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Light - Part IX |
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Images
by convex lenses First
rule
: A ray of light parallel to the principal axis XY, on refraction passes
through the focal point F. This is shown in the figure given below. Second rule
: A ray of light passing through the optical centre O of a
convex lens, goes un-deviated along the same path. Third rule : The third rule is inverse of the first rule. When a ray of light passes through the focus F and strikes a convex lens, the refracted ray is parallel to the principal axis XY. From
the above three rules, we can now study how images are formed with the help
of convex lenses. The type of images formed depends on where the object is
positioned in front of the lens. We will study a few typical cases. Case 1 : object is placed between the optical centre O and the focus F1 Case 2 : object is placed at focus F1 Case 3 : object is placed between the focus F1 and 2 F1 Case 4 : object is placed at 2 F1 Case 5 : object is placed at a distance beyond 2 F1 Case 6 : object is placed at a far off distance beyond 2 F1 (or at infinite distance). Case
1
: Image formed by a convex lens when the object is placed between the
optical centre O and the focus F1. Consider
the following figure. Let
the object be a candle AB placed between F1 and O. Consider
two rays of light starting from A. A light ray from A, parallel to the
principal axis XY will strike
the lens at D. This ray of light, on refraction will pass through F2
(rule 1) to become ray DZ. The second light ray from A passing through AO,
will go un-deviated and will get refracted along OW (rule 2). The
other end of the candle B. is placed along the principle axis XY. So any ray
of light BO will go along BO itself (rule 2). The
rays DZ and OW reach your eye positioned appropriately. You will notice that
the rays emanating from A are intersecting at a point A’. Rays of light
from B extending behind the lens, form an image B’. The refracted image
is A’B’ behind the lens. You can see that the size of A’B’ is larger
than AB. Thus
we can conclude that for an image of an object placed between F1
and O of a convex lens :
This shows why a convex lens is used as a simple magnifying glass. It should be borne in mind that in order to have magnification effects the object viewed should be within the focal length of the lens.
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