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, 1v−1u=1f
We will be substituting only sign convention of f and based on imposed condition u will be obtained. f→−f
For a concave lens, 1v−1u=1−f ⇒1v=1u−1f ...(i)
For a real image, it must form along direction of incidence.
i.e. v→+ve
Thus, for Eq. (i) to be hold, 1u−1f>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.