Q144 / 180 · AIIMS 2006 · Physics

Assertion : In optical fibre, the diameter of the core is kept small. Reason : This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

  • Correct answer
    A
    If both assertion and reason are true and reason is the correct explanation of the assertion
  • B
    If both assertion and reason are true but reason is not the correct explanation of the assertion
  • C
    If assertion is true, but reason is false
  • D
    Both assertion and reason are false statements.

Solution

Total internal reflection will be for rays incident at an angle greater than $C$. If the fibres are thin, only rays almost parallel will pass, further for nonparaxial rays, the angle of incidence is large and therefore they also will be totally internally reflected. Both are right.

Hence, the correct answer is option: 'A'.

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