Physics 123A Waves and Modern Physics

1
Physics 123A Waves and Modern Physics
Lecture 12 (TM 32.1-4)Optical
Instruments October 30, 2008 (24 Slides)

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

2
Lecture 12 Announcements

• Lecture Homework 4 is due at 1159 PM on
  Friday, October 30 (today). Lecture Homework 5
  has been posted on WebAssign and is due on
  Friday, November 6.
• We will have Exam 2 on November 13. There will
  be assigned seating. If you want a left-handed
  aisle seat, a right-handed aisle seat, or a seat
  near the front, of if you require one of the four
  table seats in the back, send me E-mail with your
  request.
• So far 180/196 students have registered their
  clickers. A clicker response that shows up as a
  number may indicate a registration problem.

3
Lecture Schedule (Part 2)
We are here!
4
ExampleFocal Length of a Thin Lens
The lens shown is made of glass with index
of refraction n 1.50 and has radii of curvature
with magnitudes 10.0 cm and 13.0 cm as shown.
Find the focal length of the lens and its power.
Note that r1 and r2 are both positive in this
example.
5
Example Focal lengthof a meniscus lens
What is the focal length of the glass
meniscus lens shown? Is it converging or
diverging?
6
Clicker Question 1
A converging lens is made of glass that has
an index of refraction n 1.60. When the lens is
in air, its focal length is f 30 cm. When the
lens is immersed in water (nw 1.33), its focal
length will be (a) greater than 30 cm (b)
between zero and 30 cm (c) equal to 30 cm (d)
a negative value.
7
Mirror Aberrations
Because the curvature of a spherical mirror
is not correct for the focusing of large-angle
rays, the focal point is smeared. Cure use
only a small region of the spherical surface,
such that the angles involved are less than 5
degrees.
Alternative For focusing distant objects, use a
parabolic mirror.
Note that curved mirrors do not have
chromatic aberrations. All wavelengths focus at
the same focal point.
8
Lens Aberrations
Because the index of refraction depends on
wavelength, a lens will focus different
wavelengths at different focus positions. Cure
use compound lenses made of glasses with two
dispersions (or use curved mirrors).
Because the focal condition is based on the
small angle approximation, spherical lenses focus
large-angle rays at different focus
positions. Cure use small lenses or compound
lenses including elements with non-spherical
shapes.
9
Acromatic Lens
crown glass
flint glass
By combining a converging lens of crown glass
(n 1.522) with a color-compensating diverging
lens of flint glass (n 1.805), chromatic
aberration can be eliminated.
10
Lens Combinations
Optical instruments can be made by using
combinations of lenses, each of which processes
the image produced by the previous lens.
The system shown is a Keplerean telescope. A
long-focal-length objective lens views a distant
object at P and produces an intermediate image at
P. This image is viewed by a short-focal-length
magnifier eyepiece.
11
Camera Lenses
Adjustableaperture
Adjustableaperture
Zeiss Tessar Lens
Zeiss Orthometer Lens
12
Camera Lenses andExposure Times
In camera notation, the f-stop means the
ratio f/D, where f is the focal length of the
lens and D is the diameter of the aperture.
Thus, f /5.6 means that f/D 5.6. If f 50 mm,
then D 8.93 mm. The exposure time t is
proportional to (intensity)-1 or to
D-2. Example A camera needs a t1 1/100 s
exposure time to make an photograph at f/16.
What exposure time is needed at f/4?
13
Example A compoundcamera lens
A camera lens is usually a combination of
two or more single lenses. Consider a camera
lens consisting of a diverging lens with f1
-120 mm and a converging lens with f2 42 mm,
spaced 60 mm apart. A 10 cm tall object is 500
mm away from the first lens.

• What are the location, size, and orientation of
  the image?

14
Example A compoundcamera lens (continued)
(b) What is the focal length of the single lens
that could produce an image at the location if
placed at the midpoint of the lens combination?
15
The Eye
The eye is essentially a variable-focus camera
with the retina as the film.
farsighted
Visual size to the eye depends on the image size
at the retina, which in turn depends on the
angular size f.
nearsighted
16
Example Designingan eyeglass lens
A plastic (n1.59) eyeglass lens to correct
for near-sightedness is a diverging lens with a
plano-concave design. The patient needs a focal
length of -1.5 m. What is the radius R2 of
the inner surface of the lens?
17
The Simple Magnifier
18
Example A magnifying lens
A stamp collector uses a magnifying lens
that is placed 2.0 cm above the stamp. The
magnification is 4.0 What is the focal
length of the lens?
19
The Microscope
The front objective lens produces a real
magnified image I1, which is then further
magnified by the eyepiece.
20
ExampleA Compound Microscope
A microscope has an f1 1.2 cm objective
lens and an f2 2.0 cm eyepiece, with the lenses
separated by 20.0 cm.(a) Find the magnifying
power if the near point of the viewer is 25.0
cm? (b) Where should the object be placed if the
image is to be viewed at infinity?
21
Refracting Telescopes
Galilean telescope Eyepiece is a short-focus
diverging lens placed before the focus of the
long focal-length objective lens. The resulting
magnified image is upright (because the rays
dont cross).
Keplerean telescope Eyepiece is a short-focus
converging lens placed after the focus of the
long focal-length objective lens. The resulting
magnified image is inverted (because the rays
cross).
22
Reflecting Telescopes
Newtonian telescope
Cassegrain telescope
23
Telescopes
24
End of Lecture 11

• Before the next lecture, read TM, Chapter
  31.1-3.
• A week from today, on Friday, May 9, we will
  have Exam 2. It will consist of MC lecture
  questions (55 pts), and free-response questions
  based on the Laboratory (25 pts) and Tutorial (20
  pts). There will be assigned seating.
• So far 149/168 students have registered their
  clickers. A clicker response that shows up as a
  number may indicate a registration problem.