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प्रश्न
A concave lens of 20 cm focal length forms an image 15 cm from the lens. Compute the object distance.
उत्तर
Focal length of concave lens f = - 20 cm (focal length is negative for a concave lens)
Image distance v = - 15 cm ( negative sign is given because the image formed by a concave lens is on the left side of the lens)
Using the lens formula:
`1/f=1/v-1/u`
`1/-20=1/-15-1/u`
`1/u=1/20+1/((-15)`
`1/u=(3-4)/60=-1/60`
∴ u=-60 cm.
The negative sign shows that the object is on the left side of the concave lens.
Therefore, the object is at a distance of 60 cm and on the left side of the concave lens.
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