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How do the spectra of the other elements compare with the spectrum of a light bulb and...

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hardworker | Student, Undergraduate | (Level 1) Honors

Posted September 20, 2010 at 10:21 PM via web

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How do the spectra of the other elements compare with the spectrum of a light bulb and the spectrum of hydrogen?

Why is the light that is emitted by hydrogen different from the light that's emitted by the other elements? Which gases are used in street lamps, and why certain gases are chosen for certain locations?

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nessus | High School Teacher | (Level 2) Adjunct Educator

Posted July 10, 2011 at 3:34 PM (Answer #1)

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Hydrogen is the simplest atom with one electron. The electron in cold hydrogen is in its "ground state" which means it is orbitting as close to the nucleus as it can get. The electron can be given energy by heat, electricity etc. This can cause it to move away from the nucleus or jump to a higher "orbit". This is called excitation.

Satellites orbiting the Earth can make an almost infinite series of small increases in orbital radius but electrons cannot do that due to quantisation of energy at these microscopic sizes. This means that the electron can only ocupy certain discreet orbits.

An excited electron in hydrogen may jump to any "allowed' orbit (properly called orbital) and may fall straight back to the ground state when the energy it gained to make the jump is released as a single photon of definite frequency and hence colour. Or it may fall through any permutation of orbitals releasing photons of other frequencies. This is the cause of the line spectrum seen in a spectroscope. A neat homemade spectroscope can be made from a CD disc.

Every other element does this but because there are different numbers of electrons and different nuclear charge affecting their allowed orbitals they produce different line spectra.

Tungsten filament lamps (globes) emit a seemingly continuous spectrum similar to a rainbow because of the much larger number of electrons emitting a very large number of close frequencies. Individual lines can be resolved with a suitably sensitive spectroscope. This routinely done with starlight.

One useful element for economical street lamps is sodium (as an excited vapour) which has a particular electron transition (jump) that produces a bright yellow light.

Some types of street lamp use mercury vapour which emits UV photons that then excite a fluorescent material to emit visible light.

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