Question

A beam of ultraviolet light of all wavelengths passes through hydrogen gas at room temperature, in the $x$-direction. Assume that all photons emitted due to electron transitions inside the gas emerge in the $y$-direction. If A and B are the lights emerging from the gas in $x$ and $y$ directions respectively then

• A. Some of the incident wavelengths will be absent in A.
• B. Only those wavelengths will be present in $B$ which are absent in $A$.
• C. $B$ will contain some visible light
• D. $B$ will contain some infrared light.

Answer 1

Answer: (A), (C), (D)

The correct options are
A Some of the incident wavelengths will be absent in A.
C $B$ will contain some visible light
D $B$ will contain some infrared light.

Given, incident wavelengths travel in x-direction which are in U.V region, which means they can excite an electron from n=1 to higher levels. Hence, some of the incident wavelengths will be absent in A.
Photons emerging due to transitions of electrons to their ground state contains visible region wavelengths if transition is done into n=2. If transition is done into n>2, photons contain I.R wavelengths.
Therefore, A,C,D is the correct choice.