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Light - Part X |
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Case
2
: Image formed by a convex lens when the object is placed at focus F1. Consider
the following figure. Let
the object be a candle AB at F1. Consider
two rays of light from point A. AD is parallel to the principal axis XY and
AO passing through O. From rule 1 and 2, we see that ray of light AD will be
refracted along DF2Z, passing through the focal point F2.
Similarly ray AO will go un-deviated. The
two refracted rays are parallel to each other and will meet at infinity!!
Thus the image of point A will be at infinity. You can also see that the
image of A will be on the opposite side of the principle axis XY, hence the
image is also inverted besides being formed at infinity. As you extrapolate
the two lines, you will see that the image of A will be highly magnified.
The image of B will be fall on the line XY itself (rule 2). Thus
we can conclude that for an image of an object placed at the focus F1
of a convex lens :
Case
3
: Image formed by a convex lens when the object is placed between the focus
F1 and 2 F1. Consider the following figure. Let
the object be a candle AB placed between F1 and 2 F1. Consider
two rays of light emanating from point A. AD is parallel to XY. AD is
refracted along DF2Aí (rule 1). The second ray of light AO is
cutting the principle axis XY at O. Hence by rule 2, the refraction of AO
will be along the same line. Lines DF2Aí and AO meet at Aí,
which is the reflected image of A. Again,
as explained earlier, light from B will go un-deflected along XY path. The
image of AB is AíBí. Clearly the image is magnified and inverted. Thus
we can conclude that for an image of an object placed between focus F1
and 2 F1 of a convex lens :
A
slide film projector uses this type of geometry for projecting a magnified
image. The slides are always kept inverted so that the image of the slide is
erect and is captured on a screen. Case
4
: Image formed by a convex lens when the object is placed at 2 F1. Consider the following figure. Let
the object be a candle AB placed at 2 F1. As
before, consider two rays emanating from A; one AD is parallel to XY the
principal axis. The second ray is AO passing through the optical centre O.
From rule 1, we see that ray AD will be refracted by the lens and pass
through the focal point F2. The second ray AO, will go along
OAí un-deviated (rule 2). Again, as explained earlier, light from B will
go un-deflected along XY path. The image of AB is AíBí. Clearly the
image is un-magnified and inverted and is formed at 2 F2. Thus
we can conclude that for an image of an object is placed at 2 F1 of
a convex lens :
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