Chapter 26: Vision and Optical Instruments
Lenses in Combination
With lenses in combination, the image formed by one
lens is the object for the next lens.
Lens 2
Lens 1
Image by
Lens 2
Object
Collision of two
galaxies captured
with the help of
the Hubble Space
Telescope.
p1
For lens 1
1 1 1
 
p1 q1 f1
Real and Virtual Objects
p1
q1
p2  s  q1
real object
distance
p2
1
1
1
 
p2 q2 f 2
F
image from first,
object for second
where,
p2  s  q1
F
This is not a
principal ray.
Ray parallel to the optical axis passes through the Focus.
q2
image from first,
object for second
q1
For lens 2
F
image of
second
s
p2
Two Converging lenses: Ray Diagram
It is possible to have the object for the second lens be
virtual (p2<0).
object
q2
s
Image that
would have
been formed
by lens 1
p2  s  q1
Ray through the center of lens comes out un-deviated.
virtual object
distance
Ray through the Focus goes parallel to the optical axis.
Camera: Single Lens Optical Instrument
Think, Answer; Talk to your neighbors, Answer
You have a camera where you can change the position
between the lens and the CCD sensor. In the last shot, you
photographed sunrise therefore the sensor was exactly at
the focal length of the lens. Now you want to take
photograph of a friend standing nearby. You need to
A camera forms a real,
inverted image
A) the lens closer to the sensor.
B) the lens farther from the sensor.
C) No need of adjusting the lens.
For far away objects,
the ccd must be placed
on focal length from
the lens.
1
Normal Eye vs. Defective Eye
Eye: Single Lens Optical Instrument
The lens is at a fixed
distance from the retina
(unlike in some cameras
where this is adjustable).
retina
The lens has a variable
focal length, which is
adjusted to keep the
image distance (q)
constant as the object
distance (p) varies.
Far
point,

Near point,
N =25 cm
Normal Eye
Focuses on retina
Defective Eye
Can’t focus on retina
Near and far points change.
Correcting Shortsightedness(Myopia)
Distant vision is blurry as
lens focuses before retina.
Using suitable diverging lens
corrects the problem.
The lens forms the image of
the distant object at the far
point of defective eye.
object at 
For the correcting
virtual image of the correcting lens.
lens:
virtual object for the eye.
p
q = - far point of the myopic eye, f  ?
Think, Answer; Talk to your neighbors, Answer
At my last check up, I was told my far point is 50 cm
near point is 20 cm. To correct my vision doctor
prescribed glasses. My glasses create an image of
distant objects
A.
B.
C.
D.
Correcting Farsightedness(Hyperopia)
Near vision is blurry as lens
focuses after retina.
object at
N =25 cm
virtual image formed by the correcting
lens at near point of defective eye.
virtual object for the eye.
object at 25 cm
For correcting converging lens:
Refracting Power of glasses:
p = 25 cm (N).
1 unit is 1/m called
q = - near point of the eye.
P
Diopters (D).
f
f ?
Problem: Harry can’t clearly see objects closer than
1.0 m. What refractive power his glasses should be?
at my near point.
at my far point.
at infinity.
at 25 cm from my eye.
Problem: What is the focal length of my glasses? Power?
2
Think, Answer; Talk to your neighbors, Answer
Magnifying Glasses
A converging lens
with object inside
the focal point.
If eyes focus light from distant object like as shown in
this sketch. What problem the eyes have and what
lens is used for correction?
A)
B)
C)
D)
E)
virtual
image
long sightedness, converging.
long sightedness, diverging.
short sightedness, converging.
short sightedness, diverging
none of these.
Image
The Compound
Microscope
F
h
near
point
Angular
aided
magnification, M  
unaided
Problem: Keesha is looking at a beetle with a magnifying
glass. She wants to see an upright, enlarged image at a
distance of 25 cm. The focal length of magnifying glass is
+5.0 cm. Assume that Keesha’s eye is close to the
magnifying glass. (a) What should be the distance between
magnifying glass and beetle? (b) What is the angular
magnification?
(a) q  25 cm
f  5.0 cm. p  ?
1 1 1
 
f p q
fq
p
 4.2 cm
q f
N
25 cm
(b) M 

 6.0 cm
p
4.2 cm
angular
 aided
size
F object
Beetle
Here, aided 
h
p
M
N
h N


p
p h
 unaided 
h
N
M
N
f
 unaided
when object is very
close to F,
p  f.
Think, Answer; Talk to your neighbors, Answer
A magnifying glass magnifies tiny objects when it is held
close to the object. What would be the most suitable
lens for such magnifier?
A)
B)
C)
D)
E)
converging lens with long focal length.
diverging lens with long focal length.
converging lens with short focal length.
diverging lens with short focal length.
either converging or diverging lens would work.
A refracting telescope:
The Telescope
 obj 
The angular magnification is
M 
h
f obj
 eye 
h
f eye
 eye
f
  obj .
 obj
f eye
3