Question

A concave mirror produces a three times enlarged image of an object placed at 10cm infront of it .find c

But if we take the magnification to be positive we get [virtual imageas produced if the object is placed between o and f]then we get c as 30cm
but if we take magnification as negative[real image,as produced if onject is placed between c and f] then we get c as 15

Now among this which should be considered

Answer 1

u= -10cm m=+3= -v/u =)v=30cm 1/f=1/v+1/u =)1/f=u+v/uv =)1/f = -10+30/-300 =20/-300 =)f= -15cm R=2f= -30cm as it is pretty clear from the solution given above , that u have to take m as +ve. We always take m as positive whenever we use this formulae because we have already assigned -ve in this formulae itself look clearly u'll find -v/u .there is a negative already in it