Fluorescence in discharge tubes
The fluorescence observed on the walls of a discharge tube is a result of the interaction between the particles present in the tube and the gas or vapor within it. Here's a detailed explanation of how each option relates to the fluorescence:
A: Cathode rays
- Cathode rays are streams of electrons emitted from the cathode (negative electrode) in the discharge tube.
- When these high-speed electrons collide with gas atoms or molecules, they transfer energy to them.
- This energy excites the electrons in the gas atoms, causing them to move to higher energy levels.
- As the excited electrons return to their original energy levels, they release energy in the form of light.
- This emitted light is what causes the fluorescence on the walls of the discharge tube.
B: Anode rays
- Anode rays are positively charged ions that are accelerated towards the cathode in the discharge tube.
- These ions do not contribute to the fluorescence observed on the walls of the tube.
- They may interact with the gas atoms, but their effect is not responsible for the fluorescence.
C: Canal rays
- Canal rays are positively charged ions that are accelerated towards the anode in the discharge tube.
- Similar to anode rays, canal rays do not result in the fluorescence observed on the walls of the tube.
- Their interaction with the gas atoms does not produce the emitted light.
D: None of the above
- This option is incorrect because fluorescence in a discharge tube is indeed caused by cathode rays.
In conclusion, the correct answer is A: Cathode rays. When these high-speed electrons collide with gas atoms or molecules, they excite the electrons in the gas, leading to the emission of light and the fluorescence observed on the walls of the discharge tube.

The correct answer is option 'A': Cathode rays.

Explanation:
The fluorescence on the walls of a discharge tube is due to cathode rays. Let's understand why this is the case.

1. Discharge Tube:
A discharge tube is a sealed glass tube that contains a gas at low pressure. It consists of two electrodes, namely the cathode (negative electrode) and the anode (positive electrode).

2. Cathode Rays:
Cathode rays are a stream of electrons that are emitted from the cathode when a high voltage is applied across the electrodes in a discharge tube. These electrons are negatively charged particles.

3. Fluorescence:
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. When cathode rays strike the walls of the discharge tube, they transfer their energy to the atoms or molecules of the gas present in the tube.

4. Excitation of Atoms/Molecules:
The energy transferred by the cathode rays excites the atoms or molecules of the gas, causing them to move to higher energy levels. This excitation is temporary, and the atoms or molecules quickly return to their ground state.

5. De-Excitation and Light Emission:
As the excited atoms or molecules return to their ground state, they release the excess energy in the form of photons (light particles). The emitted light may have different colors depending on the gas used in the discharge tube.

6. Fluorescent Coating:
To enhance the visibility of the emitted light, the walls of the discharge tube are often coated with a fluorescent material. This material absorbs the emitted light and re-emits it at a longer wavelength, making it more visible to the human eye.

7. Observation of Fluorescence:
When the discharge tube is energized and the cathode rays strike the walls coated with a fluorescent material, the walls emit visible light. This light can be observed as fluorescence.

In conclusion, the fluorescence on the walls of a discharge tube is due to the excitation of atoms or molecules by cathode rays, followed by their de-excitation and the emission of visible light.

Option b