High School
Optics
by SSL Technologies
Part 1 /2
Physics Ex-52
PART-1 /2
LENSES
Whereas mirrors produce images by reflection, lenses produce
images by refraction. Depending upon their “thickness”, lenses
are classified as “thick” or “thin”. In this course, we will only
consider “thin” lenses. In effect, therefore, we disregard the
thickness of the lens assuming it to be a line.
Convex
Concave
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Physics Ex-52
PART-1
There are two main types of lenses called convex (also known
as converging ) and concave (also known as diverging ).
Convex lenses are thicker in the middle than at the edges while
concave lenses are thicker at the edges than in the middle.
As illustrated below, both convex and concave lenses each have
three variations.
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Physics Ex-52
PART-1
When representing “thin” lenses in a diagram, it is sometimes more
convenient to simply draw an arrow rather than the actual lens.
Using this method, an arrow is used to represent the lens as
illustrated below.
Convex Lens
Concave Lens
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Physics Ex-52
Convex lenses are thicker in
the middle and thus
converge light rays.
Concave lenses are thinner
in the middle and thus
diverge light rays.
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Physics Ex-52
PART-1
Just as with concave mirrors, the characteristics of the image
formed by a converging lens depend upon the location of the object.
There are six "strategic" locations where an object may be placed.
For each location, the image will be formed at a different place and
with different characteristics. We will illustrate the six different
locations and label them as CASE-1 to CASE-6.
Case-1:
Case-2:
Case-3:
Case-4:
Object
Object
Object
Object
at infinity
just beyond 2 F’
at 2F’
between 2F’ and F’
Case-5: Object at F’
Case-6: Object within focal length (f)
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Physics Ex-52
CASE-1 : Object at “infinity”
Infinity simply means
“far away”.
No image
Object
NOTE
Since the object is at “infinity”,
all rays arrive parallel.
No image formed
(All rays pass through F)
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Physics Ex-52
CASE-2 :
Object
Object just beyond
NOTE
In order to establish
2F’
an image point, all
we need are two
Note-1
intersecting
rays.
Note-2
A ray thatNote-3
comes parallel
AAray
that
goes
through
ray
that goesthrough
throughF.the
F’
is refracted
vertex
goes right
through.
is refracted
parallel.
Image
Image is real (formed by refracted rays)
Inverted (upside down)
Reduced (smaller than object)
Located between F and 2F
This ray is extra
in locating the
image.
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Physics Ex-52
CASE-3 :
Object at 2F’
Object
Again:
In order to establish
an image point, all
we need are two
intersecting rays.
Image
Image is real (formed by refracted rays)
Inverted (upside down)
Same size as object
Located at 2F
This ray is extra.
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Physics Ex-52
CASE-4 :
Object between 2F’ and F’
Object
Image is real (formed by refracted rays)
Inverted (upside down)
Magnified (larger than object)
Located beyond 2F
Image
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Physics Ex-52
CASE-5 :
Object at F’
Object
No image
No image is formed
(rays refract parallel)
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Physics Ex-52
CASE-6 :
Object is within focal length
Image
Image is virtual
Object
(formed by extended rays)
Upright
Magnified
Located on same side as object
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Physics Ex-52
Physics Ex-52
One case only!
Object
Image
Image is always virtual
(formed by a direct ray and an extended ray)
Always upright
Always reduced
Always located on same side as object
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Physics Ex-52
PART-1
Be sure to follow the following sign convention when
solving problems concerning lenses.
 Sign convention:
1- Object distance is always positive.
2- Image distance is positive if the image is on the side of the
lens where light emerges (from lens).
3- Image distance is negative if the image is on the side of the
lens where the light enters (the lens).
4- The focal length of a convex (converging) lens is positive.
5- The focal length of a concave (diverging) lens is negative.
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Physics Ex-52
Question-1
A lens that is thicker in the middle than at the ends is known as:
A
Convex
lens or a
Converging
lens.
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Physics Ex-52
Question-2
A lens that is thicker at the ends than in the middle is known as:
A
Concave
lens or a
Diverging
lens.
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Physics Ex-52
Question-3
Trace the rays that emerge from the following glass mediums:
a)
b)
c)
d)
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Physics Ex-52
Question-4
For each case below, draw the appropriate lens that will produce
the indicated rays.
a)
Concave (diverging)
b)
Convex (converging)
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Question-5
Physics Ex-52
How is the image formed by a mirror different from the image
formed by a lens?
A mirror forms an image by reflection whereas
a lens forms a an image by refraction.
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Question-6
Physics Ex-52
For each convex lens illustrated below, draw the image.
a)
Image
Image is real, inverted, reduced
and located between F and 2F.
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Question-6
Physics Ex-52
For each convex lens illustrated below, draw the image.
b)
Image
Image is real, inverted, same
size object and located at 2F.
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Question-6
Physics Ex-52
For each convex lens illustrated below, draw the image.
c)
Image
Image is real, inverted, magnified
and located beyond 2F.
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Question-6
Physics Ex-52
For each convex lens illustrated below, draw the image.
d)
Parallel
All rays refract parallel,
no image is formed.
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Question-6
Physics Ex-52
For each convex lens illustrated below, draw the image.
e)
Image
Image is virtual, upright, magnified
and located on same side as object.
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Physics Ex-52
Question-7
For each concave lens illustrated below, draw the image.
REMEMBER
Concave lenses always produce images
that are virtual, upright and reduced.
Image
Image is virtual, upright, reduced
and located on same side as object.
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Physics Ex-52
Question-7
For each concave lens illustrated below, draw the image.
Image
Image is virtual, upright, reduced
and located on same side as object.
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Physics Ex-52
Question-8
An object that is 7 cm high is placed 20 cm in front of a convex
(converging) lens whose focal length is 15 cm. Determine the
characteristics of the image:
Type (real or virtual):
Real
_______________
Location:
60 cm
_______________
Magnification:
- 3 (inverted and 3x larger than object)
_______________
Height:
21 cm
_______________
Negative sign indicates inversion.
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Physics Ex-52
Question-9
An object whose height is 4 cm is placed 50 cm from a concave
(diverging) lens. If the focal length of the lens is 30 cm, determine
the characteristics of the image:
Type (real or virtual):
Virtual
_______________
Location:
18.75 cm
_______________
Magnification:
0.375 (upright and smaller than object)
_______________
Height:
1.5 cm
_______________
Note that diverging lenses
have a negative focal length.
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Physics Ex-52
Question-10
An object that is 5 cm high is placed 70 cm in front of a convex
(converging) lens whose focal length is 20 cm. Determine
the characteristics of the image:
Type (real or virtual):
Real
_______________
Location:
28 cm
_______________
Magnification:
- 0.04 (inverted and smaller than object)
_______________
Height:
2 cm
_______________
Negative sign indicates inversion.
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Physics Ex-52
Question-11
A lens produces the following optical effect.
Which group of lens produces the above effect?
Double concave
A)
Plano concave
B)
C)
D)
Double convex
Plano convex
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Question-12
Physics Ex-52
Beams of light rays are traveling through air parallel to the
principal axis of four different lenses. The light rays enter
the lenses and are refracted.
In which diagram are the light rays correctly illustrated?
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