Page 173 - Science Class 08

M M
v If the object is beyond the focal length, but A C
at less than double the focal length C 1 C 2 B C1 B C1
2F 1
F 1
F 2 2F 2 2F 1 F 1 F 2 2F 2
(between F and 2F), its image shall be N n A
f
inverted, real and larger, but on the u v
Case (i) Object at infinity Case (ii) Object at beyond 2f
opposite side of the lens.
M M
A A
v If the object is at the focus (F), no image is
2F 2
formed as the rays come out of the lens in B 2F 1 F 1 F 2 C 2 B 2F 1 C 1 F 1 O F 2 2F 2 B
C 2
C 1
a parallel posi on. It is believed that a real, A
N N
inverted and much larger image will be A
Case (iii) Object at 2f Case (iv) Object in between f and 2f
formed at infinity. A
M M
A
v If the object lies between the focus and the
B O F 2 2F 2 A F 2 2F 2
op cal centre (at less than the focal length) 2F 1 F 1 C 2 B 2F 1 F 1 B C 2
C 1 C 1
of the lens the image will be unreal, erect N
and larger than the object. It will be Case (v) Object at f Case (vi) Object distance < f
formed at same side of the lens as the Images formed by convex lenses at different
posi on of object.
object itself but behind it.
OPTICAL DEVICES
The table given below sums up the proper es
Magnifying Glass
of the images formed through convex lenses.
A magnifying glass is, in fact, a convex lense of
Posi on Posi on Nature of Size of
of Object of Image Image Image short focal length. It is fi ed inside a frame
(generally round) fi ed with a handle so that it
Beyond 2F Between real, inverted smaller may be easily handled and used. The things
F and 2F
At 2F At 2F real, inverted small size viewed through it are generally small. They
appear erect and magnified. Watch-repairers
Between Beyond 2F real, inverted large
F and 2 F also use magnifying glasses.
At F At Infinity real, inverted very lerge
On the Rays Spreading
F and C unreal, act large
same side Out

Image Through Concave Lenses
Virtual F Object F
In case of the concave lens, we always get erect image
images, diminished images and virtual images.

Eye
Posi on of Posi on of Nature of Size of Convex Lens
Object Image Image Image
A magnifying glass can be used to focus the
Virtual and Highly
Infinity At F rays of the sun on something like paper or rag
2
Erect Diminished and make it catch fire instantly.
Between Op cal Microscope
Beyond
F and Virtual Diminished
1
infinity The word ‘microscope’ is made up of two parts
Op cal and Erect
and 0 micro + scope = very small + range. So, this
centre

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