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How does a telescope work?
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Illustration comparing a reflecting telescope, which uses mirrors to focus light, and a refracting telescope, which uses lenses, both mounted on tripods and pointed at the night sky. Galileo's telescopes Side-by-side comparison of the Carina Nebula, with the James Webb Space Telescope image on the left showing sharp, detailed stars and gas clouds, and the Hubble image on the right with softer, more diffuse colors. Comparison between the Hubble and Webb space telescopes.
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Illustration comparing a reflecting telescope, which uses mirrors to focus light, and a refracting telescope, which uses lenses, both mounted on tripods and pointed at the night sky.
reflecting and refracting telescopes A reflecting telescope focuses light rays by reflecting them off a curved mirror. In the Newtonian type of reflector, left, a secondary mirror deflects the light rays through the eyepiece in the side of the tube. A refracting telescope, right, bends the light rays with a glass lens.

How does a telescope work?

Written by
Elijah Tan
Elijah Tan was an Editorial Intern at Encyclopædia Britannica. He is studying Astrophysics and Science Communication at the University of Chicago, where he writes for the Chicago Maroon.
Elijah Tan
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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•History

Telescopes have upended astronomy again and again by illuminating what was previously invisible to the naked eye.

Our eyes and our telescopes “see” in the same way, by collecting light rays and focusing them to a point. Our eyes collect light through our pupils, tiny openings only a few millimeters in diameter, and a lens inside our eyes focuses the light onto an optic nerve.

The difference between our naked eyes and a telescope is that a telescope collects far more light than our eyes, enabling it to capture much smaller and fainter objects. The more light a telescope can collect, the more powerful it is.

Galileo's telescopes
Galileo's telescopesTwo of Galileo's first telescopes; in the Museo Galileo, Florence.

The first telescopes increased the size of this opening beyond that of the pupil, allowing far more light to pass through, and used lenses to focus that light onto an eyepiece. Galileo’s telescope, which he used to make many astronomical discoveries, let light in through a circular opening 1.5 centimeters (0.6 inches) in diameter. These telescopes are known as refracting telescopes.

Most modern telescopes are reflecting telescopes, which means they collect and focus light using curved mirrors. (Lenses have chromatic aberration, meaning light of different colors gets bent differently, causing colored fringes around objects.) The Hubble Space Telescope collects all the light that meets its large primary mirror, 2.4 meters (7.9 feet) in diameter, and redirects it toward its scientific instruments. The James Webb Space Telescope has an even larger mirror, 6.5 meters (21.3 feet) in diameter, allowing it to pick up much more detail than the Hubble Space Telescope.

Elijah Tan