Question

Which of the following is not an electromagnetic wave?
A. Light Rays
B. X-Rays
C. Alpha Rays
D. Gamma Rays

Answer 1

Hint: Think of the regions of the electromagnetic spectrum and attempt to deduce which one of the above options is not part of the continuum. Think of the ways the rays are produced and what they look like when emitted.

Complete step-by-step answer:
Let us begin by first understanding what electromagnetic waves are.
Electromagnetic or EM waves are the synchronized oscillations of electric and magnetic fields propagating through space. EM waves propagate in a direction that is perpendicular to its electromagnetic oscillations and are hence termed to be transverse waves. Classically, EM waves are emitted from a point source and thus form a spherical wave front. Together, all EM waves form an EM spectrum that encompasses all EM radiation that we know of.
Now that we’ve established an understanding of EM waves, let us look at each of the options presented to us.
From a first glance we can easily deduce that the options fall under the classification of EM radiation based on wavelength. On this basis, the EM spectrum is divided roughly into seven regions (in the order of increasing wavelength or decreasing frequency):

Radiation	Wavelength (m)	Frequency (Hz)
Gamma Rays	10-14 – 10-10	1022 - 1018
X-Rays	1 x 10-10 – 3 x 10-8	1018 - 1017
Ultra-violet (UV) Rays	3 x 10-8 – 4 x 10-7	1017 – 8 x 1014
Visible Light	4 x 10-7 – 8 x 10-7	8 x 1014 – 4 x 1014
Infra-Red (IR) Rays	8 x 10-7 – 3 x 10-5	4 x 1014 - 1013
Microwaves	10-3 – 0.3	1011 - 109
Radiowaves	10 - 104	107 - 104

Now looking at the options we have:
Light rays are basically radiation from the visible region of the EM spectrum.
X-rays are also high energy EM radiations that have great penetration ability.
Alpha rays are energetic streams of alpha “particles” (Helium-4) that consist of two protons and two neutrons bound together produced as a result of radioactive decay of an unstable nucleus.
Gamma rays are the shortest and most energetic EM radiation, also produced by radioactive nuclear disintegration.
From the above discussion we can successfully conclude that alpha rays are not a part of the EM spectrum and hence are no EM waves.
So, the correct answer is “Option C”.

Additional Information: Recall that if EM waves (or even guitar string vibrations for that matter) are transverse waves that propagate in a direction perpendicular to its electromagnetic oscillations then longitudinal waves are those vibrations that are propagating in the same direction as their oscillations. An example of longitudinal waves would be sound waves or the compressions moving along a slinky.

Note: We know that both alpha and beta rays are produced as a result of radioactive nuclear disintegration, so it might be natural to assume that beta rays must consist of particles just like alpha rays since the parent nucleus is decaying into a daughter nucleus in both processes. However, it is crucial to remember that though both processes entail decay reactions, the by-products of the reaction are different, which form the alpha particles and gamma rays respectively.