Thin Lens
Optics
36: 4
Quiz 23:1 Did you complete
the reading assignment 36:4?
A. Yes Thin Lens Equation:
Convex= focusing; Concave diverging.
B. No
3- cases: [2 with to converging
paying attention to signs
Images (focusing) 1 with diverging.]
OR 1 real image (inverted)
2 virtual image (upright)
Drawing lines
Using a sphere to focus (if time)
Quiz: Which on of
the 3 is the illusive dollar bill?
A. Real, inverted
B. Virtual upright large
C. Virtual upright small
Note: in what follows Object is blue.
How is mirror oriented?
Where are object and image relative to
focus?
A
B
C
Exam 2 ended Sat.
►
Saturday
Today: 1. Doing
with lens what we
did mirrors
Quiz 23-2 If an image formed by a
lens is on the same side of the
lens as the object, the image is
A. real or
B. virtual
Converging Lens
+ if surface
is convex
- if surface
is concave
Diverging Lens
+ if surface
is convex
- if surface
is concave
Lens Makers’ Equation
(my version)
 1
1
1 
=( n − 1)  ±
±

f
 R1 R2 
+ if surface is convex
- if surface is concave
 1
1
1 
=
( n − 1)  + 
f
 R1 R2 
 1
1
1 
=( n − 1)  −
−

f
 R1 R2 
Which of the Quiz 23-3
following is the
 1
correct lens
1
1 
(1) =
( n − 1)  + 
f
makers’ equation
 R1 R2 
for the lens shown
 1
1
1 
( 2 ) =( n − 1)  − + 
below?
f
 R1 R2 
 1
1
1 
( 3) =
( n − 1)  − 
f
 R1 R2 
 1
1
1 
( 4 ) =( n − 1)  − − 
f
 R1 R2 
Real Image
Quiz 23-4 (participation point)
Consider a real image formed by a
converging lens. If we block out the
top half of the lens, what happens to
the image?
A. top half disappears
B. bottom half disappears
C. none of the image disappears
Ray Diagram
1. Parallel in, out thru F
2. In thru F, out parallel
3. Straight thru center
lens equation:
1 1 1
+ =
p q f
q
−
M=
p
In a Jules Verne novel, a piece of ice is
shaped to form a magnifying lens to focus
sunlight to start a fire. Is that possible?
• Assume you have a shallow soup bowl
which is a section of a sphere 0.5 meter in
diameter. The bowl is 30 cm across. If I
freeze water in it I can make a planoconvex
lens. Ice’s n is 1.31 (see problem 8). What
is the focal point of the lens in m?
____0.806m____ This is based on question
21 from the back of chapter 36.
http://wildwoodsurvival.com/survival/fire/i
ce/rb/rbfirefromice3a.html
Thick Lens
Thick lensmaker’s equation:
Wikipedia version
R1 is the radius of curvature of the lens surface
closest to the light source,
R2 is the radius of curvature of the lens surface
farthest from the light source, and
d is the thickness of the lens (the distance along
the lens axis between the two surface vertices).
Sphere R1=-R2=R; d=2R ►1/f = (n-1)(2/R)/n
f = nR/[2(n-1)] ; example ice n=1.31; f= 2.11R
http://en.wikipedia.org/wiki/Lens_(optics)
Review: Compare Mirrors
1 1 1
+ =
p q f
q
−
M=
p
Which is true?
A. p >0, q>0
B. p >0, q<0
C. p <0, q>0
D. p <0, q<0