Laser

Laser beams exhibit three distinct and defining properties:

1. Monochromaticity: Laser emits light of a single frequency or wavelength.

$$\lambda = \frac{c}{f}$$

2. Coherence: Waves are in constant phase relationship (both spatial and temporal coherence).
3. Collimation: Laser beam travels in a narrow, nearly parallel path with minimal divergence angle $\theta$:

$$\theta \approx \frac{\lambda}{\pi w_0}$$

where $w_0$ is the beam waist radius.

Lasers are classified based on the gain medium used for amplification:

1. Solid-State Lasers: Use doped crystal or glass (e.g. Nd:YAG). Doping introduces ions like Neodymium ($\text{Nd}^{3+}$).
2. Gas Lasers: Use gas mixtures (e.g., He-Ne, CO₂). Operate via electrical excitation of gas:

$$\text{Electrical energy} \rightarrow \text{excited gas atoms} \rightarrow \text{stimulated emission}$$

Example: He-Ne laser emitting at $\lambda = 632.8 \, \text{nm}$.

3. Liquid Lasers: Use dye solutions (e.g., Rhodamine 6G). They are tunable and emit in the UV-visible range.
4. Semiconductor Lasers: Use p-n junctions as the gain medium. When forward biased, they emit coherent light:

$$e^- + h^+ \rightarrow \text{Photon} \quad (E = hf)$$

Demonstrating Collimation

Use a laser pointer and move a screen away from the source. Observe minimal beam spread over distance to verify collimation.

Demonstrating Monochromaticity

Shine a red laser through various colored filters. The red laser passes through the red filter but is blocked or attenuated by others, demonstrating single-wavelength nature.

1. Industrial: Precision cutting, welding, drilling of hard materials with minimal thermal distortion.
2. Medical: Laser surgery (e.g., LASIK for eyes), tumor removal, and lithotripsy for kidney/gall stones.
3. Holography: Creation of 3D images by recording phase and amplitude of light:

$$\text{Interference pattern} \Rightarrow \text{3D image reconstruction}$$

4. Military: Targeting, range finding, missile guidance, and directed energy weapons.
5. Printing: Laser printers use focused beams to pattern photoconductive drums.
6. Data Storage: CD/DVDs use focused laser beams to read digital pits and lands representing binary data.
7. Retail: Barcode scanning via helium–neon lasers reflecting off printed barcodes.