Free Electromagnetic Waves MCQs with Answers

1. Electromagnetic waves consist of

• A. vibrating particles of a medium
• B. oscillating electric and magnetic fields at right angles to each other and to the direction of travel
• C. a stream of charged particles
• D. longitudinal compressions and rarefactions

Explanation: A changing electric field generates a magnetic field and vice versa, so the two sustain each other and the wave propagates without needing any medium, which is why light crosses empty space. Being transverse, electromagnetic waves can be polarised. All of them travel at 3 times 10 to the 8 metres per second in a vacuum.

2. Which of the following electromagnetic waves has the longest wavelength?

• A. Gamma rays
• B. X rays
• C. Visible light
• D. Radio waves

Explanation: The spectrum runs from radio waves at the long wavelength, low frequency, low energy end through microwaves, infrared, visible light and ultraviolet to X rays and gamma rays at the other extreme. Since all travel at the same speed, the longest wavelength necessarily has the lowest frequency. Photon energy is proportional to frequency, which is why gamma rays are the most damaging.

3. The speed of all electromagnetic waves in a vacuum is

• A. 3 times 10 to the 8 m per second, whatever their frequency
• B. different for each frequency
• C. 340 m per second
• D. dependent on the intensity

Explanation: In a vacuum every electromagnetic wave from radio to gamma travels at the same speed c, so wavelength and frequency vary inversely. In a material medium the speed falls and does depend slightly on frequency, which is the dispersion that splits white light in a prism. The value 340 m per second is the speed of sound in air.

4. Microwaves are used in a microwave oven because their frequency

• A. is absorbed strongly by water molecules, which then rotate and heat the food
• B. ionises the food
• C. reflects off water
• D. changes the chemical composition of the food

Explanation: The oscillating field makes polar water molecules flip back and forth, and the friction between them heats the food from within rather than only at the surface. Microwaves are non ionising, since their photon energy is far too low to remove electrons, so they do not make food radioactive. Metal containers are unsuitable because they reflect the waves and can cause arcing.

5. Ultraviolet radiation is more damaging to skin than visible light because ultraviolet photons

• A. travel faster
• B. have a longer wavelength
• C. carry more energy, enough to break chemical bonds in DNA
• D. are more numerous in sunlight

Explanation: Photon energy is proportional to frequency, so the shorter wavelength ultraviolet carries enough energy per photon to damage the bonds in biological molecules, which is why it causes sunburn and raises skin cancer risk. Visible photons simply cannot deliver that much energy however intense the beam. All electromagnetic waves travel at the same speed, so speed is not the difference.

6. The wave that has the highest frequency and penetrating power is

• A. X-rays
• B. Ultraviolet rays
• C. Gamma rays
• D. Microwaves

Explanation: Gamma rays sit at the extreme short wavelength end of the electromagnetic spectrum, so they carry the highest frequency and photon energy and need several centimetres of lead to attenuate them. X rays are next and overlap gamma rays in energy, the difference being origin: gamma from the nucleus, X rays from electron transitions or deceleration. Microwaves are at the low energy end.