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Question
Why do diamonds sparkle?
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Solution
Critical angle for diamond is quite less and is only 24°. Besides these diamonds are cut very sharp, making a large number of the refracting surfaces. Hence when a ray of light enters a diamond, it suffers a series of total internal reflections on account of a very small critical angle. This makes a ray of light literally trapped within the diamond for a little time and this trapped light energy makes a diamond sparkle.
RELATED QUESTIONS
In the given figure PQ and PR are the two light rays emerging from an object P. The ray PQ is refracted as QS.
- State the special name given to the angle of incidence ∠PQN of the ray PQ.
- What is the angle of refraction for the refracted ray QS?
- Name the phenomenon that occurs if the angle of incidence ∠PQN is increased.
- The ray PR suffers partial reflection and refraction on the water-air surface. Give reason.
- Draw in the diagram the refracted ray for the incident ray PR and hence show the position of image of the object P by the letter P’ when seen vertically from above.
Plot a graph between
Sine of angle of incidence versus sine of angle of refraction,
Plot a graph between
Angle of incidence versus angle of refraction.
What is meant by the statement, ‘the critical angle for diamond is 24°’?
The diagram shows a point source P inside a water container. Three rays A, B, and C starting from P are shown up to the water surface. Show in the diagram the path of these rays after striking the water surface. The critical angle for the water-air pair is 48°.

Answer the following question.
What are the advantages of optical fibre communication over electronic communication?
Answer the following question.
Why is prism binoculars preferred over traditional binoculars? Describe its working in brief.
The angle of incidence at which the angle of refraction is Q£ is called the critical angle.
Find the value of θ in the given figure.

- Assertion (A): Propagation of light through an optical fibre is due to total internal reflection taking place at the core-cladding interface.
- Reason (R): Refractive index of the material of the cladding of the optical fibre is greater than that of the core.
