1.

An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens? Is it a converging lens or diverging lens?

Answer»

Solution :GIVEN:
Object distance (U) = 60 cm
Image distance (v) = 20 cm
(virtual image)
To find: Focal length
Type of lens
Solution :`1/v - 1/u = 1/F `
`1/-20 - 1/-60 = 1/f `
`1/20 + 1/60 = 1/f `
`(-3 + 1)/(60) = 1/f`
`(-2)/60 = 1/f`
`-1/30 = 1/f `
f = -30
`therefore` Focal length = 30 cm
Type of lens = DIVERGING - CONCAVE lens


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