CBSE Class 12 · 2005 · All India · Geometrical OpticsShort Answer

A right-angled crown glass prism with critical angle

4 1^{\circ}

is placed before an object

P Q

in two positions as shown in the figures
(i) and
(ii). Trace the paths of the rays from

P

and

Q

passing through the prisms in the two cases.

Solution

The refracting faces are perpendicular, so light from

P

and

Q

enters normally and meets the hypotenuse at

4 5^{\circ}

. Since

4 5^{\circ} > θ_{c} = 4 1^{\circ}

, \emph{total internal reflection} occurs at the hypotenuse.

∙

\textbf{Position (i)} (prism acting as an erecting/inverting prism): the rays suffer two reflections / a

18 0^{\circ}

turn and the image

P^{'} Q^{'}

is inverted.

∙

\textbf{Position (ii)}: the rays are deviated through

9 0^{\circ}

, turning the beam by a right angle.
This is exactly how totally-reflecting prisms are used in periscopes and binoculars.

CBSE Class 12 Board · 2005 · All India · Subjective