CBSE Class 10 Science 2017 · Paper 31/1/1 · 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 in support of your answer.
(b)Write the serial number of that observation which is not correct. How did you arrive at this conclusion ?
(c)Take an appropriate scale to draw ray diagram for the observation at S. No. 4 and find the approximate value of magnification.

S. No.	Object distance $u$ (cm)	Image distance $v$ (cm)
1	−90	+18
2	−60	+20
3	−30	+30
4	−20	+60
5	−18	+90
6	−10	+100

Answer

$f = + 15$ cm from observation 3; observation 6 is incorrect (object inside $f$ cannot give a real image); $m \approx - 3$ for observation 4.

(a) The focal length is

15

cm (

f = + 15

cm). In observation 3 the object (

u = - 30

cm) and the image (

v = + 30

cm) are at equal distances on opposite sides of the lens. A convex lens does this only when the object is at

2 F_{1}

, the image then forming at

2 F_{2}

, so

2 f = 30

cm and

f = 15

cm. (b) Observation 6 is not correct. Its object distance,

10

cm, is less than the focal length,

15

cm, so the object lies between the optical centre and

F_{1}

. A convex lens then forms a virtual, erect, enlarged image on the same side as the object, with a negative image distance; so with the object at

10

cm it cannot form a real image at

+ 100

cm on the other side. (c) Drawn to the scale 1 cm =

5

cm with the object at

20

cm (between

F_{1}

and

2 F_{1}

): the ray parallel to the axis refracts through

F_{2}

and the ray through O goes straight on; they meet about

60

cm behind the lens, where the image is real, inverted and about three times the height of the object (see the diagram). The magnification is approximately

- 3

🔒 Step-by-step solution

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

Answer

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