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How did Isaac Newton’s reflecting telescope differ from Galileo’s refracting telescope?
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Telescope
Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.
The primary difference between Isaac Newton’s reflecting telescope and Galileo’s refracting telescope lies in how they collect and focus light.
- Refracting Telescopes: Galileo’s design, known as a refractor, uses lenses to bend (refract) light and focus it onto an eyepiece. A major limitation of this design is chromatic aberration, where different colors of light bend at different angles, causing color distortion in the image.123
- Reflecting Telescopes: Newton’s design, developed in 1668, uses a curved mirror to collect and focus light. Because reflected light does not disperse according to wavelength, reflecting telescopes are not subject to chromatic aberration. To view the image, Newton placed a small plane mirror at a 45° angle inside the telescope tube, which redirected the focus to the side, preventing the observer's head from blocking the incoming light.145
While refractors rely on the properties of glass lenses, reflectors utilize mirrors, which allows for shorter, more compact telescope tubes and avoids the color-fringing issues inherent in early lens-based systems.5