Question

A rectangular block of glass is placed on a printed page lying on a horizontal surface. The minimum value of the refractive index of glass for which the letters on the page are not visible from any of the vertical faces of block is –(a) ≥ √3(b) ≥ 1.55(c) ≥ √2(d) ≥ 1.38

Answer 1

Light will no merge from the vertical angle BC as shown in figure

Where

sin i > sinθc (θc = 1/μ)

According to the snell's law at point O

sinθ = μ sin (90°-i)

      = μ cos i

cos i = sinθ/μ

and sin i = √1-cos²i

 = √1 - sin²θ/μ²

Moreover, the condition with no light emerge BC is

√1 - sin²θ/μ² > 1/μ

μ² = 1+sin²θ

So μ² = √2

Therefore the refractive index of a glass  is √2

Answer 2

I think it would help you