Finally, an absorption spectrum , like the spectrum of sunlight shown in the bottom of Fig. Absorption spectra are produced when light from a hot object travels through a cooler, dilute gas. When a photon with exactly the right wavelength encounters an atom of the cool gas, it is absorbed and its energy used to kick an electron into a higher orbit; if enough atoms of gas are present, all the photons of that wavelengths are absorbed, while photons with other wavelengths get through.
The atmospheres of stars produce absorption spectra. An element produces bright and dark lines with the same wavelengths. For example, hydrogen has three prominent lines with wavelengths of nm, nm, and nm; these appear dark if the hydrogen is absorbing light, and bright if it is emitting light, but the same three wavelengths are seen in either case. In some situations, we find spectra which mix different kinds of features: for example, a continuous spectrum with bright emission lines superimposed.
Some stars, as they age, produce continuous spectra with dark absorption lines and bright emission lines; this is usually a sign that the star is ejecting gas in a stellar wind. In the lab, we will explain how to use spectroscope, and how to adjust it so you can measure wavelengths accurately. You will then have a chance to view different types of spectra. We will set up several different discharge tubes, in which various elements are excited electrically.
You will be asked to identify these elements by looking at the light they produce using your spectroscope. The elements in question will be among those featured in Fig. We will also set up a light source which produces a bright spectral line, and ask you to measure the wavelength of this line. Once you've done this, you can identify the element involved by looking at the table printed on your spectrometer. Finally, you should take the spectroscope home for a week to look at various light sources and sketch their spectra.
In each case, classify the type of spectrum continuous, emission, absorption, or mixed and measure the wavelengths of any bright or dark lines you can see. You should look at:. Use this chart to sketch spectra of different light sources. If you see lines or other features, place them at the appropriate wavelength using the scale, and list the wavelengths you measure.
Thus, each element emits its own set of colours. Which spectrum is continuous? A continuous spectrum contains many different colors, or wavelengths, with no gaps. Perfectly white light shined through a prism causes dispersion of the light, and we see a rainbow. This is a continuous spectrum. Which type is the sun's visible light spectrum and why? Notice that the Sun's spectrum appears brightest or most intense in the yellow-green region. What is called Spectrum? A spectrum plural spectra or spectrums is a condition that is not limited to a specific set of values but can vary, without steps, across a continuum.
The word was first used scientifically in optics to describe the rainbow of colors in visible light after passing through a prism. What do emission lines tell us? From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star.
The spectral line also can tell us about any magnetic field of the star. The width of the line can tell us how fast the material is moving. In , a Danish physicist, Niels Bohr —; Nobel Prize in Physics, , proposed a theoretical model for the hydrogen atom that explained its emission spectrum. Using classical physics, Niels Bohr showed that the energy of an electron in a particular orbit is given by.
In that level, the electron is unbound from the nucleus and the atom has been separated into a negatively charged the electron and a positively charged the nucleus ion.
In this state the radius of the orbit is also infinite. The atom has been ionized. In his final years, he devoted himself to the peaceful application of atomic physics and to resolving political problems arising from the development of atomic weapons.
As n decreases, the energy holding the electron and the nucleus together becomes increasingly negative, the radius of the orbit shrinks and more energy is needed to ionize the atom. Because a hydrogen atom with its one electron in this orbit has the lowest possible energy, this is the ground state the most stable arrangement of electrons for an element or a compound , the most stable arrangement for a hydrogen atom.
Any arrangement of electrons that is higher in energy than the ground state. Except for the negative sign, this is the same equation that Rydberg obtained experimentally. Because a sample of hydrogen contains a large number of atoms, the intensity of the various lines in a line spectrum depends on the number of atoms in each excited state. In contemporary applications, electron transitions are used in timekeeping that needs to be exact.
Telecommunications systems, such as cell phones, depend on timing signals that are accurate to within a millionth of a second per day, as are the devices that control the US power grid. Global positioning system GPS signals must be accurate to within a billionth of a second per day, which is equivalent to gaining or losing no more than one second in 1,, years. Quantifying time requires finding an event with an interval that repeats on a regular basis. To achieve the accuracy required for modern purposes, physicists have turned to the atom.
The current standard used to calibrate clocks is the cesium atom. Supercooled cesium atoms are placed in a vacuum chamber and bombarded with microwaves whose frequencies are carefully controlled. When the frequency is exactly right, the atoms absorb enough energy to undergo an electronic transition to a higher-energy state. Decay to a lower-energy state emits radiation. In , the second was defined as the duration of 9,,, oscillations of the resonant frequency of a cesium atom, called the cesium clock.
Research is currently under way to develop the next generation of atomic clocks that promise to be even more accurate. Such devices would allow scientists to monitor vanishingly faint electromagnetic signals produced by nerve pathways in the brain and geologists to measure variations in gravitational fields, which cause fluctuations in time, that would aid in the discovery of oil or minerals. Calculate the wavelength of the lowest-energy line in the Lyman series to three significant figures.
In what region of the electromagnetic spectrum does it occur? When those atoms are given energy, the electrons absorb the energy and move to a higher energy level. These energy levels of the electrons in atoms are quantized, meaning again that the electron must move from one energy level to another in discrete steps, rather than continuously.
An excited state of an atom is a state where its potential energy is higher than the ground state. An atom in the excited state is not stable. When it returns back to the ground state, it releases the energy that it had previously gained in the form of electromagnetic radiation.
So how do atoms gain energy in the first place? One way is to pass an electric current through an enclosed sample of a gas at low pressure. Since the electron energy levels are unique for each element, every gas discharge tube will glow with a distinctive color, depending on the identity of the gas see below.
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