In my book, it is written that when an atomic gas or vapour is excited either by heating or electrifying it the electrons get excited and immediately de excite and emit light of specific wavelengths. This is how an emission line spectrum is obtained. An emission line spectrum can be considered a fingerprint for identification of a particular element. But I have a question. When we excite the atomic gas or vapour, the electrons in it get excited and when de-excite by jumping down to a lower energy level they emit energy in the form of electromagnetic radiations. But I also studied that when an electron emits a photon by jumping down to a lower energy level it can emit the photon in any random direction. Show all the atoms in the atomic gas or vapour will emit photons in random directions. Thus if we put a screen behind the atomic gas or vapour we won't observe any emission line spectrum because the photons are emitted in random directions, right?
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see a specific experiment, how the material is excited and how the emission spectra are recorded. http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/atspect.html – anna v Jul 08 '21 at 07:33
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The hydrogen emission spectrum, in the visible range, is shown here: https://physics.stackexchange.com/a/768678/313612. We do not need to collect all the emitted light, as in Roger Vadim’s answer below. – Ed V Jul 19 '23 at 23:50
2 Answers
It is correct that atoms will emit photons in random direction. However, some of these photons will be emitted in the direction of the screen, and this will be enough. If, for simplicity, we assume that the vapor fills a cubic volume, and one of the walls of this volume is the screen, then one atom out of 6 will be emitting in the direction of the screen. Note also that we are talking about a macroscopic number of atoms, i.e., the number of the order of the Avogadro constant, $N_A\approx 10^{23}$.
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Then if you try to obtain absorption spectrum, we will obtain a continuous spectrum instead, right? If they come back to the ground level in one transition they will emit photon which has the exact wavelength of a photon that they absorbed. Thus we won't say see dark lines in a coloured background as the electron emitted the photon of the exact same wavelength with it absorb which we see as dark lines in a coloured background, right? – RIPAN BARUAH Jul 08 '21 at 06:21
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You mean to try to obtain the absorption specrum immediately after the emission one is recorded? – Roger V. Jul 08 '21 at 07:10
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no. A completely new experiment to obtain an absorption spectrum. – RIPAN BARUAH Jul 10 '21 at 04:30
The direction of a photon has nothing to do with its energy. Regardless of direction, a given photon has a well-defined energy that is analogous to its colour. A spectrometer, e.g. a glass prism, can tell you what energies of photon are boiling off the atomic gas by looking at the photons that happen to hit the spectrometer.
The only sense in which direction matters is for a phenomenon known as Doppler broadening. At finite temperature, the atoms are moving around at random - some will move away from the detector, others towards. When you plot light intensity against frequency, this broadens the peak you get:
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