7. Optical instruments

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7. Optical instruments
1) Cameras
There is a certain range of distances over which
objects will be in focus this is called the
depth of field of the lens. Objects closer or
farther will be blurred.
The amount of light that enters the camera
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• 1a) Basic parts of a camera. Camera adjustments.
• Focusing adjusting the position of the lens so
  that the image is positioned on the film
• Shutter speed controls the amount of time light
  enters the camera
• f-stop controls the maximum opening of the
  shutter. This allows the right amount of light
  to enter to properly expose the film, and must be
  adjusted for external light conditions.

A digital camera uses CCD sensors instead of
film. The digitized image is sent to a processor
for storage and later retrieval.
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Example Suppose that a correct exposure is 1/250
s at f/11. Under the same conditions, what
exposure time would be needed for a pinhole
camera if the pinhole diameter is 1.0 mm and the
film is 7.0 cm from the hole? The pinhole camera
uses a tiny pinhole instead of a lens.
Example What is the focal length of the eye-lens
system when viewing an object (a) at infinity,
(b) 33 cm from the eye? Assume that the
lens-retina distance is 2.0 cm.
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2) The Human Eye
The human eye resembles a camera in its basic
functioning, with an adjustable lens, the iris,
and the retina.

• Most of the refraction is done at the surface of
  the cornea.
• The lens makes small adjustments to focus at
  different distances.

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3) Corrective Lenses
a) Nearsightedness can be corrected with a
diverging lens
b) And farsightedness with a diverging lens
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Far point Farthest distance at which object can
be seen clearly. Normal FP is at
infinity. Nearsightedness far point is too
close. Near point Closest distance at which eye
can focus clearly. Normal NP is about 25
cm. Farsightedness Near point is too far away.
To calculate lens power we use lens equation
Correction for the distance between glass and
eye
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Example A person cannot see clearly objects
more than 70.0 cm away. What power of lens
should be prescribed if the glass is to be worn
1.00 cm in front of the eye?
Example A nearsighted person wears glasses
whose lenses have power of -0.15D. What is the
person's far point if the glasses are very close
to the eyes?
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Example What power lens is needed to correct
for farsightedness where the uncorrected near
point is 75 cm?
Example A farsighted person can read a newspaper
held 25 cm from his eyes, if he wears glasses of
3.33 diopters. What is this person's near
point?
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Vision underwater
Vision is blurry underwater because light rays
are bent much less than they would be if entering
the eye from air.
This can be avoided by wearing goggles.
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4) Aberrations of Lenses and Mirrors
a) Spherical aberration rays far from the lens
axis do not focus at the focal point
Solutions compound-lens systems use only
central part of lens
b) Distortion caused by variation in
magnification with distance from the lens axis.
Barrel and pincushion distortion
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c) Chromatic aberration light of different
wavelengths has different indices of refraction
and focuses at different points
Solution Achromatic doublet, made of lenses of
two different materials
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5) Magnifying glass (Simple magnifier) A
converging lens that allows one to focus on
objects closer than the near point, so that it
makes a larger, and therefore clearer, image on
the retina.
The power of a magnifying glass is described by
its angular magnification
Note!
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Example A magnifying glass with a focal length
of 8.5 cm is used to read print placed at a
distance 7.5 cm. Calculate (a) the position of
the image (b) the angular magnification
Example A person uses a converging lens of
focal length 5.0 cm as a magnifying glass. What
is the maximum possible magnification?
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6)Telescopes A refracting telescope consists of
two lenses at opposite ends of a long tube. The
objective lens is closest to the object, and the
eyepiece is closest to the eye.
The magnification is given by
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Example A student constructs an astronomical
telescope with a magnification of 10. If the
telescope has a converging lens of focal length
50 cm, what is the focal length of the eyepiece?
What is the resulting length of the telescope?
Example A170x astronomical telescope is
adjusted for a relaxed eye when the two lenses
are 1.25 m apart. What is the focal length of
each lens?
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6a) Different types of telescopes
Astronomical telescopes need to gather as much
light as possible, meaning that the objective
must be as large as possible. Hence, mirrors
are used instead of lenses, as they can be made
much larger and with more precision.
A terrestrial telescope, used for viewing objects
on Earth, should produce an upright image. Here
are two models, a Galilean type and a spyglass
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7) Compound Microscope A compound microscope also
has an objective and an eyepiece it is different
from a telescope in that the object is placed
very close to the eyepiece.
The magnification is given by
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Example A microscope has an eyepiece with a
focal length of 3.5 cm. An object is placed 1.5
cm in front of the objective lens and its image
appears 21.0 cm behind the objective lens. The
microscope is adjusted for a relaxed normal eye.
What is the overall magnification of the
microscope after the adjustment?