When light, or other radiation, meets an interface between materials, it undergoes some combination of reflection and refraction (Figure 1). Total internal reflection occurs when it meets an interface with a less-dense material (where it would have higher speed) at an angle greater than the critical angle; otherwise, there is partial reflection. In an optical fibre, light is internally reflected, zig-zagging along the fibre and emerging only at the end (Figure 2).
Optical biopsy (Figure 3) involves imaging cells within a patient’s body so that disease can be diagnosed without invasive surgery. The technique relies on total internal reflection. A tube containing several thousand optical fibres is passed into the body, and a fine probe at its tip enters the tissue to be examined. A laser shines along the fibres and a microscopic image is relayed back to the computer, where it is displayed on a monitor.
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