Question

A ray of light travelling in air have wavelength $\lambda ,$ frequency $n$ and velocity $v$. If this ray enters into water then these parameters are ${\lambda }^{\prime },n^{\prime }$ and $v^{\prime }$ respectively. Which of the following relation is correct?

• A. $\lambda ={\lambda }^{\prime }$
• B. $n=n^{\prime }$
• C. $v=v^{\prime }$
• D. $I=I^{\prime }$

Answer 1

The correct option is B $n=n^{\prime }$

From Snell's law,
$\frac{\sin i}{\sin r}=\frac{{\mu }_2}{{\mu }_1}=\frac{{\upsilon }_1}{{\upsilon }_2}=\frac{{\lambda }_1}{{\lambda }_2}$...........(1)
For air ${\mu }_1=1$ and for water ${\mu }_2=\mu$
Eq.(1) after putting the values :
$\Rightarrow \mu =\frac{\upsilon }{{\upsilon }^{\prime }}=\frac{\lambda }{{\lambda }^{\prime }}$
since, $\mu >1$
$\therefore v>v^{\prime }\&\lambda >{\lambda }^{\prime }$
Therefore, only frequency doesn't change, as it is only dependent on source of light.
Hence, correct option is $\left(b\right)$

Tips: Frequency of light does not get altered during reflection or refraction, because it is only the property of source producing the light.