Which of the following most correctly explains how a holograph is different from a photograph? (A) A holograph shows an image that changes with time, whereas a photograph shows an image that does not change with time. (B) A holograph shows a three-dimensional image, whereas a photograph shows a two-dimensional image. (C) A holograph shows objects at their actual size, whereas a photograph shows the size of the image of the object.
To see which of these answer options is correct, let’s clear some space at the top of our screen. And let’s say that we want to take both a holograph and a photograph of this object here, a tree. When we take a photograph of the tree, say placing our camera here, what we do is collect light from a number of points in space where the camera is pointed. A digital camera will separate out that light over a two-dimensional grid. For example, the camera will keep track of how much light lands in this grid space and how much lands in this grid space and in this one and in this one, and so on. Each one of these grid spaces is called a pixel or picture element. A pixel makes a brighter or darker part of an image depending on how much light lands on it.
Notice that in the picture that is formed, we have information about our object, the tree, in the vertical direction as well as the horizontal direction. However, we don’t have any depth information about the tree. For example, we couldn’t tell from our image that this part of the tree is closer to the camera than the tree trunk.
If, instead of taking a photograph of the tree, we take a holograph, that involves using a coherent source of light, like a laser, splitting the beam of that laser in half to create two different pathways of light, sending one of these pathways to the object of interest, our tree, so that the light can reflect off of it and then be absorbed by a light-sensitive plate here, while that light interferes with light from the other beam path. What we get at this light-sensitive plate then is a mixture of light from these two beams, an interference pattern. At some points, that pattern will include constructive interference, and at others destructive interference.
What we have then is an image that forms on our two-dimensional plate that includes phase difference information between our two beams of light. Those phase differences can be decoded so that now, in addition to knowing vertically and horizontally what our object looks like, we also have a third dimension, a depth dimension, available to us. We see then a primary difference between holographs and photographs. While a photograph gives us two dimensions of information about our object of interest, a holograph gives us three dimensions. This corresponds to answer option (B).
Note that answer option (A) is not correct because a holograph doesn’t show an image that changes with time. Option (C) is not correct because just as a photograph can have its size changed to be larger or smaller, so can a holograph be projected onto different surfaces to appear bigger or smaller than the actual size of the object. The most correct explanation of the difference between a holograph and a photograph is that a holograph shows a three-dimensional image, whereas a photograph shows a two-dimensional image.