Total internal reflection (page 255) can be used to convey light through a thin glass fibre (Fig. 24.24), with very little loss of energy, provided the angle of incidence is greater than the critical angle. A bundle of such fibres forms a flexible light guide.
The glass cladding of slightly lower refractive index than that of the core prevents surface damage to the core fibre which otherwise would allow light to escape.
Light guides can be used to illuminate an otherwise inaccessible space since the
light is kept inside the guide even if the guide is slightly curved. Conversely, if an objective is placed at one end of a light guide and an eyepiece at the other, the arrangement forms an endoscope. This is a collective name for a group of such instruments which are used in medicine and internal surgery to examine various body cavities. Endoscopes also have industrial applications.