Ncert Exercises & Solutions

2.5 Yellow light emitted from a sodium lamp has a wavelength (λ) of 580 nm. Calculate the frequency (ν) and wavenumber () of the yellow light.

From the expression,
$λ = \frac{c}{v}$
We get,
$λ = \frac{c}{v}$ ……………………………………. (i)
Where, ν = frequency of yellow light

c = velocity of light in vacuum = 3 × $10^{8}$ m/s
λ = wavelength of yellow light = 580 nm = 580 × $10^{- 9}$ m
Substituting the values in expression (i):
$v = \frac{3 x 10^{8}}{580 x 10^{- 9}} = 5.17 x 10^{14} s^{- 1}$
Thus, frequency of yellow light emitted from the sodium lamp =
5.17 × $10^{14} s^{- 1}$
Wave number of yellow light $\vec{v} = \frac{1}{λ}$
$= \frac{1}{580 x 10^{- 19}} = 1.72 x 10^{6} m^{- 1}$

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