Dual Nature Of Radiation And Matter

Step 1: Understanding the Concept:
A photo-cell is a sensor or transducer that detects light. Its electrical properties (like current, voltage, or resistance) change when it is exposed to light. The fundamental principle behind its operation is the photoelectric effect, where electrons are emitted from a material (photosensitive surface) when light of a suitable frequency shines on it.
Step 2: Detailed Explanation:
Definition of a Photo-cell:
A photo-cell is an electronic device whose operation is based on the photoelectric effect. It consists of a photosensitive cathode that emits electrons when illuminated, and an anode to collect these electrons. When light falls on the cathode, the emitted photoelectrons are attracted to the anode, completing an electric circuit and allowing a small current (photocurrent) to flow. The magnitude of this current is proportional to the intensity of the incident light.
Two Applications:
1. Automatic Street Lighting:
Photo-cells are used as light sensors in automatic streetlights. During the day, the ambient light is strong, causing the photo-cell to conduct and keep the light circuit open (lights OFF). As evening approaches and the ambient light level drops, the photo-cell's conduction ceases, which triggers a relay to close the light circuit, turning the streetlights ON.
2. Burglar Alarms:
In security systems, a photo-cell is used with a light source (often invisible ultraviolet or infrared light). A beam of light is directed across a doorway or window onto a photo-cell. As long as the light beam is uninterrupted, a current flows, and the alarm is silent. If an intruder breaks the beam, the light no longer reaches the photo-cell, the current stops, and this interruption triggers the alarm.
Other applications include smoke detectors, automatic door openers, and sound reproduction in motion pictures (reading the optical soundtrack on film).
Step 3: Final Answer:
A photocell is a light-to-electricity converter based on the photoelectric effect, used in applications like burglar alarms and automatic streetlights.

Step 1: Understanding the Concept:
LASER stands for Light Amplification by Stimulated Emission of Radiation. The core principle behind a laser is creating a condition where stimulated emission is more probable than absorption or spontaneous emission.
Step 2: Detailed Explanation:
For light amplification to occur, there must be more atoms or molecules in a higher energy (excited) state than in a lower energy (ground or intermediate) state. This non-equilibrium condition is known as population inversion or number inversion.
When population inversion is achieved, a photon passing through the medium is more likely to trigger a stimulated emission (creating an identical photon) than to be absorbed. This leads to a chain reaction and amplification of light.
Let's analyze the other options:
(A) High temperature: Generally leads to thermal equilibrium, where lower energy states are more populated, preventing population inversion.
(B) Semiconductor: While semiconductors are used as the gain medium in diode lasers, they are not a requirement for all types of lasers (e.g., gas lasers, crystal lasers).
(C) High pressure: Not a general requirement. In gas lasers, pressure is a critical parameter but is not always high. High pressure can sometimes hinder laser action due to increased atomic collisions.
Step 3: Final Answer:
Population inversion (or number inversion) is the fundamental and necessary condition for laser action. Therefore, option (D) is correct.

Step 1: Understanding the Concept:
This question deals with the wave-particle duality, specifically the de Broglie wavelength for matter particles (like electrons) and the properties of photons. We need to compare different physical quantities for an electron and a photon that share the same wavelength .
Step 2: Key Formula or Approach:
The key relationship connecting wavelength and momentum is the de Broglie relation, which applies to both matter particles and photons.
where is momentum, is Planck's constant, and is the wavelength.
Step 3: Detailed Explanation:
Let's analyze each quantity for an electron and a photon with the same wavelength .
(A) Velocity: A photon always travels at the speed of light, . An electron has rest mass, so its speed must be less than . Therefore, their velocities are not the same.
(B) Energy: - The energy of a photon is given by . - The kinetic energy of a non-relativistic electron is . Since the formulas are different, their energies will not be the same.
(C) Momentum: - The momentum of a photon is given by . - The de Broglie momentum of an electron is given by . Since both and are the same for the electron and the photon, their momenta must be equal.
(D) Angular momentum: This quantity is not inherently defined just by wavelength and would depend on other conditions (like orbital motion), so it is not guaranteed to be the same.
Step 4: Final Answer:
For the same wavelength , both the electron and the photon will have the same momentum, . Thus, option (C) is correct.