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A needle placed 50 cm from a lens forms an image on a screen placed 75cm on the other side of the lens. What is the type of the lens? Find it's focal length. If the length of the Needle is 5cm, what is the length of image?
Asked by ananyamohanty2005 | 28 Dec, 2020, 11:06: PM
we have lens equation :-  (1/v) - (1/u) = 1/f

where v = 75 cm , lens-to-image distance ,

u = -50 cm , lens-to-object distance by considering cartesian sign convention and

f is focal length of lens.

1/f = (1/75)  + (1/50) = (1/30)  , hence focal length of lens is 30 cm.

Type of lens is converging lens or convex lens .

Magnification m = v/u = 75/(-50) = -1.5  ;   Hence imag size = -1.5 × 5 = -7.5 cm

image is real and inverted
Answered by Thiyagarajan K | 29 Dec, 2020, 08:51: AM

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