New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Optics regained
Consider an ordinary 35 mm slide placed in a projector (figure la). Clearly, one important
plane is that of the slide and another is that of the screen. It is also obvious that (assuming the
projector to be a good one) there will be a very close relationship between the intensity distribution
at the slide and at the screen. But suppose we take any other plane parallel to the slide and
located anywhere between the slide and the screen (figure 1 b). For every position there will be a
different distribution - some may not resemble the slide in any way - others may be blurred
representations of the slide (figure IC). But, if you pause to think about it for a moment, it must
be obvious that the same information (i.e. all the information relating to the picture on that
particular slide) must be present in every one of the intermediate planes. Let us go one stage
further. Suppose we place the slide in the projector but we remove the projector lens (figure 2).
The distribution of light on the screen wil be a more or less uniform patch of light and yet it
must again follow that all the information about the slide must be there,(compare with the planes
between the slide and the lens in figure 1). The patch of light on the screen can properly be
described as a hologram since every point on the screen contains information about every point
on the slide. This is not a very useful kind of hologram and we shall see later what we have to do
to make it into the powerful and exciting device that most people think of in this connection.
Figure 2. Patch of light on the screen at A with the arrangement as for figure la but with the projector lens removed; again, the
information about the slide must be present in some form on the screen.
What have we really established so far in terms of physics? Merely that the first stage in one
particular image-forming process is that light interacts with the object and all the information
needed to form the image is encoded in the light. In the second stage, the lens sorts out all this
information and places it in the appropriate places on the screen to form an image. We may
describe these two processes as encoding and decoding, as scattering and recombination, as
diffraction and focusing, or even (loosely) as Fourier transformation and Inverse Fourier transformation.
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