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Question

An unidentified gas X when heated, is found to radiate light of wavelengths 423 nm, 520 nm, and 640 nm. In another situation, after white light was passed through a cooled sample of gas X, certain wavelengths were found missing. The missing wavelengths could be -


A

495 nm

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B

423 nm

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C

570 nm

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D

640 nm

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Solution

The correct options are
B

423 nm


D

640 nm


When a gas radiates upon heating, typically, we find the radiation to consist of a discrete set of lines - called the emission spectrum. Also, when white light is passed through the same gas in a cooler temperature, we find that the light that comes out has a few missing wavelengths - the absorption spectrum.

Interestingly, they exactly correspond to each other - the missing lines in the absorption spectrum are literally the emission spectral lines, like the negative of a photographic film (I'll not be surprised if some of you don't know what the negative of a photo film is - ask your parents for some 90's wisdom!).

Look at the spectra of hydrogen gas to know what I mean. Both spectra have lines at exactly the same wavelengths (dark lines - absorption spectrum; bright lines - emission spectrum), as shown.


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