Download Class 16

Physics 1230: Light and Color
Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
[email protected]
Phone: 303-492-7277
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
[email protected]
Class # 11
Midterm Exam Results
The remaining lectures:
• Ch. 5 (the eye),
We
are
here
• Ch. 6 (optical instruments),
• Ch. 7 (Retina and visual perception),
• Ch. 9 & 10 (color & color perception).
3
Optical instruments we’ll cover:
• Single lens instruments
We
are
here
– Eyeglasses
– Magnifying glass
• Two lens
– Telescope & binoculars
– Microscope
4
Ray tracing a convex lens:
object inside focus
The image appears larger (and farther away) than the object.
This is a magnifying glass.
Does this match what you saw with your magnifying glass? Water drop?
5
Ray tracing a convex lens:
object OUTSIDE focus
The image appears smaller, inverted, and is a real image.
Does this match what you saw with your magnifying glass?
6
What can you tell me about focal length of bottle filled with water?
Which person is wearing a convex lens?
A) The one on the left
C) Both
B) one on right
D) ???
Object close to lens appears magnified Object far away looks smal
Nearsighted (myopic; can’t see far) = concave
lenses
Diverging Lens:
f<0
Farsighted (hyperopic; can’t see near) = convex len
f>0
Eyeglasses: Our most common optical
instrument
Normal vision: you can focus from 25 cm to infinity ()
For nearsighted people (can’t focus far away)
Eyeglasses are diverging (thinner in middle)
For farsighted people (can’t focus up close)
Eyeglasses are converging (thicker in middle)
18th century HUGE
improvement in quality of life
Demo: eyeglasses
9
Thin concave (diverging) lens: three
easy (?) ray rules
1) A ray parallel to the axis
is deflected as if it came
from the focus
2) A ray through the center
of the lens continues
undeviated
3) A ray aimed at the focus
on the other side comes
out parallel
1
2
3
F
F’
Ray might have to be extended
For diverging lens focal length defined to be negative
But we’ll stick with convex lenses for ray tracing 10
Ray tracing shows that for FARAWAY object,
an image is created that is CLOSER and
SMALLER than the object
Nearsighted lenses
are concave:
Faraway objects look
closer and smaller
Eyeglass prescription is in diopters
Optometrists use diopters to measure the power
of a lens
Diopters [or D] = 1 / (focal length in meters)
Example: f = 50 cm or f = 0.5 m
D = 1/f = 2 diopters (units are 1/meters)
Does this lens fix myopia or hyperopia? Hint: Concave lenses
have a negative focal length
13
How thin lenses add
Ftot = final focal length
F1 = focal length lens 1
F2 = focal length lens 2
Diverging lenses (concave) have
negative focal lengths
This is the same as adding powers:
Dtot = D1 + D2
Demo: put together some lenses
14
A Question
You have two focusing lenses, each with a focal
length of F. You put them close together to make
them behave as a single lens. The new ‘doublet
lens’ has a focal length of:
A)
B)
C)
D)
2*F because the diopters add.
F/2 because the diopters add.
Still F for this special case.
Something else happens.
Is there an experiment you
can try?
15
How thin lenses add
Ftot = final focal length
F1 = focal length lens 1
F2 = focal length lens 2
Diverging lenses (concave) have
negative focal lengths
This is the same as adding powers:
Dtot = D1 + D2
1
1 1 2
  
Ftotal F F F
OR
Demo: put together some lenses
Ftotal
F

2
16
A Question
You have two focusing lenses, each with a focal
length of F. You put them close together to make
them behave as a single lens. The new ‘doublet
lens’ has a focal length of:
A)
B)
C)
D)
2*F because the diopters add.
F/2 because the diopters add.
Still F for this special case.
Something else happens.
Is there an experiment you
can try?
Physics is always based
on the experiment!!
17
You have a lens with a short focal length and you
wish it was longer. You can make it longer by
using a second lens. The correct choice for this
case is:
A) A focusing lens of negative power
B) A diverging lens of positive power
C) A focusing lens of positive power
D) A diverging lens of negative power
1
Recall:
fTOTAL

1 1

f1 f 2
OR
DTOTAL  D1  D2
18
The Magnifying glass (again): Another view
The eye perceives via focused images:
25 cm
Typical closest focus is 25 cm from the eye.
A magnifying glass is like READING GLASSES:
It lets you focus on closer things.
19
The Microscope
Hooke’s discoveries
The cell
Detailed structure of creatures.
Example: The flea (plague).
From Robert Hooke’s Micrograp
21
Robert Hooke’s
microscope, also
circa 1660
A TWO LENS system.
Discovered: Blood cells,
Microbes, etc.
van Leeuwenhoek
October 24, 1632
Born
Delft, Netherlands
No existing pictures
of Hooke…
22
van Leeuwenhoek’s microscope
Years developed: 1660s
Tiny lens with 2 mm focal length
(a lens cannot be much bigger than the focal length)
Magnification =
25 cm
 125
0.2 cm
Technology edge: Outstanding single lenses
Problem: image was still small, and very dim.
23
van Leeuwenhoek’s microscope
Focal length, approximately
25 cm, reading distance,
approximately
Single convex lens: Similar to a magnifying glass 24
Simple Microscope
An unmounted and unstained blood smear
Great image quality is possible!
http://www.brianjford.com/wavr
Robert Hooke’s two-lens microscope
A magnifying glass (the eyepiece) magnifies
the first image further.
lens 2 image
lens 1 image
lens 2
eyepiece
An image of an
image!
object
lens 1
Nosepiece
or
Objective
The first lens, the
nosepiece, is used as
a projection lens.
26
Modern
binocular
microscope is
very much the
same as
Hooke’s.
A beamsplitter, a halfsilvered mirror, sends
half the light to each
eyepiece.
27
Many optical instruments can be
understood step by step, as we did
for Hooke’s microscope:
The first lens collects light and
produces an image.
The second lens produces a
new image of the first image.
The third lens produces a new
image of the second image…
And so on.
28
Telescopes
Hubble Ultra Deep Field
Galileo’s
telescope
30 x magnification
Tiny lens means not
much light entered,
so image is dim.
Discoveries:
Sunspots
Craters on the Moon
Phases of Venus
Moons of Jupiter
31
Galileo’s telescope (~1600)
An image of an
image!
Negative lens for eyepiece gives rightside-up image.
32
Kepler’s telescope (~1600)
lens 2 image
lens 1 image
lens 2
eyepiece
An image of an
image!
lens 1
Objective
Positive lens for eyepiece gives upside down image.
It’s upside down, but brighter and is easier to see.
33
Astronomical telescope (Kepler type)
http://hyperphysics.phyastr.gsu.edu/hbase/geoopt/teles
Telescope kit makes a:
A) Galilean telescope
B) Keplerian telescope
C)Another type that we have not seen.
35
European Extremely Large Telescope
(42 m dia.) would use many smaller mirrors
36
Telescope drives
• Telescopes must rotate once every 24 hrs (approximately) to follow the
stars, or the pictures will have streaks.
North
star
37
Student telescopes
View of
galaxy
NGC1300
38
Catadioptric telescope
Also called Schmidt-Cassegrain
Front glass lens corrects aberrations
Why buy this? It’s shorter.
39
Hubble Space Telescope
image
Compare to student
telescope image.
40
Viewgraph projector
Mirror
Projection
lens
Fresnel lens (condenser)
and viewgraph location
Curved mirror
41
Slides moved to the end
A near-sighted or MYOPIC eye produces an
image that is not far enough behind the lens, so
is blurry on the retina. Therefore, the eye lens
focal length is:
A) Too long for a focused image.
B) Too short for a focused image.
C) Actually, the iris is closed too much
D) None of these.
43
Astigmatism
Vertical and horizontal lines focus differently
This problem is fixed by a cylinder lens
Sharply
focused
Out of
focus
Focuses in one
direction, but not the
other!
44
Action of a cylinder lens
Focuses in one
direction, but not the
other!
If a cylinder lens is needed for your eyeglasses,
your cornea and eyelens is curved more in one
direction than in the other!
45
Reminder: Lens Power (or diopters)
Lens power: D = 1/F
Units of D are 1/meters, also called diopters
Eyeglass lenses are measured in diopters.