Question

True conditions for sustained interference of light waves are:
A. Two interfering sources must be coherent.
B. Two interfering waves must be propagated along the same line.
C. Two interfering waves must have equal amplitude.
D. If the interfering waves are polarized, they must be in the same state of polarization.
Choose the correct answer from the options given below:

• A. A, B and D only
• B. A, B and C only
• C. A, B, C and D
• D. B, C and D only

Hint:

The three "Cs" of interference are Coherence, Collinearity (or near-collinearity), and Comparable amplitudes. A fourth condition, related to polarization, is also crucial for light waves.

Answer 1

The Correct Option is C

Step 1: Analyze each condition for sustained interference. - A. Coherence: The sources must be coherent, meaning they must maintain a constant phase relationship. This is the most fundamental condition for interference to be observable. If the phase relationship changes randomly, the interference pattern will wash out. So, A is true. - B. Propagation along the same line: For interference to occur, the waves must overlap in space. While they don't have to be perfectly collinear, they must be propagating in nearly the same direction to produce a stable interference pattern. So, B is considered a necessary condition. - C. Equal amplitude: For maximum contrast between bright and dark fringes (i.e., perfect destructive interference where intensity is zero), the amplitudes of the interfering waves should be equal. While interference can be observed with unequal amplitudes, the condition for ideal, sustained interference often includes equal amplitudes for best visibility. So, C is a condition for ideal interference. - D. Same state of polarization: Two light waves with perpendicular polarization cannot interfere. For interference to occur, the electric field vectors of the waves must have components along the same direction. Therefore, they must be in the same state of polarization. So, D is true.
Step 2: Conclude the set of true conditions. All four statements (A, B, C, and D) represent the ideal conditions required to observe a clear, stable, and high-contrast interference pattern.