Question

A plane polarised light is incident normally on a tourmaline plate. Its $E$ vector make an angle of ${60}^{\circ }$ with the optic axis of the plate. Find the percentage difference between initial and final intensities.

• A. $50$%
• B. $25$%
• C. $75$%
• D. $90$%

Answer 1

Answer: (C)

$75$%

Key Concept According to law of Malus, when a beam of completely plane polarised light is incident on an analyser, the resultant intensity of light (l) transmitted from an analyser varies directly as the square of cosine angle between plane of analyser and polariser.
Let initial intensity is $I$ and final intensity is $I^{\prime }$,
then
$I^{\prime }=I{\cos }^{2}60$
$=I{\left(\frac{1}{2}\right)}^2=\frac{I}{4}$
Now, % difference between initial and final intensities,
$=\frac{I-\frac{I}{4}}{I}\times 100=\frac{3I}{4}\times \frac{1}{r}\times 100$
$=\frac{3}{4}\times 100=75$%.