Question

an object is placed at a distance of 10 cm from a lens of power -4 diopters find the position and nature of image formed

Answer 1

u = -10 cm

P = - 4 D

So f = -25 cm

Using lens formula , 1/f = 1/v -1/ u

1/-25 = 1/v - 1/ - 10

By calculation we have v= - 7.14 cm

Image formed is Virtual and Erect

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Answer 2

is it mirror or lens?

i admit my mistake. it is a lens. magnification = -image distance/object distance = +2 (i assume positive). therefore, image distance is -20 cm, i.e., it is a virtual, erect image. therefore, the focal length will be 20 cm i.e., 0.2m, therefore, power will be -1/(focal length in meters), if i remember the formula correctly. this gives you the result -5 dioptre for power

i just now searched and i find the definition to be P = 1/f, with a positive sign!

then the answer could be +5 dioptre also

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