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Geometrical Optics question

2020 · 2 Sep · Shift 2 · Q60
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Geometrical Optics question

2020 · 2 Sep · Shift 2 · Q60

JEE MainPhysicsGeometrical OpticsNumerical+4 / −1
A light ray enters a solid glass sphere of refractive index μ\muμ=3\sqrt 33​ at an angle of incidence 60o. The ray is both reflected and refracted at the farther surface of the sphere. The angle (in degrees) between the reflected and refracted rays at this surface is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 90

  1. Refraction at the first surface

A ray enters the glass sphere from air.

  • Refractive index of air: n1=1n_1=1n1​=1
  • Refractive index of glass: n2=μ=3n_2=\mu=\sqrt{3}n2​=μ=3​
  • Angle of incidence at first surface: i=60∘i=60^\circi=60∘

Using Snell’s law: n1sin⁡i=n2sin⁡rn_1\sin i=n_2\sin rn1​sini=n2​sinr 1⋅sin⁡60∘=3sin⁡r1\cdot \sin 60^\circ=\sqrt{3}\sin r1⋅sin60∘=3​sinr 32=3sin⁡r\frac{\sqrt{3}}{2}=\sqrt{3}\sin r23​​=3​sinr sin⁡r=12\sin r=\frac{1}{2}sinr=21​ So, r=30∘r=30^\circr=30∘

Thus, inside the sphere, the ray makes an angle 30∘30^\circ30∘ with the normal at the first surface.


  1. Angle of incidence at the second surface

For a sphere, the normals at the two points of intersection are along the radii. A standard result for a ray inside a sphere is that the angle the ray makes with the normal at the second surface is the same as the refracted angle at the first surface.

Hence, angle of incidence at the farther surface is: i2=30∘i_2=30^\circi2​=30∘


  1. Reflection and refraction at the farther surface

At the second surface, the ray goes from glass (3)(\sqrt{3})(3​) to air (1)(1)(1).

Using Snell’s law again: 3sin⁡30∘=1⋅sin⁡r2\sqrt{3}\sin 30^\circ = 1\cdot \sin r_23​sin30∘=1⋅sinr2​ 3⋅12=sin⁡r2\sqrt{3}\cdot \frac{1}{2}=\sin r_23​⋅21​=sinr2​ sin⁡r2=32\sin r_2=\frac{\sqrt{3}}{2}sinr2​=23​​ So, r2=60∘r_2=60^\circr2​=60∘

The reflected ray makes the same angle with the normal as the incident ray: angle of reflection=30∘\text{angle of reflection}=30^\circangle of reflection=30∘


  1. Angle between reflected and refracted rays

At the second surface:

  • reflected ray makes 30∘30^\circ30∘ with the normal inside the glass
  • refracted ray makes 60∘60^\circ60∘ with the normal outside the glass

These two rays lie on opposite sides of the normal, so the angle between them is: 30∘+60∘=90∘30^\circ + 60^\circ = 90^\circ30∘+60∘=90∘


  1. Final answer

90\boxed{90}90​

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