Young's Double Slit Experiment and He-Ne Laser Principles

Young's Double Slit Experiment (YDSE)

Concept: In 1801, Thomas Young performed an experiment where he passed light through two narrow slits and observed an interference pattern on a screen. This pattern consists of alternating bright and dark fringes.

Experimental Setup

• Monochromatic Light: Light of a single wavelength falls on a primary slit S.
• Slit Arrangement: From slit S, light travels to two secondary slits, S₁ and S₂.
• Diffraction and Interference: Light diffracts from S₁ and S₂ and interferes on the screen.
• Constructive Interference: This occurs where waves meet in phase, resulting in a bright fringe.
• Destructive Interference: This occurs where waves meet out of phase, resulting in a dark fringe.

Fringe Width Derivation (W)

Let:

• D = Distance between the slits and the screen
• d = Distance between slits S₁ and S₂
• λ = Wavelength of light
• y = Distance of the n^th fringe from the center

Path Difference: Δx = d · sin(θ) ≈ d · θ (since θ is small).
For a bright fringe:
→ d · θ = nλ
→ θ = nλ / d

Now,
y = D · θ
→ y = (D · nλ) / d

Fringe Width (W): This is the distance between two consecutive bright fringes:
→ W = y_n+1 – y_n
→ W = Dλ / d

Final Fringe Width Formula

Fringe Width (W) = Dλ / d

He-Ne Laser (Helium-Neon Laser)

Construction of He-Ne Laser

• Tube: A long glass tube (approximately 80 cm) filled with Helium (He) and Neon (Ne) gases at low pressure, typically in a ratio of 10:1.
• Electrodes: Electrodes are placed at both ends of the tube to provide a high voltage supply.
• Mirrors:
  • One end features a 100% reflective mirror.
  • The other end features a partially reflective mirror, which serves as the output for the laser.
• Power Supply: A high voltage (~10kV) DC supply is used to excite the gas atoms.

Working Mechanism

1. Excitation: The high voltage supply causes the Helium (He) atoms to become excited.
2. Energy Transfer: These excited Helium atoms transfer their energy to Neon (Ne) atoms through collisions (due to resonance and matching energy levels).
3. Population Inversion: Neon atoms are raised to an excited state, allowing population inversion to be achieved.
4. Spontaneous and Stimulated Emission: As the excited Neon atoms return to a lower energy state, they emit coherent photons.
5. Amplification: Photons reflect back and forth between the mirrors, triggering further stimulated emission and forming a concentrated laser beam.
6. Output: A red laser light (632.8 nm) is emitted through the partially reflective mirror.