CBSE Class 10 Science 2017 · Paper 31/1 · Question 22

Light - Reflection and Refraction 5 marks Hard

Analyse the following observation table showing variation of image-distance (

v

) with object-distance (

u

) in case of a convex lens and answer the questions that follow without doing any calculations :

(a)What is the focal length of the convex lens ? Give reason to justify your answer.
(b)Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion ?
(c)Select an appropriate scale and draw a ray diagram for the observation at S.No.2. Also find the approximate value of magnification.

Answer

(a) $f = + 20$ cm; (b) S.No. 6 is incorrect; (c) the image is real, inverted and half the size, $m \approx - 0.5$ .

(a)

f = + 20

cm. Reason: in S.No. 3 the object and image distances are equal (

u = - 40

cm,

v = + 40

cm), which happens only when the object is at

2 F_{1}

and the image at

2 F_{2}

; so

2 f = 40

cm and

f = 20

cm. (b) S.No. 6 is not correct. Its object distance,

15

cm, is less than the focal length,

20

cm, so the object is between

F_{1}

and

O

; a convex lens then forms a virtual, erect image on the same side as the object, so

v

must be negative, not

+ 120

cm. (c) With the scale

1

= 10

cm (object at

6

cm,

F

2

cm), the rays meet

3

cm from the lens, that is

v = + 30

cm, between

F_{2}

and

2 F_{2}

; the image is real, inverted and about half the size of the object, so the magnification is approximately

m = - 0.5

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