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Name: Jacob
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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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