Question

The predominant yellow line in the spectrum of a sodium vapour lamp has a wavelength of $590nm$. What minimum accelerated potential is needed to excite this line in an electron tube having sodium vapours?

• A. $8.44$ volt.
• B. $2.11$ volt.
• C. $6.11$ volt.
• D. $4.22$ volt.

Answer 1

The correct option is B $2.11$ volt.

Energy$=\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{590\times {10}^{-9}}=3.369\times {10}^{-19}J$

Ex-potential $=$ Energy.

$\therefore$ Potential required (min)$=\frac{3.369\times {10}^{-19}}{1.6\times {10}^{-19}}v\approx 2.11v$