1i m not even getting how to start [2]...iska jitna aapse bana utna toh solution batao
A plano-convex lens has a thickness of 4 cm. When placed on a horiz. table , with the curved surface in contact with it , the apparent depth of the bottom most pt. of the lens is found to be 3 cm. If the lens is inverted such that the the table , the apparent depth of the centre of the plane face of the lens is found to be 25/8 cm. Find the focal length of the lens.
I m gettin the ans 75 cm
while the ans given is 72 cm and 75 cm....
can som1 explain how the ans can be 72 also??
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6 Answers
1
6:P
look what i did is this:
In the 1st case , the real thickness = 4 cm and apparent thickness = 3 cm.
therefore , ref. index = 4/3
In the 2nd case , the rays travel frm Pole thru glass . They r refracted on the spherical surface , and give its image at a distance 25/8 cm frm point P on the curved surface.
so u = - 4 cm
v = -25/8 cm
r is radius of curvature
so usin the formula : \frac{\mu }{v} - \frac{1}{u} = \frac{(\mu -1)}{r}
r = -25 cm.
then applyin lens maker's formula ,
focal length comes out to be 75 cm.
but what abt the other one , 72??
1Who gave 72 cm?
I have done this question before and the answer is only 75cm.