Question

the filament of a lamp is 80cm from a screen and a converging lens forms an
image of it on a screen,magnified three times.Find the distance of the lens from the filament and the focal length of the lens​

Answer 1

Answer:

The filament of a lamp is 80 cm from a screen and a converging lens forms an image of it on a screen, magnified three times. Find the distance of the lens from the filament and the focal length of the lens.

Explanation:

-u + v = 80 cm -------(1)

m = -3 (The image is real, since it form on a screen)

m = v/u =- 3

v = -3u

-u – 3u = 80

-4u = 80

U = - 20 cm

Distance of lens from filament is 20 cm

V = - 3u = 60 cm

Lens formula: 1/v – 1/u = 1/f

1/60 – 1/-20 = 1/f

1/f = (1 + 3)/60 = 4/60

f = 15 cm

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