Question Video: Explaining Why Coherent Light Is Used in the Recording and Playback of Holograms | Nagwa Question Video: Explaining Why Coherent Light Is Used in the Recording and Playback of Holograms | Nagwa

# Question Video: Explaining Why Coherent Light Is Used in the Recording and Playback of Holograms Physics • Third Year of Secondary School

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Which of the following most correctly explains why coherent light is used in the recording and playback of holograms? [A] Incoherent light waves do not have phase differences proportional to their path differences. [B] Incoherent light waves produce destructive interference that prevents an image from being produced. [C] Incoherent light waves have lower intensities than coherent light waves.

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### Video Transcript

Which of the following most correctly explains why coherent light is used in the recording and playback of holograms? (A) Incoherent light waves do not have phase differences proportional to their path differences. (B) Incoherent light waves produce destructive interference that prevents an image from being produced. (C) Incoherent light waves have lower intensities than coherent light waves.

In our question statement, we’re told that it’s coherent light that is used in the recording and playback of holograms. We can recall that waves are coherent if they have the same wavelength and also have a constant phase relationship. Having a constant phase relationship doesn’t necessarily mean that two coherent waves are in phase with one another. For example, these two waves are coherent, but they have a phase difference of 180 degrees or 𝜋 radians. The key is that as these waves move along, that phase difference is constant.

Since these two waves are not in phase, we might expect that they haven’t been emitted from the same source. It’s possible though that these two waves did come from the same point. If that were true, then the phase difference between them would mean that they had traveled different distances to get to these points in space. That is, we could say that the phase difference between these two waves is because of the difference in path length that the two waves had to travel. In fact, if these two waves interfered with one another on this surface that we’ve drawn here, then we can work backwards from the interference pattern to solve for any path length difference between the two waves. It’s this property of being able to link phase differences with path length differences that makes coherent light so useful for forming holograms.

Let’s imagine for a moment though that rather than working with coherent light, we have incoherent light waves. If that’s true, then there’s no longer any connection we can make between phase differences of interfering rays of light and differences in the lengths of the paths that those rays may have followed. Incoherent light waves do not exhibit a constant phase relationship. And so a phase difference in an interference pattern may or may not mean that those waves traveled different distances to get to that interference point. This is the main reason why coherent light, rather than incoherent radiation, is used in the recording and playback of holograms.

Considering our other answer options, option (B) claims that incoherent light waves produce destructive interference patterns. In fact, incoherent waves of light can interfere constructively as well as destructively. They don’t interfere only destructively as claimed in option (B). Answer choice (C) says that incoherent waves of light have lower intensities than coherent light waves. This may be true in a particular case but is not generally true. What determines the intensity of a ray of light is not whether it is incoherent or coherent, but rather the wave amplitude.

For our final answer, we choose option (A). Incoherent light waves do not have phase differences proportional to their path differences. This is why coherent light is used in the recording and playback of holograms.

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