Problem 81 - Master Resource Book in JEE Main Physics

Question

A wrong argument of the particle nature of cathode rays is that they (a) produce fluorescence (b) travel through vacuum (c) get deflected by electric and magnetic fields (d) cast shadow

Step-by-Step Solution

Answer

Traveling through vacuum is not a unique argument for the particle nature.

Step 1: Understanding cathode rays

Cathode rays are streams of electrons observed in vacuum tubes. They are produced when a high voltage is applied across the electrodes of a vacuum tube. These rays exhibit characteristics associated with both waves and particles.

Step 2: Exploring Particle Properties

To determine if an argument supports the particle nature, we consider how particles behave. Particle-like properties include casting shadows, being deflectable by fields, and being able to produce a physical impact, such as fluorescence.

Step 3: Analyzing Each Option

We evaluate each option: - (a) Cathode rays produce fluorescence by exciting the material they strike, which can be considered a particle-like interaction since it involves energy transfer to specific points. - (b) Traveling through a vacuum does not specifically indicate particle nature, as both particles and waves travel in a vacuum. - (c) Deflection by electric and magnetic fields is a behavior of charged particles, like electrons in cathode rays. - (d) Casting shadows is indicative of particles since particles can block light and create shadows.

Step 4: Identify the Non-Particle Argument

Option (b), traveling through a vacuum, does not uniquely demonstrate the particle nature of cathode rays since wave phenomena can also occur in a vacuum. The other options (a, c, d) indicate particle-like behavior.

Key Concepts

Particle Nature

Cathode rays are streams of electrons that show distinct particle-like characteristics. Understanding these characteristics helps to identify the particle nature of cathode rays. For example, they are capable of casting shadows, which is a trait typically associated with physical objects or particles.
This means they have a definite shape and size that can block light, similar to how a shadow occurs. Furthermore, cathode rays can cause fluorescence, which is the emission of light by a material when it absorbs energy from the particles.
This interaction again highlights their particle nature because it involves a direct energy transfer from the particles to the material. Understanding these properties makes appreciating the particle nature of cathode rays clearer.

Electric and Magnetic Deflection

Cathode rays are also famous for their deflection by electric and magnetic fields. These rays are made up of electrons, which are negatively charged particles.
When they pass through electric or magnetic fields, their path can be altered. This behavior is consistent with how charged particles respond to these types of fields. For instance:

• Electric Fields: The electrons in the cathode rays experience a force when moving through an electric field, causing them to deflect.
• Magnetic Fields: Similarly, they are deflected in a magnetic field, following the right-hand rule where the movement is determined by the charge and direction of the field.

This deflection strength is measured by the degree of change in their path, providing a quantitative confirmation of their particle nature.

Vacuum Tubes

Cathode rays are typically produced within vacuum tubes, an essential setup for observing their behavior. Vacuum tubes are glass tubes from which air has been removed to create a vacuum.
The absence of air molecules in the tube allows the electrons, or cathode rays, to travel without being obstructed or scattered by other particles. This provides a clear path for observation. Here’s how they work:

• High Voltage Application: A high voltage is applied across the electrodes at each end of the tube.
• Electron Emission: This voltage causes electrons to be emitted from the cathode, forming the cathode rays.

Vacuum tubes are crucial so that the electrons can be manipulated and studied without interference.

Fluorescence

Fluorescence occurs when a material emits light after it absorbs energy, typically seen with cathode rays. When these electrons strike certain materials, they transfer energy to the material, causing it to glow.
This glowing is what we refer to as fluorescence. The process provides visual proof of the particle nature because it involves direct, point-to-point energy interactions typical of particle behavior. Understanding fluorescence involves:

• Energy Transfer: It occurs when electrons hit a material, causing atoms in the material to become excited.
• Light Emission: The excited atoms release energy in the form of light as they return to a stable state.

This property is not only interesting but also practically significant, helping in the development of different technologies like cathode ray tube displays.

Wave-Particle Duality

A unique characteristic of matter at a small scale, like cathode rays, is wave-particle duality. This concept states that particles, such as electrons, can exhibit both particle and wave-like properties.
Cathode rays demonstrate this duality primarily through their behavior in different conditions, showing how:

• Wave-like Properties: In certain scenarios, cathode rays can demonstrate wave behavior, such as diffraction patterns that suggest they can move in waves.
• Particle-like Properties: As discussed, features like casting shadows and causing fluorescence highlight their particle nature.

Wave-particle duality is a fundamental principle in quantum mechanics and shows how complex the natural world can be, especially at the microscopic level.