Video Transcript
Which of the following most correctly explains how a holograph is different from a
photograph? (A) A holograph viewed from one position shows the image of an object viewed from
every position around the object. (B) All parts of a holograph are sharply defined, with no blurring. Photographs showing objects at different distances must show some of the objects more
blurred than others. (C) When a holograph is viewed from different positions, the positions from which the
images in the holograph are viewed also change.
This question asks how to define the difference between a hologram, or holograph, and
a photograph. To do this, we are going to go over the main characteristics of holograms and those
of photographs.
Recall that a photograph is a recording of a real image that is present at a
surface. Light rays from an imaged object are focused by a convex lens. A light ray from an imaged object that does not travel to the convex lens cannot
become part of the image produced. And therefore any parts of the surface of the object from which there is no path that
light rays can travel that will reach the lens will not be imaged. This means that a photographic image of an object cannot, for example, include parts
of the surface of the object that face in the opposite direction to the convex lens
producing the image.
A holographically recorded image is produced differently to a photographically
recorded image. We can see that no convex lenses are used and light rays are not focused. A holographic image is formed by the interference of coherent light directly from a
coherent light source and light from the same source that has been reflected from
the object to be imaged. When viewed at a very high resolution, the surfaces of objects that appear smooth to
the human eye are in fact irregular. This results in diffuse reflection of light from the surface rather than specular
reflection.
As no focusing of diffusely reflected light from the surface of an object occurs, the
light rays from a point on the surface of the object may take many different
paths. A convex lens would redirect these rays to a single point. This means that for any point on the holographic plate that records the image of the
object, light from many different points on the surface of the object may be
incident at that point. A point at the center of the holographic plate, for example, may have received light
rays from any points on the surface of the object, including points very far from
the center of the object.
For a holographic image to be viewed, the holographic plate on which the image was
recorded must be illuminated by a reference beam. Light from the points on the holographic plate are then focused by an observer’s eye
to form a real image at the observer’s retina. The various parts of the real image produced are determined by the interference
between the light waves from the holographic plate that are focused at a given part
of the observer’s retina.
If the position of the observer’s eye changes while the holographic image is viewed,
the interference produced by light waves at the new position will be different to
the interference produced by light waves at the previous position. This change will occur for all the parts of the image produced at the observer’s
retina. This means that if the observer changes the position from which they view the
holographic image, they appear to see the imaged object as it would appear if viewed
from the position to which the observer has moved.
The same kind of changes in the image produced would be observed if the direction of
the playback beam was changed and the observer did not change position. This means that when a holograph is viewed from different positions, the positions
from which the images in the holograph are viewed also change. This is what option (C) describes. Option (C) is correct.
