15-10-2012, 01:18 PM
Optical Instruments
Optical Instruments.pdf (Size: 3.03 MB / Downloads: 79)
From eyeglasses to
microscopes and telescopes,
our everyday world is filled
with optical instruments,
devices that aid our senses
by using lenses and mirrors
to form images we wouldn’t
be able to see, or see as well,
with our eyes alone.
Chapter Goal: To
understand some common
optical instruments and their
limitations.
The Camera
• A camera “takes a picture” by
using a lens to form a real, inverted
image on a light-sensitive detector
in a light-tight box.
• We can model a combination lens
as a single lens with an effective
focal length (usually called simply
“the focal length”)
• A zoom lens changes the effective
focal length by varying the spacing
between the converging lens and
the diverging lens.
Zoom Lenses
• When cameras focus on objects that are more that 10 focal
lengths away (roughly s > 20 cm for a typical digital
camera), the object is essentially “at infinity” and s' ≈ f.
• The lateral magnification of the image is
• The magnification is much less than 1, because s >> f, so
the image on the detector is much smaller than the object
itself.
• More important, the size of the image is directly
proportional to the focal length of the lens.
Controlling the Exposure
• The amount of light passing through the lens is controlled
by an adjustable aperture, also called an iris because it
functions much like the iris of your eye.
• The aperture sets the effective diameter D of the lens.
Vision
• The human eye is roughly spherical, about 2.4 cm in
diameter.
• The transparent cornea and the lens are the eye’s
refractive elements.
• The eye is filled with a clear, jellylike fluid called the
aqueous humor and the vitreous humor.
• The indices of refraction of the aqueous and vitreous
humors are 1.34, only slightly different from water.
• The lens has an average index of 1.44.
• The pupil, a variable-diameter aperture in the iris,
automatically opens and closes to control the light
intensity.
• The f-number varies from roughly f/3 to f/16, very similar
to a camera.