Question

What are coherent sources? Why are they necessary for observing a sustained interference pattern?

Hint:

Laser light is an example of a coherent source, which is why it is commonly used in interference experiments.

Answer 1

Coherent Sources and Their Role in Observing a Sustained Interference Pattern

What Are Coherent Sources?

Coherent sources are two or more light sources that maintain a constant phase relationship with each other over time. In other words, the phase difference between the waves from coherent sources remains fixed.

There are two main conditions for sources to be coherent:

• Same Frequency: The sources must emit waves with the same frequency. If the frequencies are different, the phase difference between the waves keeps changing, making the sources incoherent.
• Constant Phase Difference: The phase difference between the waves from coherent sources should remain fixed or constant over time. This allows the waves to add or cancel out in a predictable manner, creating a stable interference pattern.

Why Are Coherent Sources Necessary for Observing a Sustained Interference Pattern?

Interference is the phenomenon where two or more waves superpose to form a resultant wave. When two coherent sources produce waves, their **superposition** leads to interference. This interference can be:

• Constructive Interference: When the waves are in phase (i.e., their crests align), the amplitude increases.
• Destructive Interference: When the waves are out of phase (i.e., the crest of one wave aligns with the trough of the other), the amplitude decreases or cancels out.

For a **sustained interference pattern**, the phase relationship between the waves must remain stable over time. This is only possible when the sources are coherent. Here's why:

• Stable Phase Difference: If the sources are coherent, the phase difference remains constant, leading to a consistent interference pattern (e.g., alternating bright and dark fringes in a double-slit experiment). Without coherence, the phase difference keeps changing, causing the interference pattern to fade or become unstable.
• Same Frequency: Coherent sources emit waves of the same frequency, which ensures that the constructive and destructive interference occur at regular intervals, creating a stable, repeated pattern over time.

Example of Coherent Sources:

A common example of coherent sources is light from a **laser**. Lasers emit light of a single frequency and maintain a fixed phase relationship over time, making them ideal for producing stable interference patterns.

Conclusion:

Coherent sources are necessary for observing a sustained interference pattern because they maintain a constant phase relationship and emit waves of the same frequency. Without coherence, the interference pattern would not be stable, and the effects of constructive and destructive interference would not persist over time.