Which physicist discovered the electron and showed that cathode rays are streams of negatively charged particles?

Introduction / Context:
The electron is a fundamental particle carrying negative electric charge. Identifying it as a component of all atoms changed our understanding of matter. This question again focuses on which physicist demonstrated that cathode rays consist of negatively charged particles, thereby discovering the electron. Even though there are many famous names listed, only one is directly associated with this specific discovery.

Given Data / Assumptions:

• The context is cathode ray experiments.
• The particle discovered is the electron.
• We must pick the correct physicist among several Nobel laureates.
• We assume prior exposure to atomic models and early 20th century physics history.

Concept / Approach:
In evacuated tubes, applying a high voltage between electrodes produces a glowing beam called cathode rays. J. J. Thomson measured how these rays were deflected by electric and magnetic fields and concluded they were made of small, negatively charged particles. He calculated their charge-to-mass ratio, showing that they were much lighter than atoms. This work established the electron as a basic constituent of matter. To answer, we must separate Thomson's role from those of Bohr, Einstein, and Rutherford, who made different key contributions to physics.

Step-by-Step Solution:

Verification / Alternative check:
If you remember the sequence of discoveries, it becomes easy to verify your answer. Thomson's discovery of the electron came first, followed by Rutherford's nuclear model and Bohr's quantized orbits. Einstein's work on the photoelectric effect involved electrons but did not discover them. Standard exam keys and textbooks clearly associate the phrase "discovered the electron" with J. J. Thomson alone. This cross-check confirms that option B is the correct choice.

Why Other Options Are Wrong:
Niels Bohr refined the model of the atom but used the electron concept that Thomson had already established. Albert Einstein explained how light can eject electrons from metal surfaces (photoelectric effect), providing evidence for the quantum nature of light but not discovering the electron itself. Ernest Rutherford discovered the atomic nucleus and proposed the nuclear model of the atom, but he built on the earlier understanding that electrons exist. These roles are all crucial but are not the same as discovering the electron.

Common Pitfalls:
Because all the listed scientists are very famous, some students may feel tempted to choose Bohr or Rutherford by mistake, thinking of atomic structure in general. Others might confuse Einstein's work on electrons in the photoelectric effect with the initial discovery of the electron. To avoid these errors, memorize the specific association: "J. J. Thomson – discovery of electron via cathode rays." This clear link helps you quickly answer similar questions even when names are shuffled or presented in different combinations.

Final Answer:
The physicist who discovered the electron and showed that cathode rays are negatively charged particles is J. J. Thomson.