First slide

Dispersion

Question

Light from sodium lamp is passed through cold sodium vapours, the spectrum of transmitted light consists of

Moderate

A

$A line at 5890 Å$
B

$A line at 5896 Å$
C

Sodium doublet lines
D

No spectral features

Solution

$We know that \frac{δ_{v} - δ_{r}}{δ_{mean}} = ω \Rightarrow Angular dispersion = δ_{v} - δ_{r} = θ = {ωδ}_{mean}$
According to Kirchhoff's law, a substance in the unexcited state will absorb these wavelengths which it emit in de-excitation. Hence no spectral features.

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