A student placed a candle flame at different distances from a convex lens and focused its image on a screen.…
A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below :
S.No. | Distance of flame from the lens (cm) | Distance of the image from the lens
1 | -90 | +18
2 | -60 | +20
3 | -40 | +24
4 | -30 | +30
5 | -24 | +40
6 | -20 | +60
7 | -18 | +90
8 | -12 | +120
Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :
(a) What is the focal length of the convex lens used ? Give reason to justify your answer.
(b) Which one of the sets of observations is not correct and why ?
(c) Draw ray diagram to show image formation for any correct set of observation.
Solution
(a) Focal length = + 15 cm (Ignore units)
Reason: when object distance and image distance are same i.e. object is at 2F, image is also formed at 2F on the other side
∴ 2f = 30 cm
(b) Observation No. 8
Reason: here the object is between optical centre and principal focus of the lens hence image is formed on the same side as the object and v is not equal to + 120 cm. (it should be -120 cm)
(c) Ray diagram (Any one ray diagram)