What are Coherent Sources of Light?

Two sources of light are said to be coherent if they emit light waves of:

1. The same frequency (same color/wavelength).
2. The same amplitude (ideally).
3. And most importantly, they must have a Constant Phase Difference (meaning the waves are perfectly in step with each other, and if one wave lags behind the other, it lags by the exact same amount forever).

A common exam question is: "Can two identical 60W light bulbs act as coherent sources?"

No, they cannot. Light is emitted by millions of individual atoms inside a bulb. These atoms emit light in rapid, random bursts lasting less than a billionth of a second. Because the emission from the two bulbs is completely uncoordinated, the phase difference between their light waves changes millions of times per second.

Instead of a clear pattern of bright and dark bands, our eyes just see a general, blurry brightness.

Since two separate light bulbs won't work, physicists use a clever trick: They split a single light wave into two.

1. Young's Double Slit Experiment (YDSE): Thomas Young took a single source of light and passed it through a board with two tiny slits. The light emerging from the two slits acts as two new sources. Because they originally came from the exact same wave, they are perfectly coherent and create a beautiful interference pattern.
2. Lloyd's Mirror / Fresnel's Biprism: These devices use reflections and refractions to split a single light source into two virtual coherent sources.

Today, we don't always need slits and mirrors. LASERs (Light Amplification by Stimulated Emission of Radiation) are artificial devices designed to force atoms to emit light completely in sync. A laser beam is highly monochromatic (one color) and highly coherent, making it perfect for holography and optical experiments.