JEE quesn doubt in ans

: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??

actually i hav FIITJEE archive which has given 2 answers , 72 and 75.

I have Brilliant-tutorials archive and they gave only 75 ;)