What are the main characteristics of laser light that make it different from ordinary light?

In one line

Laser light is highly monochromatic, coherent, directional and intense, unlike light from a bulb, which is mixed in wavelength, random in phase and spreads in all directions.

Explained simply

An ordinary bulb is like a crowd chatting in a hall: many voices, different pitches, everyone facing different ways. A laser is like a trained choir singing one note together, facing one direction. That is why a 1 mW laser pointer spot looks far brighter than a 60 W bulb at the same distance.

How it works

1. Monochromaticity: the cavity allows only a very narrow band of frequencies, so \(\Delta\lambda\) is tiny compared with ordinary sources.
2. Coherence: stimulated photons have the same phase, giving long coherence length (temporal coherence) and a fixed phase across the beam (spatial coherence).
3. Directionality: only axial photons are amplified, so divergence is very small. Divergence angle \(\theta = \dfrac{r_2 - r_1}{d_2 - d_1}\), where \(r_1, r_2\) are beam radii at distances \(d_1, d_2\).
4. High intensity: all energy is concentrated in a narrow beam of small cross-section, so power per unit area is very high.

Key formula

Divergence: \(\theta = \dfrac{r_2 - r_1}{d_2 - d_1}\) (radians) with radii and distances in metres. Intensity: \(I = P/A\) in W/m², \(P\) the power, \(A\) the beam area.

Quick example

A laser beam has diameter 2 mm at 1 m and 4 mm at 2 m. Radii are 1 mm and 2 mm, so \(\theta = (2-1)\times10^{-3}/(2-1) = 10^{-3}\) rad = 1 milliradian. A 1 mW beam of radius 1 mm has \(I = 10^{-3}/(\pi\times10^{-6}) \approx 318\) W/m², while a 60 W bulb spreading equally in all directions gives about 4.8 W/m² at 1 m even if all its power were light.

Common mistakes

• Using diameters instead of radii in the divergence formula (or mixing them).
• Saying laser light has exactly one wavelength. It has a very narrow, not zero, spread.
• Listing "high power" instead of high intensity; many lasers have low power but high intensity.

How MU asks it

Asked as "State and explain characteristics of laser" (5 marks) or "Compare laser light with ordinary light" (3 marks). For full marks explain all four with one line of reason each, give the divergence formula, and add a comparison table.