The variation of image distance (v) with the object distance (u) for a convex lens is given in the following…
The variation of image distance (v) with the object distance (u) for a convex lens is given in the following observation table. Analyse it and answer the questions given below :
S.No. | Object distance (u) cm | Image distance (v) cm
1 | -150 | +30
2 | -75 | +37.5
3 | -50 | +50
4 | -37.5 | +75
5 | -30 | +150
6 | -15 | +37.5
(a) Without calculation, find out the focal length of the given convex lens. Justify your answer.
(b) Which one of the observations given in the table is not correct and why ?
(c) (i) Find out the value of magnification for u = -30 cm. Write the nature of the image formed. Give reason for your answer.
Solution
(a) In observation no.3. The object distance is equal to image distance,
which is only possible when the object is at 2F (C) =50 cm. Therefore, the
focal length is 25 cm.
(b)
• Observation no. 6
• Because when object is kept between F and O, image formed by
lens will be on same side of object. Hence, image distance(v)
should be negative.
v
(c) (i) m =
u
= −30
m=−5
• Nature of image: Image will be real and inverted
• Reason: because the value of m is negative.
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.
OR
(c) (ii)
(deduct ½ mark for no or incorrect direction of rays)