(a) The variation of image distance (v) with object distance (u) for a convex lens is given in the following…
(a) The variation of image distance (v) with object distance (u) for a convex lens is given in the following observation table. Analyse it and answer the questions that follow :
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
(i) Without calculation, find the focal length of the convex lens. Justify your answer.
(ii) Which observation is not correct ? Why ? Draw ray diagram to find the position of the image formed for this position of the object.
(iii) Find the approximate value of magnification for u = – 30 cm.
Solution
(a)
(i) S. No. 3, 2f is 50 cm. ∴ 2f = 50 cm, or f = 25 cm.
Justification: Object distance(u) and image distance (v) are same so it implies that object is placed at 2F.
(ii) S. No. 6, is not correct.
Reason: For u = −15 cm, sign of v must be – ve ( as the image is formed on the same side of the lens as the object)
(iii) Magnification: m = v/u = (+150 cm)/(−30 cm) = −5 cm
(deduct ½ mark if the direction of the rays are not shown)