Question

The greatest thickness of a plano-convex glass lens appears to be 2 cm when observed normally through the plane face, and when observed through the curved face the greatest thickness appears to be 20/9 cms. If real thickness is 3 cm then the radius of curvature of the spherical surface is -

• A. 6.67 cm
• B. 10.0 cm
• C. 20.0 cm
• D. 2.22 cm

Answer 1

Answer: (B)

10.0 cm

Answer 2

When seen through plane face

d_apparent = Ž 2 = $\frac { 3 } { \mu }$ ...(1)

when seen through curved face

apply, $\frac { \mu _ { 2 } } { \mathrm { v } } - \frac { \mu _ { 1 } } { \mathrm { u } } = \frac { \mu _ { 2 } - \mu _ { 1 } } { \mathrm { R } }$ ...(2)

where m₂ = ref index of air

m₁ = ref index of glass i.e. m

v = $\frac { - 20 } { 9 }$cm ; u = – 3cm

Apply in 2 equation & then solve (1) & (2)

radius of curvature = 10 cm