A convex lens forms an image of an object on a screen 30 cm from the lens. When the lens is moved 90 cm towards the object, then the image is again fo

Question

A convex lens forms an image of an object on a screen 30 cm from the lens. When the lens is moved 90 cm towards the object, then the image is again fo

A convex lens forms an image of an object on a screen 30 cm from the lens. When the lens is moved 90 cm towards the object, then the image is again formed on the screen. Then, the focal length of the lens is
A. 13 cm
B. 24 cm
C. 33 cm
D. 40 cm

Monjur Alahi

14 views

2025-01-04 04:42

1 Answers

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Salim Ahmed Kawsar · Accepted Answer · 2023-02-05T15:29:48+06:00

Salim Ahmed Kawsar

Correct Answer - B
(b) We have , `1/v-1/u=1/f`
`because 1/30-1/((-u))=1/for 1/30+1/u=1/f`
Similarly, , `1/120+1/(u-90)=1/f`
On solving Eqs. (i) and (ii), we get
The focal length of the lens, f= 24 cm

14 views Answered 2023-02-05 15:29