Progressive refraction through glass layers

JEE Advanced 2017 Paper 1, Question 10

A monochromatic light is traveling in a medium of refractive index n=1.6. It enters a stack of glass layers from the bottom side at an angle \theta=30^{\circ}. The interfaces of the glass layers are parallel to each other. The refractive indices of different glass layers are monotonically decreasing as n_{m}=n-m \Delta n, where n_{m} is the refractive index of the m^{\rm th} slab and \Delta n=0.1 (see the figure). The ray is refracted out parallel to the interface between the (m-1)^{\rm th} and m^{\rm th} slabs from the right side of the stack. What is the value of m?

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Solution

Consider the trajectory of …

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Three glass cylinders

JEE Advanced 2019 Paper 2, Question 6

Three glass cylinders of equal height H=30 \, {\rm cm} and same refractive index n=1.5 are placed on a horizontal surface as shown in figure. Cylinder I has a flat top, cylinder II has a convex top and cylinder III has a concave top. The radii of curvature of the two curved tops are same (R=3 \, {\rm m}). If H_{1}, H_{2}, and H_{3} are the apparent depths of a point X on the bottom of the three cylinders, respectively, the correct statement(s) is/are:

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  1. H_{2}>H_{1}
  2. H_{3}>H_{1}
  3. H_{2}>H_{3}
  4. 0.8 \, {\rm cm} < (H_{2}-H_{1}) < 0.9 \, {\rm cm}

Solution

We can get some intuition for what happens in each case by drawing some simple ray diagrams, as …

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Total internal reflection from a prism

JEE Advanced 2019 Paper 2, Question 11

A monochromatic light is incident from air on a refracting surface of a prism of angle 75^{\circ} and refractive index n_{0}=\sqrt{3}. The other refracting surface of the prism is coated by a thin film of material of refractive index n as shown in figure. The light suffers total internal reflection at the coated prism surface for an incidence angle of \theta \leq 60^{\circ}. What is the value of n^{2}?

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Solution

First we trace the given incident ray through the prism to find the angle at which it is incident on the opposite face (see figure below). For …

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Delay in a fiber optic cable

JEE Advanced 2019 Paper 1, Question 18

A planar structure of length L and width W is made of two different optical media of refractive indices n_{1}=1.5 and n_{2}=1.44 as shown in figure. If L \gg W, a ray entering from end {\rm AB} will emerge from end {\rm CD} only if the total internal reflection condition is met inside the structure. For L=9.6 m, if the incident angle \theta is varied, the maximum time taken by a ray to exit the plane \mathrm{CD} is t \times 10^{-9} s. Determine t.

[ Speed of light c=3 \times 10^{8} m/s. ]

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Solution

To ensure that the ray spends the maximum possible time inside the media, we should arrange it so …

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