Question

What is the meaning of coherent sources of light?
• Sources that emit light with high intensity
• Sources that emit light at the same color only
• Sources that maintain a constant phase difference between them
• Sources that emit light continuously without interruption

• A. Sources that emit light with high intensity
• B. Sources that emit light at the same color only
• C. Sources that maintain a constant phase difference between them
• D. Sources that emit light continuously without interruption

Hint:

Coherent sources must:
• Have same frequency
• Maintain constant phase difference This condition is necessary for sustained interference.

Answer 1

The Correct Option is C

Concept: Coherent sources are extremely important in interference phenomena such as:
• Young's double slit experiment
• Interference of light
• Holography For sustained interference fringes to be observed, the two light waves must maintain a fixed relationship in phase.

Step 1: Understand the definition of coherent sources. Two light sources are said to be coherent if:
• They emit waves having the same frequency (or wavelength)
• They maintain a constant phase difference with time Thus the defining condition is: \[ \boxed{ \text{Constant phase difference} } \] This ensures stable constructive and destructive interference.

Step 2: Analyze Option 1 carefully. Option 1 says: “Sources that emit light with high intensity” Intensity has no direct relation with coherence. A source may:
• Be highly intense but incoherent
• Be low intensity but coherent Therefore Option 1 is incorrect.

Step 3: Analyze Option 2 carefully. Option 2 says: “Sources that emit light at the same color only” Same color means same wavelength. Although coherent sources generally have same frequency, this alone is not sufficient. The crucial requirement is: \[ \boxed{ \text{Constant phase difference} } \] Hence Option 2 is incomplete and incorrect.

Step 4: Analyze Option 3 carefully. Option 3 says: “Sources that maintain a constant phase difference between them” This is exactly the correct definition of coherent sources. Hence: \[ \boxed{ \text{Option 3 is correct} } \]

Step 5: Analyze Option 4 carefully. Option 4 says: “Sources that emit light continuously without interruption” Continuous emission does not guarantee coherence. Ordinary bulbs emit continuously but are not coherent sources. Thus Option 4 is incorrect.

Step 6: Choose the correct answer. Therefore the correct answer is: \[ \boxed{ \text{Sources that maintain a constant phase difference between them} } \] Hence the correct option is: \[ \boxed{(3)} \] Additional Understanding: Interference fringes remain stable only when:
• Phase difference remains constant
• Frequencies are same If phase changes randomly:
• Fringes disappear
• Stable interference cannot occur Final Conclusion: Coherent sources are: \[ \boxed{ \text{Sources maintaining constant phase difference} } \] Hence, the correct answer is: \[ \boxed{(3)} \]