An emission spectrum is formed when a substance emits light, appearing as bright lines against a dark background. An absorption spectrum is formed when a substance absorbs some of the light passing through it, appearing as dark lines on a continuous spectrum. For a given element, the lines in both spectra lie at the same wavelengths because they arise from the same energy levels of the atom.
To sum it up
- Radiative: The atom releases energy and emits a photon – we see bright colored lines on a dark background.
- Absorption: the atom absorbs a photon and moves to a higher energy level – dark lines appear in the continuous spectrum.
- Conditions: Emission is generated by excited, rarefied gas; absorption is generated by cooler gas in front of a continuous-spectrum light source.
- General property: The position of the lines is the "fingerprint" of the element, which is the same in both spectra.
Where do spectral lines come from?
Electrons in an atom can only have certain, discrete energies—we call them energy levels . An electron's transition between two levels involves the emission or absorption of a photon with an energy exactly equal to the difference between these levels. The photon's energy determines its wavelength, or, in the visible spectrum, its color.
Each element has its own level structure and, therefore, its own unique set of lines. This is the basis spectroscopy , or spectral analysis: the lines can be used to determine the composition of a sample - even if it is light years away from the star.
Radiation spectrum – how is it created?
In order for an atom to emit light, it must first be excited , meaning it needs to be energized—by heating in a flame, an electric discharge, or exposure to light. The excited electron returns to a lower energy level and emits a photon. By scattering this light using a prism or diffraction grating, we see separate bright lines on a dark background .
Real life examples include: gas discharge lamps Sodium street lamps emit a characteristic yellow-orange light, while neon tubes emit a reddish-orange light. The same mechanism is used in school flame tests : sodium salts color the flame yellow, and copper salts color it green or blue-green.
Absorption spectrum – how is it formed?
To form an absorption spectrum, two things are needed: continuous spectrum light source (for example, a hot solid body or the hot interior of a star) and colder environment, through which light passes. Atoms in the medium absorb photons only with energies corresponding to their transitions and move to higher energy levels.
As a result, in the rainbow band of the continuous spectrum appear dark lines — "missing" wavelengths. The most famous example is Fraunhofer lines in the solar spectrum, described by Joseph von Fraunhofer in 1814. They arise when gases in the solar atmosphere absorb some of the light emitted by the photosphere.
Comparison of emission and absorption spectra
| Characteristic | Emission spectrum | Absorption spectrum |
|---|---|---|
| Appearance | Light lines on a dark background | Dark lines against a continuous spectrum |
| Process in an atom | The electron moves to a lower level, emitting a photon. | The electron moves to a higher level, the photon is absorbed. |
| Energy direction | An atom releases energy. | An atom absorbs energy |
| Source needed | Excited luminous substance itself | External continuous spectrum source |
| Typical conditions | Hot, rarefied gas | The cooler gas is in front of the hotter source. |
| Line location | Element characteristics | The same wavelengths as in the radiation spectrum. |
| Examples | Neon, sodium lamp, flame test | Fraunhofer lines in the solar spectrum |
It's worth remembering that an absorption spectrum typically doesn't contain all the lines of the emission spectrum. In a cold gas, most atoms are in their ground state, so it absorbs primarily photons corresponding to transitions from the lowest energy levels.
Continuous spectrum is the third type
In addition to linear spectra, there are continuous spectrum — a smooth, continuous transition of all colors. These spectra are emitted by hot solids, liquids, and dense gases, such as the filament of a standard light bulb. This combination of three spectral types is described Kirchhoff's laws , formulated in the mid-19th century based on the research of Gustav Kirchhoff and Robert Bunsen. They noticed that dark lines in the solar spectrum overlap with light lines of heated elements, which helped explain the composition of the solar atmosphere.
What are both spectra used for?
- In astronomy The absorption spectra of stars allow us to determine their chemical composition and temperature, and the shift of their lines allows us to measure their velocity (the Doppler effect). In the 19th century, a line of an unknown element was discovered in the spectrum of the Sun—thus, helium was discovered, even before it was found on Earth.
- Analytical chemistry – absorption and emission spectrometry – used to determine elements, such as metals, in samples of water, soil or food products.
- Nebulae — luminous clouds of gas around hot stars — produce emission spectra that, among other things, allow the identification of hydrogen.
- Lighting and technology – the color of gas discharge lamps depends on the emission lines of gas or metal vapor inside them.
Frequently Asked Questions (FAQ)
How to distinguish an emission spectrum from an absorption spectrum in a figure?
If you see a few colored lines on a black background, that's an emission spectrum. If you see a full rainbow crossed by black lines, that's an absorption spectrum.
Why do the emission and absorption lines of a given element lie in the same place?
Because they correspond to the same transitions between energy levels. A photon with a given energy is emitted when an electron moves from a higher to a lower level, and is absorbed when it moves from a lower to a higher level.
Is the solar spectrum emission or absorption?
Sunlight has a continuous spectrum with dark absorption lines (Fraunhofer lines) superimposed on it. Therefore, it is said to have an absorption spectrum.
What is a linear spectrum?
This is a spectrum consisting of individual lines, rather than a continuous band of colors. Both emission spectra (light lines) and absorption spectra (dark lines) can be linear. They are typical of atomic gases.
How does a continuous spectrum differ from a linear spectrum?
A continuous spectrum contains all wavelengths in a given range and results from radiation from hot solids or dense gases. A linear spectrum contains only selected wavelengths characteristic of a given element.
See also
- What is the difference between an optical microscope and an electron microscope?
- What is the difference between OLED and QLED?
- What is the difference between a telescope and a regular telescope?
- What is the difference between a meteor and a meteorite?