Electromagnetic waves carry energy through space without needing a material medium. They include radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. In every case, changing electric and magnetic fields support each other as the wave travels.
This is why signals from an antenna can move through air, vacuum, and open space at extremely high speed.
In a traveling electromagnetic wave, the electric field, magnetic field, and direction of motion are all perpendicular to one another. An antenna creates changing electric currents, which produce changing electric and magnetic fields that detach and spread outward as radiation. In vacuum, all electromagnetic waves travel at the speed of light, c = 3.00 x 10^8 m/s.
The wavelength and frequency determine the wave type and are connected by v = fλ.
Key Facts
- In vacuum, electromagnetic waves travel at c = 3.00 x 10^8 m/s.
- Wave speed is related to frequency and wavelength by v = fλ.
- For light in vacuum, c = fλ.
- The electric field E is perpendicular to the magnetic field B.
- Both E and B are perpendicular to the direction of propagation.
- For an electromagnetic wave in vacuum, E = cB when E is in V/m and B is in tesla.
Vocabulary
- Electromagnetic wave
- A traveling disturbance made of oscillating electric and magnetic fields that can move through vacuum.
- Electric field
- A field that describes the force per unit charge on a positive test charge at a location.
- Magnetic field
- A field produced by magnets or moving charges that can exert forces on moving charges and magnetic materials.
- Wavelength
- The distance from one point on a wave to the next identical point, such as crest to crest.
- Frequency
- The number of wave cycles that pass a point each second, measured in hertz.
Common Mistakes to Avoid
- Drawing E and B in the same direction is wrong because the electric and magnetic fields in a plane electromagnetic wave are perpendicular to each other.
- Forgetting that the wave direction is perpendicular to both fields is wrong because the wave propagates in the direction given by the orientation of E crossed with B.
- Using the speed of sound for a radio or light wave is wrong because electromagnetic waves in vacuum travel at c = 3.00 x 10^8 m/s, not at about 343 m/s.
- Mixing up frequency and wavelength is wrong because frequency counts cycles per second while wavelength measures distance per cycle.
Practice Questions
- 1 A radio station broadcasts at 100.0 MHz. Assuming the wave travels at 3.00 x 10^8 m/s, what is its wavelength?
- 2 A laser emits light with wavelength 650 nm in vacuum. What is its frequency?
- 3 An electromagnetic wave travels to the right. Its electric field oscillates up and down. What direction must the magnetic field oscillate, and why?