# a convex lens of power 4D is placed at a distance of 40cm from the wall . at what distance from the lens should a candle be placed so that is's imsge is formed on the wall , solve and explain why we have not taken image distance negative in this ques. also explain when we take image distance positive or negative

### Asked by Sanchit Sharma | 2nd Mar, 2015, 07:20: PM

Expert Answer:

### Given that the power of the lens, P= 4D
Therefore the focal length of the lens, f = 1/ 4 =0.25 m = 25 cm
Given that the image distance, v = 40 cm
According to lens formula:
So the candle should be placed at a distance of 66.67 cm from the lens so as to form an image on the wall.
According to sign convention:
The distance measured from the optical centre to the left side are taken as negative, hence the object distance, u will be negative.
On the other hand the distance measured from optical centre of the lens to the right side are taken as positive.As the image is formed to the right side of the lens on the wall, the image distance,v is taken positive.

### Answered by Jyothi Nair | 3rd Mar, 2015, 11:06: AM

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