Cathode rays cause an object placed behind a perforated anode to cast a shadow on the screen. This observation shows that the rays

Analysis of the Question

The question asks us to identify the physical property of cathode rays based on the observation that they cast a shadow of an object placed in their path.

Concept Explanation

Cathode rays are streams of electrons emitted from the cathode in a vacuum tube. To understand why they cast a shadow, we look at the principles of wave and particle motion:

• Rectilinear Propagation: Light and certain particle beams travel in straight lines. If an opaque object is placed in the path of such a beam, the rays are blocked by the object, preventing them from reaching the screen behind it.
• Shadow Formation: A shadow is formed only when a beam of radiation is obstructed by an object. If the rays were able to bend or curve around the object, a clear shadow would not be cast. Therefore, the presence of a sharp shadow is a classic experimental proof that the rays travel in a straight line.

Step-by-Step Solution

1. Examine the setup: We have a perforated anode and an object placed in the path of the rays.
2. Observe the effect: The rays hit the object and are blocked. Because the rays cannot "bend" around the object, the area behind the object remains dark (a shadow).
3. Relate to properties: • If they were simply charged, they would be deflected by electric or magnetic fields, but that doesn't explain the shadow. • If they had mass, that would be proven by their ability to rotate a paddle wheel, not by casting a shadow. • The formation of a shadow is the direct consequence of the rays moving in a straight-line path.

Conclusion

The observation that cathode rays cast a shadow confirms that they travel in straight lines. While cathode rays are indeed negatively charged (electrons) and possess mass, the specific observation of a "shadow" is the experimental evidence for their rectilinear (straight-line) propagation.

Exam Tip

When you see questions about cathode ray properties, remember these three key experiments:

1. Shadow formation: Proves they travel in straight lines.
2. Paddle wheel rotation: Proves they possess mass/momentum.
3. Deflection by electric/magnetic fields: Proves they are negatively charged.