Problem 80 - Master Resource Book in JEE Main Physics

Question

In a cathode ray oscillograph, the focusing of beam on the screen is achieved by (a) convex lenses (b) magnetic field (c) electric potential (d) All of these

Step-by-Step Solution

Answer

The focusing of the beam is achieved by magnetic field and electric potential.

Step 1: Identify Key Components

First, let's understand the components mentioned in the problem. A cathode ray oscillograph is an instrument (similar to an oscilloscope) used to display and analyze the waveform of electronic signals. It uses a cathode ray tube to achieve this. The focus here is specifically on the mechanism used to focus the electron beam onto the screen.

Step 2: Analyze Each Option

We will examine each option: - **(a) Convex lenses**: These are used in optics to focus light, but cathode ray tubes use electron beams, not light. Thus, it's unlikely they're used for focusing in this context. - **(b) Magnetic field**: Magnetic fields can deflect and focus charged particles, such as electrons in a cathode ray tube. - **(c) Electric potential**: Electric fields can also be used to deflect and focus electron beams in a cathode ray tube. - **(d) All of these**: This implies using a combination of all the above, but considering the nature of electron beams, only magnetic and electric fields are typically used.

Step 3: Evaluate Feasibility and Common Practices

In practice, focusing an electron beam typically employs electric and magnetic fields rather than optical components like convex lenses. This is because electric and magnetic fields directly interact with the charged particles (electrons) in the beam, allowing precise control over their path and focus.

Step 4: Determine the Correct Answer

Based on the analysis, the focusing of the electron beam in a cathode ray oscillograph is commonly achieved using magnetic fields and electric potentials. Convex lenses are not employed for focusing electron beams as they work with light rather than charged particles.

Key Concepts

Electron Beam Focusing

In a cathode ray oscillograph, to focus an electron beam requires a specific method suited to handle charged particles. Unlike traditional optics that use convex lenses to focus light, electron beams in a CRT (Cathode Ray Tube) benefit from different principles. Magnetic and electric fields are exemplary in focusing because they interact directly with electrons, which are charged particles.

• Magnetic fields can apply a force perpendicular to the velocity of electrons, affecting their trajectory and allowing for beam convergence or divergence, much like focusing.
• Electric fields can adjust electron velocity and path directly by accelerating or decelerating the electrons.

Combining these fields provides a dual mechanism for effectively focusing the beam. This precise control ensures sharp and stable images on the CRT screen.

Magnetic Field in Electronics

The role of magnetic fields in electronics is crucial, especially in devices like cathode ray tubes. Magnetic fields influence electrons significantly due to the Lorentz force, which acts on moving charges. Electrons traveling through a magnetic field experience deflection, which can be used to steer the beam in specific directions. More importantly:

• Magnetic fields are responsible for focusing the electron beam by adjusting its spread. This involves using magnetic coils placed around the CRT tube, known as deflection coils.
• The strength and orientation of the magnetic fields can be varied to bring the electron beam into focus, ensuring that it converges accurately on the desired spot on the screen.

The ability to influence electrons makes magnetic fields indispensable in electron beam focusing in cathode ray oscillographs.

Electric Potential in CRT

Electric potentials are essential in controlling the behavior of electrons in a cathode ray tube. By applying a voltage across the CRT, one can manipulate the speed and direction of electron flow. In a CRT, electric fields serve multiple purposes such as:

• Accelerating electrons: Electric potential differences create an electric field that accelerates electrons toward the screen.
• Focusing the beam: Electric fields act to straighten and focus the trajectory of electrons to ensure they hit a precise area on the screen, enhancing clarity and resolution.

The strategic use of electric potentials ensures that the electron beam maintains its path and focus, working alongside magnetic fields for optimum performance.