A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam, at 10cm from the screen. It is found that the screen has to be moved 8 cm further away from the lens to obtain a sharp image. Find the focal length and nature of lens.

A converging beam of light forms a sharp image on a screen. A lens is

1.	A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam, at 10cm from the screen. It is found that the screen has to be moved 8 cm further away from the lens to obtain a sharp image. Find the focal length and nature of lens.
Answer» Solution : As SHOWN in figure, in this problem, object Ois virtual while image I REAL, so that u = 10 cm and v=(10+8)= 18 cm and hence from lens FORMULA, `(1)/(v) -(1)/(u ) =(1)/(f) ` we have `(1)/(18) -(1)/(10) =(1)/(f)` which on simplification gives `f= -(90)/(4) 22.5cm ` i.e., the lens is diverging (CONCAVE) of focal length 22.5 cm

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A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam, at 10cm from the screen. It is found that the screen has to be moved 8 cm further away from the lens to obtain a sharp image. Find the focal length and nature of lens.

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