The Radian

In Chapter 2 of Reference [1], one of the footnotes has a definition of a radian. For a unit circle with a radius of one unit of length called $ u$, one radian is the angle corresponding to an arc-length of $ u$. In degrees, one radian is approximately 57 degrees, which is perhaps easier to visualize. A diagram of one radian is included here for reference.


[1] Konrad Knopp. Theory and Application of Infinite Series. Dover Publications. 1990.

Christina Daniel

I am a research assistant in theoretical physics. This website,, serves to organize my ongoing learning and research as well as to provide a resource to other learners around the world.

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