Question

which two options are reasons that electromagnetic waves can be used effectively in communication devices?
a. they can travel through either air or other types of matter.
b. any type of electromagnetic wave can be used for communication.
c. the speed at which they carry information can be varied.
d. they require no wires to carry information.

Explanation:

• Option A: Electromagnetic waves can travel through various media like air (and also vacuum). This property is crucial as communication devices often operate in different environments (e.g., mobile phones work in air - filled spaces).
• Option D: Since they don't need wires (unlike some wired communication methods), it allows for more flexible and widespread communication setups (e.g., wireless internet, satellite communication).
• Option B is incorrect because not all electromagnetic waves are suitable for communication. For example, gamma rays are harmful and not used for regular communication purposes.
• Option C is incorrect as the speed of electromagnetic waves in a given medium (e.g., speed of light in vacuum \(c = 3\times10^{8}\ m/s\)) is constant for that medium and not varied for the purpose of carrying information in the way described in the option.

Answer:

A. They can travel through either air or other types of matter, D. They require no wires to carry information.