An Image Formed on a Screen is Three Times the Size of the Object. the Object and Screen Are 80 Cm Apart When the Image is Sharply Focussed. Calculate Focal Length of the Lens. - Science

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed.

Calculate focal length of the lens.

Solution

Magnification (m) = 3
Object distance (u) = ?
Image distance (v) = ?
Focal length (f) = ?
Distance between image and object (v + u) = 80 cm
v = 80-u
We know that:

m=v/u

or 3=(80-u)/u

or 3u=80-u

or 4u=80

or u=20cm

thus, v=80-u

=80-20=60 cm

Putting these values in lens formula, we get:

1/v-1/u=1/f

1/60-1/-20=1/f

(1+3)/60=1/f

f=60/4=15 cm

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APPEARS IN

Lakhmir Singh Class 10 Physics (Science)
Chapter 5 Refraction of Light
Q 20.2 | Page 257