If cathode rays are projected at right angles to a magnetic field, their trajectory is:

Options

A. Ellipse
B. Circle
C. Parabola
D. None of these

Answer

To determine the trajectory of cathode rays when they are projected at right angles to a magnetic field, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cathode Rays**:
- Cathode rays are streams of electrons emitted from a cathode in a vacuum tube. These electrons are negatively charged particles.

2. **Direction of Magnetic Field**:
- In this scenario, we assume that the magnetic field (B) is directed out of the page (or screen). This can be represented by dots (•) indicating the direction of the magnetic field lines.

3. **Direction of Cathode Rays**:
- The cathode rays (electrons) are projected at right angles to the magnetic field. For simplicity, we can assume they are moving along the positive x-axis with a velocity (v).

4. **Applying the Right-Hand Rule**:
- To determine the force acting on the electrons due to the magnetic field, we use the right-hand rule. Since electrons are negatively charged, the force will be in the opposite direction to what the right-hand rule indicates.
- Point your thumb in the direction of the velocity of the electrons (positive x-direction), and your fingers in the direction of the magnetic field (out of the page). The palm will then face the direction of the force acting on a positive charge. For electrons, the force will be in the opposite direction.

5. **Resulting Motion**:
- The force acting on the electrons will be perpendicular to both their velocity and the magnetic field. This results in a centripetal force that causes the electrons to move in a circular path.
- Therefore, the trajectory of the cathode rays will be circular.

6. **Conclusion**:
- The correct answer is that the trajectory of cathode rays projected at right angles to a magnetic field is circular.

### Final Answer:
The trajectory of cathode rays projected at right angles to a magnetic field is circular.