Which of the following types of waves is a part of the electromagnetic spectrum? (A) S-waves, (B) ultrasonic waves, (C) longitudinal waves, (D) gamma rays.
Let’s start by recalling that the electromagnetic spectrum is a continuous range of radiation that we categorize based on properties such as energy and wavelength. And this radiation is made up of oscillations in the electric and magnetic fields, which as a whole we call electromagnetic waves.
Now, let’s look at answer choice (A) S-waves. These are secondary seismic waves which, like electromagnetic waves, do carry energy. But S-waves travel by means of the Earth’s crust, not the electric and magnetic fields. These are two entirely different types of waves. So answer choice (A) is incorrect.
Next, (B) says ultrasonic waves. Sonic means sound, and ultrasonic just refers to sounds with frequencies beyond the human threshold of hearing. Recall that sound waves need a physical medium, such as air, to travel through. This is in contrast to electromagnetic waves, which can travel through a vacuum, like empty space. So answer choice (B) is incorrect as well.
Now, it turns out that sound waves are longitudinal waves, which means they oscillate parallel to the wave’s direction of propagation. For example, if a longitudinal wave, such as a sound wave, is traveling to the right, the wave is wiggling the air left and right. But looking at our diagram, we can tell this is not the case for electromagnetic waves. If this wave here is traveling to the right, the electric and magnetic fields are wiggling up and down and forward and back, but not left and right. This means they’re transverse waves, not longitudinal. So answer choice (C) is incorrect. Besides, all waves in the electromagnetic spectrum are transverse. So the direction of oscillation is not unique to any one specific part of the spectrum.
Now, answer choice (D) says gamma rays, which is a category of electromagnetic radiation characterized as having the highest range of energy and frequency. So gamma rays refers to any radiation in this part of the electromagnetic spectrum. Therefore, (D) is correct in that gamma rays are a type of wave that makes up a part of the electromagnetic spectrum.