# Question Video: Choosing the Hologram Image Seen by an Observer Physics

The diagram shows a laser being used to record a holographic image of a cylindrical object and then to display the image recorded on the holographic plate. Which of the virtual images would be observed by a viewer at the position shown?

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### Video Transcript

The diagram shows a laser being used to record a holographic image of a cylindrical object and then to display the image recorded on the holographic plate. Which of the virtual images would be observed by a viewer at the position shown? (A) Virtual image A. (B) Virtual image B. (C) Both virtual images simultaneously. (D) Each virtual image would alternate. (E) Neither virtual image.

In the top part of our diagram, we see coherent light from a laser being split into two separate beams. One of these beams interacts with the object of interest, our cylindrical object, and the other provides a reference for that object beam. When the two beams interfere with one another at the holographic plate, they record a hologram of our object. Interestingly, to see that hologram, it’s necessary to illuminate the holographic plate with the same laser light that was used to record the hologram.

In this lower part of our diagram, we see two potential virtual images formed: virtual image A and virtual image B. We want to know which of these images or both or neither is observed by this eye positioned here. The key to understanding what this observing eye will see is to know that that eye must be looking on a line of sight through the holographic plate. That is, if any image is visible to the eye, it will lie along this line. This is true regardless of where the eye is located. For example, if our observing eye was, say, up here, then the only image it could see would have to lie along the line that goes from the eye and through the holographic plate.

Looking at where our observing eye is actually located, we see that the line from this eye through the plate passes through virtual image A. Therefore, this will be the image that the observing eye sees. For our answer, we choose option (A).