# Physics Questions and Answers – Coherent and Incoherent Addition of Waves

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Coherent and Incoherent Addition of Waves”.

1. Which of the following is a form of light whose photons share the same frequency and whose wavelengths are in phase with one another?
a) Coherent sources
b) Incoherent sources
c) Electromagnetic waves
d) Sunlight

Explanation: Coherent light is a form of light whose photons share the same frequency and whose wavelengths are in phase with one another. The phase difference between the waves should be constant in case of coherent sources.

2. Which among the following is an example of coherent sources?
a) Fluorescent tubes
b) LED light
c) LASER
d) Tungsten filament lamps

Explanation: LASER is the short form for Laser Amplification by Stimulated Emission Radiation. The amplified light beam coming out of a LASER is essentially due to the emission of electrons stimulated by incident radiation consisting of photons. As a result, this causes the coherent behavior of the LASER beam.

3. Pick the odd one out.
a) LASER
b) LED
c) Sound waves

Explanation: LED is the odd one out. LED is short for light-emitting diode. LED is not a coherent source, whereas, others are examples of coherent sources. The light emitted from an LED is neither spectrally coherent nor even highly monochromatic.
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4. Scattering of waves can be coherent and incoherent.
a) True
b) False

Explanation: Yes, the scattering of waves can be coherent and incoherent. The scattering of a wave is coherent and constructive if the phase delay is the same for all the scattered waves. If it varies randomly, then it is considered to be incoherent.

5. Identify the factor is not the same for coherent waves.
a) Frequency
b) Phase difference constant
c) Amplitude
d) Wavelength in phase with each other

Explanation: Coherent waves are the waves with the same frequency and the wavelength of the waves are in phase as well. Therefore, the phase difference is constant. But the coherent waves do not have the same amplitude. Since the amplitude is different, there will be no complete constructive interference where they meet, so they will contribute poorly to an interference pattern.
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6. Which of the following is the formula for calculating coherence time?
a) Τc = $$\frac {\lambda^3}{(c\Delta \lambda)}$$
b) Τc = $$\frac {\lambda}{(c\Delta \lambda)}$$
c) Τc = $$\frac {\lambda^2}{(c\Delta \lambda)}$$
d) Τc = $$\frac {\lambda^2}{(c\Delta \lambda)}$$

Explanation: The formula for calculating coherence time is given as:
Τc = $$\frac {\lambda^2}{(c\Delta \lambda)}$$
Where Τc is the coherence time, λ is the wavelength, ∆λ is the spectral width of the source, and c is the speed of light in a vacuum (i.e. 3 × 108 m/s).

7. When is the wave interference strong?
a) When the paths taken by all of the interfering waves are greater than the coherence length
b) When the paths taken by all of the interfering waves are lesser than the coherence length
c) When the paths taken by all of the interfering waves are equal than the coherence length
d) When the paths taken by all of the interfering waves are independent of the coherence length

Explanation: Coherence length is defined as the propagation distance over which a coherent wave maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves are lesser than the coherence length.

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