# A Screen is Placed 90 Cm from an Object. the Image of the Object on the Screen is Formed by a Convex Lens at Two Different Locations Separated by 20 Cm. Determine the Focal Length of the Lens. - Physics

Numerical

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

#### Solution

Distance between the image (screen) and the object, D = 90 cm

Distance between two locations of the convex lens, d = 20 cm

Focal length of the lens = f

Focal length is related to d and D as:

"f" = ("D"^2 - "d"^2)/4"D"

= ((90)^2 - (20)^2)/(4 xx 90)

= 770/36

= 21.39 cm

Therefore, the focal length of the convex lens is 21.39 cm.

Concept: Refraction at Spherical Surfaces and Lenses - Refraction by a Lens
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Chapter 9: Ray Optics and Optical Instruments - Exercise [Page 346]

#### APPEARS IN

NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 9.20 | Page 346
NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 20 | Page 347
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