. The lowest frequency of electromagnetic radiation used for communication purposes is at 76 Hz. This lies in the extremely low frequency (ELF) part of the radio spectrum.
1. Who uses this frequency and for what purpose?
2. What is the wavelength of this radiation?
3. How long would a dipole antenna have to be to broadcast at this frequency?
4. How does the antenna length you calculated in part c. compare to the circumference of the Earth?
5. How long is the actual antenna used for ELF communication?