Under what conditions are cathode rays produced in a discharge tube?

Analysis of the Question

This question tests your understanding of the gas discharge phenomenon. To produce cathode rays (which are essentially a stream of electrons), you need to facilitate the ionization of gas molecules within a discharge tube.

Concept Explanation

A discharge tube contains a gas at a specific pressure with two electrodes (anode and cathode) connected to a high-voltage source.

1. Why Low Pressure? At atmospheric pressure, air is a very good insulator. By reducing the pressure (using a vacuum pump), the distance between gas molecules increases. This allows electrons to gain enough kinetic energy to collide with gas molecules and cause ionization without being stopped by frequent collisions.
2. Why High Voltage? A very high potential difference is required to provide the strong electric field necessary to accelerate the electrons and strip them from the cathode (or from the gas atoms) to sustain the flow of current.

If the pressure is too high, the electrons collide too often and lose energy. If the voltage is too low, the electrons don't have enough energy to ionize the gas or be emitted from the cathode.

Step-by-Step Solution

• Step 1: Identify the requirements for gas discharge. We need to create a path for electrons to flow.
• Step 2: Evaluate the pressure requirement. To allow electrons to travel freely without constant collisions with gas particles, the pressure must be significantly reduced (low pressure).
• Step 3: Evaluate the voltage requirement. To overcome the resistance of the gas and pull electrons from the cathode, a very strong electric field is needed (high voltage).
• Step 4: Combine these conditions. The production of cathode rays occurs specifically at low pressure and high voltage.

Conclusion

Comparing our findings with the given options, we see that Option D matches the physical requirements for the production of cathode rays.

Exam Tip

Think of a discharge tube like a crowded hallway (high pressure). If you want to run (electrons moving), you need the hallway to be empty (low pressure) and you need a strong push (high voltage) to get moving fast!