Question

380 450 590 O 620 750

a) A photograph of an absorption spectrum is mostly bright with black spectral lines. A

photograph of an emission spectrum is black with bright spectral lines. Why are they different? Explain

in 4-6 sentences.

b) Electron are accelerated between parallel plates in a tube containing hydrogen gas at

ambient temperature. The potential difference across the plates is 12.9 V. What spectral lines will be visible in the emission spectrum? Make an energy diagram to support your answer.

Answer 1

a) in the absorption spectra we see those lines which are absorbed by an atom that,s why they apper black

similarly in emission spectra we see only those particular lines those an atom is emmiting .This shows that emission and absorption spectra are descrete not continuous and fails the theory of blackbody radiation and supports the quantum nature of light .

b)In hydogen atom only balmer series spectra is only fall in visible region

$\lambda=B\frac{n^2}{n^2 -2^2}$

B is constant which is equal to

B=364 .5

so, for n=3

$\lambda=$656.3 nm in red region

for n=4

$\lambda=$486.2 nm in aqua region

for n=5

$\lambda=$434.02 nm in blue

for n=6

$\lambda=$410.17 nm in voilet

we see these for line in visible spectra.