Which of the following are not electromagnetic waves?
(a) cosmic rays
(b) gamma-rays
(c) beta-rays
(

Q: Are cosmic rays really electromagnetic waves? I read somewhere that they are protons.

You are absolutely right! Modern physics classifies primary cosmic rays mostly as high-energy protons and atomic nuclei. However, historically, the term was used broadly for radiation from space, which includes high-energy gamma photons (secondary cosmic rays). In the context of older or basic multiple-choice questions, they are often grouped with EM waves to contrast with strictly nuclear emissions like alpha and beta particles.

Q: Why are they all called 'rays' if they aren't the same thing?

It's purely historical! When scientists like Roentgen, Becquerel, and Rutherford first discovered these emissions, they didn't know their fundamental nature. They just saw energy traveling in straight lines, so they called them 'rays'. The names stuck even after we figured out that some are waves (gamma, X-rays) and some are particles (alpha, beta).

Q: Do beta rays travel at the speed of light?

No. Because beta rays are electrons, they have mass. According to relativity, no object with mass can reach the exact speed of light in a vacuum ($c$). They can travel very fast (up to 99% of $c$), but never exactly $c$.

Q: Can electromagnetic waves be deflected by a magnetic field?

No, electromagnetic waves (like gamma rays and X-rays) have no electric charge, so they pass straight through electric and magnetic fields without being deflected. Beta rays, being negatively charged electrons, are strongly deflected.

Q: What is the difference between X-rays and gamma rays if they are both EM waves?

The main difference is their origin. X-rays are typically produced by high-energy electron transitions outside the nucleus (in the electron cloud), while gamma rays are emitted from the nucleus itself during radioactive decay. Gamma rays also generally have higher frequencies than X-rays.

Question

Which of the following are not electromagnetic waves?

Options:

(A) cosmic rays

(B) $\gamma$-rays

(C) $\beta$-rays

(D) X-rays

Sigma Percentile · Accepted Answer

### The Mystery of the "Rays"

When scientists first began discovering various forms of radiation in the late 19th and early 20th centuries, they didn't immediately know what they were looking at. Whenever they detected a mysterious emission that traveled in a straight line and carried energy, they simply called it a "ray." This is why we have X-rays, gamma rays, alpha rays, beta rays, and cosmic rays.

However, as physics advanced, we realized that these "rays" are not all made of the same stuff. They fall into two completely different categories: **Electromagnetic Waves** and **Particulate Radiation**.

### What Makes an Electromagnetic Wave?

An electromagnetic wave is a ripple in the electric and magnetic fields of the universe. These waves carry energy, but they have absolutely **zero rest mass** and **zero electric charge**. They always travel at the speed of light ($c$) in a vacuum.

In our given options, **X-rays** and **$\gamma$-rays** are classic examples of high-energy electromagnetic waves. They are just like the visible light we see, but with much shorter wavelengths and much higher frequencies.

**Cosmic rays** are a slightly tricky historical term. Today, we know that *primary* cosmic rays traveling through space are mostly high-energy protons and atomic nuclei (which are particles). However, when they hit Earth's atmosphere, they create a shower of *secondary* cosmic rays, which includes a massive amount of high-energy gamma photons. In the context of classic physics questions like this one, cosmic rays are often grouped with the electromagnetic spectrum due to this high-energy photon component.

### The Odd One Out: $\beta$-rays

So, what about **$\beta$-rays**?

During the radioactive decay of unstable atomic nuclei, the nucleus can spit out high-speed particles to reach a stable state. When a nucleus undergoes beta decay, it ejects a high-speed electron (or its antimatter twin, a positron).

Because a $\beta$-ray is literally just a fast-moving electron, it has a definite **mass** ($9.1 	imes 10^{-31} 	ext{ kg}$) and a definite **electric charge** ($-1.6 	imes 10^{-19} 	ext{ C}$).

Electromagnetic waves cannot have mass or charge. Therefore, $\beta$-rays are fundamentally different from X-rays and gamma rays. They are matter particles, making them the correct answer to our question.

Animated Solution for Physics - Electromagnetic Waves: Which of the following are not electromagnetic waves?

Select Answer:

Visualized Solution

Analyzing the Options

Identifying Electromagnetic Waves

The Nature of $β$ -rays

Conclusion

The Sigma Insight: Characteristics of Electromagnetic Waves

The Mystery of the "Rays"

What Makes an Electromagnetic Wave?

The Odd One Out: $β$ -rays

Similar Questions

Electromagnetic waves are transverse in nature is evident by

Arrange the following electromagnetic radiations per quantum in the order of increasing energy. A. Blue light B. Yellow light C. X-ray D. Radio wave

Choose the correct option relating wavelengths of different parts of electromagnetic wave spectrum

In free space, the energy of electromagnetic wave in electric field is $U_{E}$ and in magnetic field is $U_{B}$ . Then

For an electromagnetic wave travelling in free space, the relation between average energy densities due to electric ( $U_{e}$ ) and magnetic ( $U_{m}$ ) fields is

During the propagation of electromagnetic waves in a medium,

A plane electromagnetic wave propagating along $y$ -direction can have the following pair of electric field ( $E$ ) and magnetic field ( $B$ ) components.

The electric field of a plane electromagnetic wave propagating along the x-direction in vacuum is $E = E_{0} \hat{j} cos (ω t - k x)$ . The magnetic field $B$ , at the moment $t = 0$ is

An electromagnetic wave in vacuum has the electric and magnetic fields $E$ and $B$ , which are always perpendicular to each other. The direction of polarisation is given by $X$ and that of wave propagation by $k$ . Then,