CBSE Class 12 · 2003 · Geometrical OpticsShort Answer

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Define critical angle with reference to total internal reflection. Calculate the critical angle for a glass--air surface, if a ray of light which is incident in air on the glass surface is deviated through

1 5^{\circ}

when the angle of incidence is

4 5^{\circ}

Answer: Critical angle $θ_{c} = 4 5^{\circ}$ .

Solution

\textbf{Critical angle:} the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is

9 0^{\circ}

; for angles larger than this the ray is totally internally reflected.
Here the ray goes air

\to

glass with

i = 4 5^{\circ}

and is deviated by

1 5^{\circ}

, so the angle of refraction is

r = 4 5^{\circ} - 1 5^{\circ} = 3 0^{\circ}

μ = \frac{sin i}{sin r} = \frac{sin 4 5 ^{\circ}}{sin 3 0 ^{\circ}} = \frac{1/ 2}{1/2} = 2

sin θ_{c} = \frac{1}{μ} = \frac{1}{2} \Rightarrow θ_{c} = 4 5^{\circ}

CBSE Class 12 Board · 2003 · Subjective

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CBSE Class 12 · 2003 · Geometrical OpticsShort Answer

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