LENSES
LEARNING OBJECTIVE
• To draw and use ray diagrams for the formation of a real image by a converging lens.
• To describe the characteristics of an image.
KEY WORDS
A real image
A virtual image
Inverted
Diminished
STARTER
What are the Factors that affect the image formed by a
lens?
Type of lens
The Distance between the
object and the lens
CONVERGING LENS - REAL IMAGE
• Lenses can be used to form images
of objects placed in front of them
• The location (and nature) of the
image can be found by drawing a
ray diagram:
THREE KEY RULES FOR CONVEX RAY DIAGRAMS
• Light passing through the centre of a lens
continues through in a straight line.
• Light travelling parallel to the principal axis is
refracted by the lens so that is passes through
the principal focus on the other side.
• Light passing through the focus into the lens
comes out travelling parallel to the principal axis.
• The image forms where the rays cross.
• If the object is far away, the image formed is much closer to the lens,
smaller, inverted and real (the rays do cross over)
• If the object is much closer to the lens, the image formed is very different.
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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Note-2
A ray that goes through the
Centre goes right through.
CASE-2 :
Object
Object just beyond
NOTE
In order to establish
2F’
an image point, all
we need are two
Note-1
intersecting
rays.
A ray that comes parallel
is refracted through F.
Note-3
Image
A ray that goes through F’
is refracted parallel.
Image is real (formed by refracted rays)
Inverted (upside down)
Reduced (smaller than object)
Located between F and 2F
Nearer to the lens than the object
This ray is extra
in locating the
image.
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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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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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CASE-5 :
Object at F’
Object
No image
No image is formed
(rays refract parallel)
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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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PLENARY
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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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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Question-3
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-4
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-5
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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