Since an absorbing gas reemits the light it absorbs, why are there dark lines in an absorption spectrum? That is why doesn't the reemitted light simply fill in the dark places?
If ALL of the absorbed light could be emitted in the same direction, then the dark places WOULD be filled in; however, the re-emitted light covers a spherical 360 degrees so much of it goes in another direction.
The presence of dark lines in an absorption spectrum can be explained by understanding the process of absorption and reemission of light by gases. When light passes through a gas, certain atoms or molecules in the gas can absorb specific wavelengths of light. This occurs when the energy of the incoming photons matches the energy required for an electron in the atom or molecule to transition to a higher energy state.
Once these atoms or molecules in the gas absorb specific wavelengths of light, the electrons are temporarily excited to higher energy levels. However, the excited electrons cannot remain in these higher states indefinitely and eventually return to their original energy levels. When this happens, the absorbed energy is reemitted in random directions, often with a different wavelength than the original absorbed light.
Now, here's why there are dark lines in an absorption spectrum: When the reemitted light is observed, the observers look at the light that has been scattered or is in the line of sight from the gas. If the reemitted light were to simply fill in the same wavelengths as the absorbed light, then we would indeed see a continuous spectrum without any dark lines. However, the chances of reemitted light being directed straight toward the observer are quite low.
Instead, the reemitted light from the gas is scattered in random directions, filling up all available space around the gas. Only a small fraction of the reemitted light is actually directed towards the observer's line of sight. So, what we observe is the scattered light that has reached our eyes or instruments.
The dark lines in an absorption spectrum correspond to the wavelengths of light that have been absorbed by the gas. These dark lines appear because the absorbed light is missing from the overall spectrum of reemitted light that reaches our observation point. Therefore, these dark lines represent the specific wavelengths of light that have been absorbed by the gas, leaving gaps in the observed spectrum.
In summary, the reemitted light from a gas does not simply fill in the dark places because it is scattered in various directions, and only a small fraction of it reaches the observer. The resulting absorption spectrum shows dark lines where specific wavelengths of light have been absorbed, representing the missing light that has been absorbed by the gas.