Would like to know how to find it. The answer is given as -67 cm.
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The focal length of a lens in meters is the reciprocal of its power in dioptres:
f = 1/(2.5 dioptre) = 0.4 m = 40 cm
then you use the lens equation:
1/o + 1/i = 1/f
You know the object distance (o) and the focal length (f). Solve the equation for the image distance (i):
1/i = 1/f - 1/o
i = 1/(1/f - 1/o)
and plug in your numbers:
i = 1/( 1/(40 cm) - 1/(25 cm) )
i = -67 cm
The negative sign means that it is a virtual image formed in front of the lens
f = 1/(2.5 dioptre) = 0.4 m = 40 cm
then you use the lens equation:
1/o + 1/i = 1/f
You know the object distance (o) and the focal length (f). Solve the equation for the image distance (i):
1/i = 1/f - 1/o
i = 1/(1/f - 1/o)
and plug in your numbers:
i = 1/( 1/(40 cm) - 1/(25 cm) )
i = -67 cm
The negative sign means that it is a virtual image formed in front of the lens