CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 36

Light - Reflection and Refraction 2 marks Medium

A student places a candle flame at a distance of about 60 cm from a convex lens of focal length 10 cm and focuses the image of the flame on a screen. After that he gradually moves the flame towards the lens and each time focuses the image on the screen.

(a)In which direction-toward or away from the lens, does he move the screen to focus the image ?
(b)How does the size of the image change ?
(c)How does the intensity of the image change as the flame moves towards the lens ?
(d)Approximately for what distance between the flame and the lens, the image formed on the screen is inverted and of the same size ?

Answer

The screen moves away, the image becomes larger and dimmer, and a same-size inverted image forms with the flame at 20 cm ( $2 F$ ).

(a) Away from the lens: as the flame comes in from 60 cm the image distance grows (12 cm, 15 cm, 20 cm and 30 cm for the flame at 60 cm, 30 cm, 20 cm and 15 cm). (b) The image becomes larger: the magnification changes from

- 0.2

through

- 1

- 2

. (c) The intensity (brightness) of the image decreases: the image grows faster than the light the lens collects from the nearer flame, so the image becomes dimmer. (d) At about 20 cm, that is at 2F, twice the focal length; the inverted image of the same size is then also 20 cm from the lens.

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CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 36

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