1.

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens ? Draw the ray diagram.

Answer»

SOLUTION :Focal lengths of concave lens (oF1)
`f=-15 cm`
Image distance, `V=-10 cm`
According to the lens formula,
`(1)/(v)-(1)/(u)=(1)/(f)`
`(1)/(u)=(1)/(v)-(1)/(f)`
`=(-1)/(10)-(1)/(-15)=(-1)/(10)+(1)/(15)=(-5)/(150)`
`u=-30 cm`.
The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the FOLLOWING ray diagram.


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