This applet shows parallel rays[br]- reflecting from a concave spherical mirror, and then[br]- refracting through a thick window.[br]The locations where light comes to a focus, both with and without the window, are shown, with relevant distances labeled.[br][br]The radius of the spherical mirror can be changed by dragging point B.[br]The incident angle of the top ray can be changed by dragging point E.[br]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.[br][br]Pan the view using Shift-drag with the mouse; zoom using Shift-mousewheel.