Question:
Assertion (A) : Light added to light can produce darkness.
Reason (R) : When two coherent light waves interfere, there is darkness at position of destructive interference.
Assertion (A) : Light added to light can produce darkness.
Reason (R) : When two coherent light waves interfere, there is darkness at position of destructive interference.
Reason (R) : When two coherent light waves interfere, there is darkness at position of destructive interference.
Show Hint
"Light + Light = Darkness" is the quintessential trademark signature exclusively of the wave nature of light, impossible to explain using classical Newtonian particle models.
Updated On: Sep 14, 2026
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false and Reason (R) is also false.
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The Correct Option is A
Solution and Explanation
Concept:
• The principle of linear superposition states that when multiple distinct waves simultaneously overlap in physical space, the resultant total displacement is the precise vector sum of their individual independent displacements.
• If two coherent light waves interact and meet exactly out of phase (specifically, a peak flawlessly meets a trough), their opposing wave amplitudes aggressively cancel each other out entirely.
• This highly specific phenomenon is officially termed destructive interference. The localized result of total destructive interference of visible light is visually complete darkness, representing zero net energy at that precise spatial spot.
Step 1: Evaluate the Assertion (A)
The assertion boldly claims that "Light added to light can produce darkness."
In classical ray optics (particle theory), adding more light always increases brightness.
However, in wave optics, the phenomenon of interference proves that two light waves can indeed completely cancel each other out if they overlap with a phase difference of precisely $\pi$, $3\pi$, $5\pi$, etc.
This cancellation genuinely produces observable dark fringes, such as those famously seen in Young's Double Slit Experiment.
Therefore, the Assertion (A) is absolutely and factually true.
Step 2: Evaluate the Reason (R)
The reason clearly states that "When two coherent light waves interfere, there is darkness at position of destructive interference."
This sentence is the textbook, rigorous physical definition of how and why dark fringes aggressively form in any coherent optical interference pattern.
Therefore, the Reason (R) is also undeniably true.
Step 3: Determine if the Reason correctly explains the Assertion
The assertion is a counter-intuitive phenomenon (light + light = dark). The reason explicitly provides the exact scientific mechanism (destructive interference of coherent waves) that physically makes this strange phenomenon possible.
Because the reason actively and flawlessly explains exactly "why" the assertion is true, Reason (R) firmly constitutes the correct and complete explanation of Assertion (A).
Step 4: Conclusion
Since both statements are perfectly true and the reason provides the necessary correct physical explanation, the correct choice is definitely option (A).
• The principle of linear superposition states that when multiple distinct waves simultaneously overlap in physical space, the resultant total displacement is the precise vector sum of their individual independent displacements.
• If two coherent light waves interact and meet exactly out of phase (specifically, a peak flawlessly meets a trough), their opposing wave amplitudes aggressively cancel each other out entirely.
• This highly specific phenomenon is officially termed destructive interference. The localized result of total destructive interference of visible light is visually complete darkness, representing zero net energy at that precise spatial spot.
Step 1: Evaluate the Assertion (A)
The assertion boldly claims that "Light added to light can produce darkness."
In classical ray optics (particle theory), adding more light always increases brightness.
However, in wave optics, the phenomenon of interference proves that two light waves can indeed completely cancel each other out if they overlap with a phase difference of precisely $\pi$, $3\pi$, $5\pi$, etc.
This cancellation genuinely produces observable dark fringes, such as those famously seen in Young's Double Slit Experiment.
Therefore, the Assertion (A) is absolutely and factually true.
Step 2: Evaluate the Reason (R)
The reason clearly states that "When two coherent light waves interfere, there is darkness at position of destructive interference."
This sentence is the textbook, rigorous physical definition of how and why dark fringes aggressively form in any coherent optical interference pattern.
Therefore, the Reason (R) is also undeniably true.
Step 3: Determine if the Reason correctly explains the Assertion
The assertion is a counter-intuitive phenomenon (light + light = dark). The reason explicitly provides the exact scientific mechanism (destructive interference of coherent waves) that physically makes this strange phenomenon possible.
Because the reason actively and flawlessly explains exactly "why" the assertion is true, Reason (R) firmly constitutes the correct and complete explanation of Assertion (A).
Step 4: Conclusion
Since both statements are perfectly true and the reason provides the necessary correct physical explanation, the correct choice is definitely option (A).
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