The electromagnetic spectrum
A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.
The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.
A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths
The electromagnetic spectrum, with the visible portion highlighted
17th century telescope
Plot of Earth's atmospheric opacity to various wavelengths of electromagnetic radiation. This is the surface-to-space opacity, the atmosphere is transparent to longwave radio transmissions within the troposphere but opaque to space due to the ionosphere.
The 60-inch Hale (debuted in 1908) considered to be the first modern large research reflecting telescope.
Plot of atmospheric opacity for terrestrial to terrestrial transmission showing the molecules responsible for some of the resonances
Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli-Fantina, Sicily
The primary mirror assembly of James Webb Space Telescope under construction. This is a segmented mirror and its coated with Gold to reflect (orange-red) visible light, through near-infrared to the mid-infrared
The amount of penetration of UV relative to altitude in Earth's ozone
Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.
Modern telescopes typically use CCDs instead of film for recording images. This is the sensor array in the Kepler spacecraft.
Hong Kong illuminated by colourful artificial lighting.
A 1.2-meter (47 in) reflecting telescope
Pierre Gassendi.
Christiaan Huygens.
The Very Large Array at Socorro, New Mexico, United States.
Thomas Young's sketch of a double-slit experiment showing diffraction. Young's experiments supported the theory that light consists of waves.
Einstein Observatory was a space-based focusing optical X-ray telescope from 1978.
The Compton Gamma Ray Observatory is released into orbit by the Space Shutte in 1991, and it would operate until the year 2000
The reflectors of HEGRA detect flashes of light in the atmosphere, thus detecting high energy particles
Equatorial-mounted Keplerian telescope
A diagram of the electromagnetic spectrum with the Earth's atmospheric transmittance (or opacity) and the types of telescopes used to image parts of the spectrum.
Six views of the Crab nebula supernova remnant, viewed at different wavelengths of light by various telescopes
The Five-hundred-meter Aperture Spherical radio Telescope in Guizhou, China, is the world's largest filled-aperture radio telescope

Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.

- Light

This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end.

- Electromagnetic spectrum

The word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors.

- Telescope

Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the telescope and microscope.

- Electromagnetic spectrum

Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io.

- Light

Optical telescopes, using visible light

- Telescope
The electromagnetic spectrum

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The wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests (on top), or troughs (on bottom), or corresponding zero crossings as shown.


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Spatial period of a periodic wave—the distance over which the wave's shape repeats.

Spatial period of a periodic wave—the distance over which the wave's shape repeats.

The wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests (on top), or troughs (on bottom), or corresponding zero crossings as shown.
Sinusoidal standing waves in a box that constrains the end points to be nodes will have an integer number of half wavelengths fitting in the box.
A standing wave (black) depicted as the sum of two propagating waves traveling in opposite directions (red and blue)
Wavelength is decreased in a medium with slower propagation.
Refraction: upon entering a medium where its speed is lower, the wave changes direction.
Separation of colors by a prism (click for animation)
Various local wavelengths on a crest-to-crest basis in an ocean wave approaching shore
A sinusoidal wave travelling in a nonuniform medium, with loss
A wave on a line of atoms can be interpreted according to a variety of wavelengths.
Near-periodic waves over shallow water
Wavelength of a periodic but non-sinusoidal waveform.
A propagating wave packet
Pattern of light intensity on a screen for light passing through two slits. The labels on the right refer to the difference of the path lengths from the two slits, which are idealized here as point sources.
Diffraction pattern of a double slit has a single-slit envelope.
Relationship between wavelength, angular wavelength, and other wave properties.

Examples of waves are sound waves, light, water waves and periodic electrical signals in a conductor.

The name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum.

Diffraction is the fundamental limitation on the resolving power of optical instruments, such as telescopes (including radiotelescopes) and microscopes.