Generation of X-ray laser flashes in an undulator: to generate the extremely short and intense X-ray laser flashes in the European XFEL, bunches of high-energy electrons are directed through special arrangements of magnets (undulators).
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in monochromators and spectrometers.
The most familiar example of light dispersion is probably a rainbow, in which dispersion causes the spatial separation of white light into components of different wavelengths (different colors). In a prism, material dispersion (a wavelength-dependent refractive index) causes different colors to refract at different angles, splitting again white light into a rainbow.
Ordinary light is made up of light of many different colors. You can see these colors if you bounce the light of a thin film on a highly reflective surface. Oil slicks show this effect, where you see a whole spectrum of colors that result from the light reflecting off the surface of the oil and whatever is underneath. Here, a thin polymer film is receding across a highly reflective silicon substrate, leaving behind small droplets in its wake. The different colors tell you the thickness of the film, and, as you can see, it changes as you go across the film. The droplets form because the polymer doesn't like the surface and tries to bead up, just like water on a nonstick pan.
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies. When a photon has about the right amount of energy to allow a change in the energy state of the system (in the case of an atom this is usually an electron changing orbitals), the photon is absorbed. Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field.
Our yellow-appearing Sun emits light of nearly every color. Here you can see all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device. This spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our white-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light. The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below.