Light Reflection, Huygens’ Law of Reflection, and Telescopes

This post covers Huygens’ Law of reflection, specular and diffuse reflection, telescopes and space telescope missions (WISE).

In the late 1600’s, Christian Huygens a Dutch Physicist,  developed the idea that reflections are caused by light bouncing off a smooth surface.  Huygens’ theory states that the angle of  reflection equals the angle of incidence.   Note that both angles must be measured from a line perpendicular to the surface of the reflector.



The physical process of reflection is in itself a complicated process. There are essentially 3 kinds of reflection, specular, and Lambertian diffuse (cosine) and Lambertian spread (cosine squared).

Specular and diffuse reflection


The main difference is the roughness of the reflecting surface.

surface roughness affects reflection


Now that we’ve covered the basic physical effect, let’s talk about how we can use this information. Specular reflection applies to mirrors which may be flat or may be curved.  Curved mirrors can sometimes focus light into an image as in a reflecting telescope. In this lesson we’ll develop the equations for focusing mirrors and you’ll see that a similar set of equations may be used to determine the focusing properties of lenses.




A replica of Newton’s second reflecting telescope, which he presented to the Royal Society in 1672.

NewtonsTelescopeReplica” by User:Solipsist (Andrew Dunn) – Licensed under CC BY-SA 2.0 via Wikimedia Commons.

The WISE Widefield Infrared Survey Explorer (under construction)

WISE telescope

WISE optics


Modern Ground based reflecting telescopes.

30 meter telescope in Hawaii

30 meter telescope


LASER re-emission of the Sodium layer in the earth’s upper atmosphere is used to flex mirror segments to adjust for atmospheric distortion (twinkle) using a technique called adaptive optics.

30 meter telescope - adaptive optics






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