Question

two lenses A and B have focal length of 20 centimetre and - 10 centimetre respectively .what is the nature of the lens A and B

Answer 1

Lens A has focal length +20 cm, as the sign of focal length is positive hence the lens is a convex lens. Similarly the lens B of focal length -10 cm, which is negative so the lens B is a concave lens.
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Answer 2

Lenses A have focal length 20 cm.They image formed behind the mirror .so it is positive according to sign convention that also known as convex mirror.

Lenses B have focal length -10 cm . They image formed front of the mirror . According to sign convention that shows negative .Hence that mirror is also known as concave mirror.

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