Question

The coherence of two light sources means that the light waves emitted have

• A. same frequency
• B. same intensity
• C. constant phase difference
• D. same velocity

Hint:

Coherence is essential for observable interference patterns.

Answer 1

The Correct Option is C

The concept of coherence in physics, particularly in optics, refers to a property that light waves should have to exhibit interference effects. Coherence can be temporal and spatial. Here, we focus on temporal coherence, which is relevant to the question.

• Temporal Coherence: This is the measure of the correlation between the phases of a light wave at different points along the direction of propagation.
• Constant Phase Difference: Two light sources are considered coherent if they maintain a constant phase difference over time. This is crucial for the superposition principle to apply effectively, allowing for the constructive and destructive interference patterns — such as those seen in the double-slit experiment.

Now, considering the given options:

• Same frequency: While having the same frequency is an essential condition for waves to maintain coherence, it is not sufficient on its own for coherence.
• Same intensity: The intensity of the light sources does not determine coherence. Two beams can have different intensities and still interfere if their phase relationship is constant.
• Constant phase difference: This is the defining condition of coherence. It is necessary for the consistent phase relationship that allows interference to occur.
• Same velocity: Light waves travel at a uniform speed in a vacuum. However, this aspect does not directly lead to coherence between multiple sources.

Based on this explanation, the correct answer is constant phase difference.

Therefore, the coherence of two light sources means that the light waves emitted have a constant phase difference.