Let ABCD be a horizontal section of a rectangular slab of glass. A pin is set up vertically at P, close to the face AB, or at a short distance from it. It is possible to see the pin P through the glass if we look through the face CD; but the pin is invisible if we look through the face AD. If, however, we look through the face CD, into the face AD, we shall see an image of the pin in AD, which acts like a mirror. The experiment illustrates total reflection of light; the explanation is easy. Let PO be a ray of light which after passing into the glass will be incident on AD at O′. Let NON′ be the normal to AB at O,

and KO′K′ be normal to AD at O′. The critical angle for glass being 41° 45′ approximately,

∴ the ray OO′ will not emerge through AD, but will be reflected in the direction O′Q, and will emerge at Q in the direction QS.