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The Balmer series consists of hydrogen transitions terminating at principal quantum number $n=2$. Its emission wavelengths obey $1/\lambda =R_H(1/4-1/n_i^2)$ for $n_i=3,4,\dots$, a special case of the Rydberg formula.

The first transitions fall in the visible part of the electromagnetic spectrum, while the high-level lines converge toward a shorter-wavelength limit with ${\lambda }_{\lim }=4/R_H$. A hydrogen discharge observed through a spectroscope shows separated lines rather than a smooth continuum because the atomic energy differences are discrete.

The same level separations can produce absorption when atoms initially occupy the lower state. Temperature, excitation and the number of atoms in that state affect line strength; a wavelength formula alone does not predict the observed relative intensities.

A chest radiograph is an X-ray transmission image through the chest. Different paths encounter different compositions and thicknesses, producing different detected intensities through attenuation. The detector records a projection through the body rather than a physically isolated slice.

For a narrow monochromatic beam, a spatially varying linear attenuation coefficient gives $I=I_0\exp [-\int \mu (s)ds]$. Real sources emit an electromagnetic spectrum of photon energies, and scattered radiation contributes additional signal. Overlapping structures can therefore be ambiguous in a single projection; detector display brightness also depends on processing and must not be confused directly with one local material density.

The electromagnetic spectrum organizes electromagnetic radiation by frequency or wavelength. In vacuum, $c=\lambda \nu$ and the photon energy is $E=h\nu$. Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays belong to the same physical field, with conventional band boundaries.

Different bands couple differently to matter and require different sources and detectors. The visible band is only a small portion of the full range. Frequency remains unchanged at a stationary interface, while wavelength generally changes with propagation speed, so a quoted wavelength needs a medium convention when that distinction matters.

Galactic multiwavelength astronomy compares observations of the Milky Way in different parts of the electromagnetic spectrum to distinguish its overlapping physical components. The neutral-hydrogen 21-centimeter line traces neutral atomic gas, carbon monoxide spectral lines trace molecular clouds, and infrared emission reveals warm interstellar dust and stars hidden at visible wavelengths. Radio waves from synchrotron radiation trace energetic electrons in magnetic fields, while X-rays reveal hot plasma and compact sources. Maps of the same sky region can therefore test whether a feature is a stellar concentration, an obscuring cloud or shocked gas. Comparing them also requires attention to differing angular resolution and foreground absorption.^[ref]