Question

Light of wavelength $5880A^o$ is incidents on a thin glass plate $(\mu =1.5)$ such that the angle of refraction in the plate is ${60}^0$. The minimum thickness of the plate, so that it appears dark in the reflected light will be

• A. $3920A^o$
• B. $4372A^o$
• C. $5840A^o$
• D. $6312A^o$

Answer 1

Answer: (A)

$3920A^o$

From equation of destructive interference,
$2tcos\theta =\frac{\lambda }{n}$
where, $n$ is refractive index.
Using other values from the problem statement,
$t=\frac{5880A^o}{1.5}=3920A^o$