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 shown 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

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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 shown 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 half of our diagram, we see laser light being split into two different beams where one of these beams interacts with our object of interest and the other provides a reference for that object beam. When these two beams interfere at our holographic plate, a hologram of our cylindrical object is formed. In order to actually see this hologram though, the holographic plate where the hologram is recorded needs to be exposed to the same light that was used to record the hologram in the first place. When that happens, an image of the hologram can become visible.

In the bottom half of our diagram, there are two candidate virtual images: virtual image A here and virtual image B here. We want to know which one, or perhaps both, or perhaps neither will this observing eye see. If we look back at how our hologram was recorded, we see that the holographic plate is located here, and relative to that our object is here. We might think then that since virtual image A is located in roughly the same position as our original object relative to our holographic plate, that this would be the image that would form and be observed by the eye. A key here, though, is that the observing eye must look on a line of sight through the holographic plate in order to see any image that’s formed. We can trace this line of sight as far as we like, and we see that no image is formed along it.

Because of the position of the observing eye relative to the holographic plate, the eye is unable to see either virtual image A or virtual image B. A line of sight from the eye through the holographic plate passes through neither one of those images. So, for our answer, we choose option (E). Of the two virtual images shown here, the observing eye sees neither virtual image because neither one is along the eye’s line of sight.

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