Coherence Condition by PHYWE

Light waves that overlap can only interfere if they are also coherent. The requirement for coherence can be represented as: d⋅sin (ε) ≪ λ or d⋅D/a ≪ λ. Thus, coherence depends on the spatial dimension (experiment width d) and the angle of reflection used. Given the small dimensions of the diffracting object (triple slit) compared to the source-light-to-diffracting-object distance a, it's true that sin(ε)/2 = tan(ε)/2 = ε/2 = (D/2)/a. This allows students to note the coherence requirement as given. To verify this formula, measure d, D, and a. The first experiment aims to verify the coherence requirement for red light. In the second experiment, students must check that, with constant light source magnitude, longer-wavelength light is more likely to meet the coherence requirement than shorter-wavelength light.

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