Microscopes
Microscopes are used to see objects that are too small to see well with
the naked eye. An ordinary microscope works by combining convex
lenses. The lens closer to the object is called the objective. The object
is between one and two focal lengths from this lens, so the lens focuses
an enlarged image of the object inside the microscope.
The other microscope lens—the one you look through—is called
the eyepiece. You use this lens to look at the image formed by the
objective. Like a magnifying glass, the eyepiece lens forms an enlarged
image of the first image.
Very small objects do not reflect much light. Most microscopes use
a lamp or a mirror to shine more light on the object.
Which types of images do the lenses in a microscope form?
Telescopes
Telescopes are used to see objects that are too far away to see well
with the naked eye. One type of telescope, called a refracting telescope,
is made by combining lenses. Another type of telescope, called a
reflecting telescope, is made by combining lenses and mirrors.
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Refracting telescopes combine convex lenses, just as microscopes
do. However, the objects are far away from the objective lens instead
of near to it. The object is more than two focal lengths from the
objective lens, so the lens focuses a reduced image of the object inside
the telescope. The eyepiece of a telescope then forms an enlarged
image of the first image, just as a microscope does. This second image
enlarges the object.
Reflecting telescopes work in the same way that refracting
telescopes do. However, there is no objective lens where light enters
the telescope. Instead, a concave mirror at the opposite end focuses an
image of the object. A small flat mirror redirects the image to the side
of the telescope. With this arrangement, the eyepiece does not interfere with light on its way to the concave mirror. The eyepiece then
forms an enlarged image of the first image.
Both refracting and reflecting telescopes must adjust for the small
amount of light received from distant objects. The amount of light
gathered can be increased by increasing the diameter of the objective
lens or mirror. Large mirrors are easier and less expensive to make
than large lenses. So reflecting telescopes can produce brighter images
more cheaply than refracting telescopes.
How is a reflecting telescope different from a refracting telescope?
564 Unit 4: Waves, Sound, and Light