Question

(c)
A student focused the image of a candle flame on a white screen by
placing the flame at various distances from a convex lens of focal length
25 cm. He noted his observations in the table given below.
S.no Distance of flame from Distance of the screen from
the lens (cm)
the lens (cm)
-100
+25
-60
-40
+40
+60
-25
+100
+30
-30
-15
+120
In which case, is the observation incorrect? Give reason for your answer​

Answer 1

Answer:no

Explanation:because distance cant go in negative

Answer 2

Answer:

The observation which is incorrect is as following:

Distance of flame from the lens - + 25cm - Distance of the screen from the lens - + 100 cm

Explanation:

Focal length is defined as the distance in which parallel rays of light divereg or converge. In convex lens the parallel rays converge along the distance. Focal length in convex lens is always positive.

Accoring to the observation it is given that a convex lens is having focal length is 25 cm. On the basis of principle of reflection of eye if any object is placed at focal length its image will form at infinity. Hence, the flame distance which is placed at +25 cm (same as focal length)  whose image is screend at the distance of +100 cm will be incorrect observation, as image should be at infinity.

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