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Question

Under what condition, a concave lens can make a real image?

A
Real object placed between optical centre O and and focus F2.
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B
Virtual object placed between optical centre O and focus F1.
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C
Real object placed at focus F2.
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D
Real object placed between and 2F2.
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Solution

The correct option is B Virtual object placed between optical centre O and focus F1.
Applying lens formula,
1v1u=1f
We will be substituting only sign convention of f and based on imposed condition u will be obtained.
ff
For a concave lens,
1v1u=1f
1v=1u1f ...(i)
For a real image, it must form along direction of incidence.
i.e. v+ve
Thus, for Eq. (i) to be hold,
1u1f>0
or, u<f and u must be positive.
u should be between optical centre O & F1.
Why this question?
It intends to challenge your fundamental understanding of sign convention of virtual object.

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