If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 D, what is its focal length in the Cartesian system?
It is a concave lens.
Power of lens = 10 D
P = 1/f[in metres]
f = 1/P
f = 1/10
f = 0.1 m
So focal length = -10 cm
Negative sign indicates concave lens.
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