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A pin of length 2 cm is placed perpendicular to the principal axis of a converging lens.An inverted image of size 1cm is formed at a distance of 40 cm from the pin . Find the focal length and its distance from the pin. { In many solutions i ‘ve seen that they take v=40 but as per my thining , the question says that u + v=40 as it says ( is formed at a distance of 40 cm from the pin. ) }

A pin of length 2 cm is placed perpendicular to the principal axis of a converging lens.An inverted image of size 1cm is formed at a distance of 40 cm from the pin.Find the focal length and its distance from the pin.
{In many solutions i ‘ve seen that they take v=40 but as per my thining , the question says that
u + v=40 as it says ( is formed at a distance of 40 cm from the pin.)}

Grade:10

3 Answers

SKgenius
13 Points
6 years ago
H = 2 cm h` = - 1 cm(image is inverted => image is real.) m = - v/u = h`/h = -1/2 u = - 2 v u is negative and v is positive -u + v = 40 cm 3 v = 40 cm => v = 40/3 cm u = - 80/3 cm 1/f = 1/v - 1/u = 3/40 + 3/80 = 9/80 f = 80/9 cm
SKgenius
13 Points
6 years ago
H = 2 cm h` = 1 cm image is inverted => image is real. m = - v/u = h`/h = -1/2 u = - 2 v u is negative and v is positive -u + v = 40 cm 3 v = 40 cm => v = 40/3 cm u = - 80/3 cm 1/f = 1/v - 1/u = 3/40 + 3/80 = 9/80 f = 80/9 cm
SKgenius
13 Points
6 years ago
h=2 h`= - 1m=h`/h = - 1/2v/u = - 1/2 => u = - 2v - - - - 1u is negative and v positiveSo, - u + v = 40 - - - - 2Solving 1 and 2, we get u= - 80/3 and v= 40/3Put in equation 1/v - 1/u = 1/f you will get f = 80/9 cm

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