🔬 Unit 3 – Part A (2-Mark Q&A)

1. Define critical angle.

The critical angle is the angle of incidence in the denser medium for which the refracted ray emerges along the surface (r = 90°). When the angle of incidence exceeds this, total internal reflection occurs.

Formula: sinθ_c = n₂/n₁, where n₁ > n₂.

2. What is interference of light?

Interference is the superposition of two coherent light waves to form alternate bright and dark fringes on a screen. It supports the wave nature of light.

Formula: Fringe width β = λD / 2d.

3. Define refractive index.

The refractive index (n) of a medium is the ratio of the velocity of light in vacuum (c) to the velocity in that medium (v).

Formula: n = c / v = sin i / sin r.

4. Define polarization of light.

Polarization is the phenomenon of restricting the vibration of the electric field vector to a single plane perpendicular to the direction of propagation. It occurs only in transverse waves.

5. Differentiate spontaneous and stimulated emission.

Spontaneous: Atom emits photon randomly, incoherent and uncontrolled. Stimulated: Incident photon triggers emission of a coherent photon with same energy, phase, and direction.

6. What is population inversion?

Population inversion is the condition where the number of atoms in the excited state (N₂) is greater than those in the ground state (N₁), i.e., N₂ > N₁. It is achieved by pumping methods (optical, electrical, or chemical).

7. List three industrial applications of lasers.

• Cutting, drilling, and welding of metals.
• Holography and alignment operations.
• Barcode scanners, CD players, and laser printers.

8. Compare homojunction and heterojunction lasers.

Homojunction: Same semiconductor material; poor confinement and efficiency.

Heterojunction: Two semiconductors of different bandgaps (e.g. GaAs/AlGaAs); better confinement, low threshold, high efficiency.

9. Calculate wavelength for E_g = 3 eV.

Using E = hc/λ, λ = (6.626×10⁻³⁴ × 3×10⁸) / (3×1.602×10⁻¹⁹) = 4.14×10⁻⁷ m = 414 nm (violet region).

10. What is optical pumping?

Optical pumping is the process of using light energy to excite atoms from a lower energy level to a higher level, creating population inversion.

11. Differentiate between laser and ordinary light.

12. Mention active medium and applications of CO₂ laser.

Active medium: Gas mixture of CO₂, N₂, and He. Wavelength: 10.6 µm (Infrared region).

Applications: Cutting, welding, engraving, and surgery.

13. Define fibre optic sensor.

A fibre optic sensor uses optical fibers to measure parameters like temperature, strain, pressure, or rotation by detecting changes in intensity or phase of light.

They are small, sensitive, and immune to electromagnetic interference.