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# Question 17: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

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Question 17: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

(a) Figure shows a cross-section of’light pipe’ made of a glass fiber of refractive index 1.68. The outer covering of the pipe is made of a material of refractive index 1.44. What is the range of the angles of the incident rays with the axis of the pipe for which total reflections inside the pipe take place, as shown in the figure.
(b) What is the answer if there is no outer covering of the pipe?

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1. Solution:

(a) Let us first derive the condition for total internal reflection. Critical angle for the interface of medium 1 and medium 2.

Condition for total internal reflection from core to cladding

Now, for refraction at first surface air to core.

Thus all incident rays which makes angle of incidence between 0° and 60° will suffer total internal reflection in the optical fiber.
(b) When there is no outer covering critical angle from core to surface.

Thus all incident rays at first surface 0° to 90° will suffer total internal reflection inside core.

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NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments