Spherical mirror & thick window

This applet shows parallel rays - reflecting from a concave spherical mirror, and then - refracting through a thick window. The locations where light comes to a focus, both with and without the window, are shown, with relevant distances labeled. The radius of the spherical mirror can be changed by dragging point B. The incident angle of the top ray can be changed by dragging point E. The position and thickness of the window can be altered by dragging points I and N; its index of refraction is controlled by slider n2. Pan the view using Shift-drag with the mouse; zoom using Shift-mousewheel.