Prisms are blocks of optical material with flat, polished sides arranged at precisely controlled angles. Because small errors can inhibit proper operation, so angle specifications are critical. Prisms deflect or deviate light, invert or rotate an image, disperse light into component wavelengths, and separate polarization states. There are many kinds of prisms, many of which make use of the property of total internal reflection (TIR). Light incident upon an air/glass boundary at angles greater than the critical angle (approximately 42° for visible light and BK7 glass) is totally reflected.