Themes > Science > Physics > Optics > Optical Instruments, Principles & Applications > Prism and Critical Angle > Prism and Critical Angle If light passes through a prism, a transparent object with flat, polished surfaces at angles to each other, the exit ray is no longer parallel to the incident ray. Because the refractive index of a substance varies for the different wavelengths, a prism can spread out the various wavelengths of light contained in an incident beam and form a spectrum. In Fig. 5, the angle CBD between the path of the incident ray and the path of the emergent ray is the angle of deviation. If the angle the incident ray makes with the normal is equal to the angle made by the emergent ray, the deviation is at a minimum. The refractive index of the prism can be calculated by measuring the angle of minimum deviation and the angle between the faces of the prism. Critical Angle Given that a ray is bent away from the normal when it enters a less dense medium, and that the deviation from the normal increases as the angle of incidence increases, an angle of incidence exists, known as the critical angle, such that the refracted ray makes an angle of 90° with the normal to the surface and travels along the boundary between the two media. If the angle of incidence is increased beyond the critical angle, the light rays will be totally reflected back into the incident medium. Total reflection cannot occur if light is traveling from a less dense medium to a denser one. The three drawings in Fig. 6 show ordinary refraction, refraction at the critical angle, and total reflection. In recent years, a new, practical application of total reflection has been found in the use of fiber optics. If light enters a solid glass or plastic tube obliquely, the light can be totally reflected at the boundary of the tube and, after a number of successive total reflections, emerge from the other end. Glass fibers can be drawn to a very small diameter, coated with a material of lower refractive index, and then assembled into flexible bundles or fused into plates of fibers used to transmit images. The flexible bundles, which can be used to provide illumination as well as to transmit images, are valuable in medical examination, as they can be inserted into various openings.