What are cathode rays?

Analyze the question You are asked to identify what cathode rays are made of.

Explain the concept Cathode rays are produced in a discharge tube (a glass tube at very low pressure with a high voltage across two electrodes). The negative electrode is the cathode, and the rays appear to come from it, which is how they got their name.

In 1897, J.J. Thomson showed that cathode rays:

• Are deflected by electric and magnetic fields, and the direction of deflection shows they carry negative charge.
• Travel in straight lines and cast shadows.
• Have the same charge-to-mass ratio (e/m) whatever gas or cathode material is used, so they are a universal constituent of matter.
• Can turn a small paddle wheel, so they carry momentum and have mass.

Solution by elimination

• Neutrons (B): These have no charge, so electric and magnetic fields would not deflect them. Cathode rays are deflected, so B is wrong.
• Protons (C): These are positively charged and would bend toward the negative plate. Cathode rays bend toward the positive plate, so C is wrong.
• Alpha particles (D): These are helium nuclei, which are positively charged and heavy. They come from radioactive decay, not from a cathode, so D is wrong.
• Electrons (A): These are negatively charged, emitted from the cathode, and deflected toward the positive plate, which matches the observations.

Answer: Cathode rays are a stream of electrons.

Exam tip: Remember "Cathode rays = Electrons" (the cathode is the negative terminal, so it repels negative electrons). Also, positive rays (canal rays) travel the opposite way and are made of positive ions. WASSCE often tests this contrast.