In atomic physics experiments with discharge tubes, cathode rays produced between electrodes are best described as which of the following?

Introduction / Context:
This question comes from the early experiments on electrical discharge through gases, where scientists discovered cathode rays in evacuated tubes. Understanding what cathode rays really are was a key step toward discovering the electron and developing modern atomic models. Here you must recall the nature of cathode rays and choose the correct description from the options given.

Given Data / Assumptions:

• Cathode rays are produced in a discharge tube under low pressure and high voltage.
• They travel from the cathode toward the anode.
• They cause fluorescence on certain materials and can be deflected by electric and magnetic fields.
• The options available describe possible physical interpretations: electromagnetic waves, alpha particles, electrons, or unspecified harmful radiation.

Concept / Approach:
Experiments by J. J. Thomson and others showed that cathode rays have several important properties. They are deflected by electric and magnetic fields, they travel in straight lines, they can cause mechanical motion of small paddles placed in their path and they have negative charge. Based on measurement of the charge to mass ratio, these rays were identified as streams of negatively charged particles with much smaller mass than atoms. These particles were later named electrons. Alpha particles are positively charged and much heavier, and electromagnetic waves such as light are not deflected by electric fields in the same way. Therefore cathode rays must be streams of electrons.

Step-by-Step Solution:
Step 1: Recall that cathode rays originate from the cathode in a discharge tube and move toward the anode. Step 2: Experimental evidence shows that cathode rays are affected by electric and magnetic fields, indicating that they carry charge. Step 3: The direction of deflection demonstrates that they are negatively charged. Step 4: Thomson measured the charge to mass ratio of the particles making up cathode rays and found it very large, meaning the particles have very small mass. Step 5: These particles were recognised as fundamental constituents of atoms and later called electrons. Step 6: Therefore cathode rays are correctly described as a stream of electrons.

Verification / Alternative check:
A simple way to check this understanding is to compare cathode rays with other known radiations. Alpha particles are helium nuclei that are positively charged and several thousand times more massive than electrons. They are produced in radioactive decay, not in discharge tubes, and are deflected in the opposite direction compared to electrons. Electromagnetic waves like light or X rays do not consist of charged particles and are not deflected by electric fields in the same way. Since cathode rays respond to electric and magnetic fields exactly as a beam of negative charges would, describing them as a stream of electrons is consistent with all experimental observations.

Why Other Options Are Wrong:
Electromagnetic waves such as visible light or X rays do not carry net electric charge and therefore are not deflected by uniform electric fields in the way cathode rays are. A stream of alpha particles would be composed of positively charged helium nuclei, which would deflect in the opposite direction from cathode rays in the same field and have a much larger mass. The phrase harmful radiations of unknown type ignores the well established identification of cathode rays as electrons and is historically outdated. Hence only the description as a stream of electrons fits the observed behaviour.

Common Pitfalls:
Students sometimes confuse cathode rays with X rays because both can be produced in similar apparatus. However, X rays are generated when high energy electrons strike a metal target and are electromagnetic in nature, whereas cathode rays themselves are the electrons. Another source of confusion is between different kinds of radiation (alpha, beta and gamma rays) emitted by radioactive substances. Beta rays are streams of electrons or positrons and are more closely related to cathode rays. To avoid confusion, remember that cathode rays and beta minus rays are both streams of electrons, while alpha rays are helium nuclei and gamma rays are high energy photons.

Final Answer:
Cathode rays in discharge tubes are best described as a stream of electrons.