1)) The distance between two point sources of light is 24cm.Find out where would you place a converging lens of focal length 9cm, so that the images formed at the same point.

 

2)) A plano convex lens of thickness 4cm, when placed on a horizontal table with the curved face in contact with it,the apparent depth of the bottommost point of the lens is found to be 3cm. If the lens is inverted such that the plane face is in contact with the table. The apparent depth of the center of the plane surface of the lens is found to be 25/8 cm.find the focal length of the lens.((Assume thickness to be negligible while finding fo.len.))

 

pls help sir!!

its urgent!!

HIII

 

answer 2nd) part 1) when the curved surface is in contact with table

 

u = 4 , v = 3

 

₁ = 1 , we have to find ₂ , R = infinity

 

as the refraction occurs frm denser to rarer medium

 

(₂ / u ) - (₁ / v ) = ₂ - ₁ / R , as R = infinity

 

 

(₂ / u ) - (₁ / v ) = 0

 

put the values , ₂ comes out to be 4/3

 

 

part 2) when the plane surface is in contact with the table

 

u = 4 , v = 25/8

 

₁ = 1 , ₂ = 4/3

 

we have to find R

 

as refraction occurs frm denser to rarer medium

 

(₂ / u) - (₁ / v) = (₂ - ₁) / R

 

put the values , we get R = 25cm

 

for plano convex lens

 

R₁ = R , = 25cm

 

R₂ = infinity

 

(1/f) = ( - 1) ((1/R₁) - (1/R₂))

 

 

 = 4/3

 

put the values , u get f = 75 cms