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Magnification and Eye
Name: Jacob
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
Sir/Madam, When a lens "magnifies" a distant object, why does
the object appear to be closer? Similarly, when viewing microscopic objects
through a microscope, why do they become visible even though their size in
reality lies below the perception threshold of the human eye? Are both of
the above descriptive of the exact same function, i.e., using the instrument
to gather up more of the reflected photons from the object and converging
them onto the human eye and thereby raising the intensity of the light
energy above the perception threshold of the eye to make the object's
reflected light energy better match the eye's sensitivity needs? Does this
imply that all aspects (even microscopic details) of an object reflecting
visible light theoretically are present at some energy level even at great
distances from the object, so that everything lying in the visible spectrum
about that object is simultaneously present at some energy level in the
light from every object?
Replies:
It might help to distinguish two functions of optical instruments --
first, as you hinted, they can gather and amplify very low levels of
light and create images that a person could not see unaided. They
might capture light in the visible spectrum, or other radiation people
cannot see (like infrared). Examples of this kind of instrument include
many types of "night vision" cameras. Second, instruments can simply
magnify an object which means the light coming from the image is bent
(refracted) to span a larger area. Traditionally this is done with
lenses, but with modern electronic equipment, other technologies are
also available. As you also suggested, extra illumination is commonly
required when magnifying an image because its brightness (light per
area) is lower when the area is increased.
Hope this helps,
Burr Zimmerman
Hi Jacob,
Lenses perform a couple of functions and it seems you may be
confusing two of them.
One function is gathering light. This is the function of the
objective lens in a microscope or telescope (the objective "lens"
can be a curved mirror).
A second function is magnification which makes an image larger than
the original object. Magnification can happen with a simple lens
but for microscopes and telescopes it is achieved with a combination
of lenses in the eyepiece (and the objective).
For telescopes, gathering more light (larger objective) allows
dimmer objects to be seen. In addition to adding more light, larger
objective lenses improve the resolution (the amount of detail) that
can be seen for a given magnification. Note that other factors will
impact resolution for telescopes, including atmospheric distortion.
Magnification means that the image formed on the back of our eyes is
larger than if we looked at the object without the benefit of the
additional lenses. The larger image means the photons coming from
the source are spread out more so if there are competing light
sources (room lights, etc) it will be difficult to separate the
image from the competing light sources. To compensate for this, the
objective gathers more of the light coming from the source, and in
the case of a microscope, additional illumination is provided to the object.
Note that in a perfectly dark room the eye can detect single photons
but, of course, single photons are not sufficient to form an
image. Thus astronomers use photosensitive detectors that can
capture and HOLD charge created by the photons striking the detector
until enough photons are capture to form a full image.
Greg Bradburn
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Update: June 2012
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