Video Transcript
Which of the following would be
observed if the object beam of a holographic recording was focused by a convex lens
and projected onto a screen? (A) A three-dimensional real image
of the object. (B) An interference pattern. (C) A two-dimensional real image of
the object. (D) A patch of uniform-intensity
light.
Here is an apparatus used for
recording a holographic image of this orange cylinder. Along with the beam splitter, here,
and the two plane mirrors, here and here, there are also lenses used in this
setup. These are concave lenses that
diverge parallel light rays. These lenses have the opposite
effect to convex lenses that converge parallel light rays at a focus.
Let’s look more closely at the beam
of light going from the object being imaged to the holographic plate, called the
object beam. We can see that the rays of this
beam are diverging and do not focus at a point. If a convex lens with the required
focal length was placed in the path of the object beam, the rays in the object beam
would focus at its focal point. The rays on the opposite side of
the focal point would then form a real image if they were incident on a screen. We see then that if an object beam
is focused with a convex lens and projected on a screen, the result would be a real
image. Two answer options refer to a real
image. But one of the options states that
the real image would be three-dimensional and the other that the real image would be
two-dimensional.
Holographic recording produces
three-dimensional images. So it might seem that the image
produced on a screen by focusing an object beam would be three-dimensional. In this case though, that is not
the right answer. The three-dimensional properties of
holographic recorded images depend on interference between the object beam and
another beam of light called the reference beam. The question only refers to an
object beam and does not say that the object beam interacts with a reference
beam. Also, holographic images are not
viewable as they are recorded, only afterward when a playback beam is used to
illuminate the recorded image.
Considering only an object beam
projected onto a screen, what would be produced would be a two-dimensional real
image. So the correct answer is option
(C).
