This simulation shows the working of a lens made of a circular segment, that is, a disc cut by a straight line. The lens works here like the objective of a camera, by concentrating an incoming planar wave at the focal point of the lens. The index of refraction of the lens is equal to 5/3 = 1.666…, so that according the the lensmaker’s equation, the focal length should be equal to 1.5 times the radius of the circle.