A concave lens has focal length 20cm at what distance from the lens 5 cm tall object to be placed so that to it will from the image at 15 cm from the lens .Also calculate size and nature of the image.
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u = ?
h = 5cm
f = - 20cm
v = - 15 cm
1 / f = 1 / u + 1 / v
1 /u = - 1/f + 1/v
1 /u = -1 / (- 20 ) + 1 /(-15)
1/u = 3 - 4 / 60
1 /u = - 1 /60
u = - 60 cm
m = h' / h = v / u
h' = h × v /u
h' = 5 × -15 / -60
h' = 75 / 60 = 1.25 cm
The image formed is diminished.
h = 5cm
f = - 20cm
v = - 15 cm
1 / f = 1 / u + 1 / v
1 /u = - 1/f + 1/v
1 /u = -1 / (- 20 ) + 1 /(-15)
1/u = 3 - 4 / 60
1 /u = - 1 /60
u = - 60 cm
m = h' / h = v / u
h' = h × v /u
h' = 5 × -15 / -60
h' = 75 / 60 = 1.25 cm
The image formed is diminished.
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