1.

A 10 mm long pin is placed vertically in front of a concave mirror. A 5 mm long image of the pin isformed at 30 cm in front of the mirror. The focallength of this mirror is(a) 30 cm (b) 20 cm (c) 40 cm (d) 60 cm​

Answer»

Answer:Apply the RELATION - (v / u) = m = hi / ho asAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}Answer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmApply mirror formula asAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmApply mirror formula as= (1 / v) + (1 / u)= 1 / fAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmApply mirror formula as= (1 / v) + (1 / u)= 1 / f= 1 / (- 60) + 1 / (- 30)= 1 / fAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmApply mirror formula as= (1 / v) + (1 / u)= 1 / f= 1 / (- 60) + 1 / (- 30)= 1 / f= 1 / (- 20) = 1 / f or f = - 20 cmAnswer:Apply the relation - (v / u) = m = hi / ho as= (- 5) / 10 = - {(- 30) / u}=u = - 60 cmApply mirror formula as= (1 / v) + (1 / u)= 1 / f= 1 / (- 60) + 1 / (- 30)= 1 / f= 1 / (- 20) = 1 / f or f = - 20 cmHence , the CORRECT choice is b) -20 CM. The (-) ve sign indicates that the FOCAL length lies in front of the mirrorSo option b is correct


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