Can a light beam be bent? Einstein said that light is affected by gravity and bent in the proximity of a large mass (general theory of relativity). Well, that’s not an effect that one can easily observe in everyday life. However, there is another way to bend light, namely when light travels through a medium with a refractive index that changes smoothly. In the picture below a green laser beam enters a water tank. The water is colored with fluorescein to make the laser beam visible from the side. The water in the bottom part of the tank has a higher refractive index because of added suger. The interface between the sugar water in the bottom part and the sugar free top part is smooth. When the laser beam enters the region of changing refractive index, the top part of the laser beam travels at a higher speed than the bottom part (low refractive index = high speed of light). Therefore, the wavefront of the laserbeam is continuously changing the direction towards the higher refractiv index region (sugar water).
This effect demonstrated below is not just fun to observe but is actually used in optical fibers. So called graded index (GRIN) fibers have a refractiv index that is high in the fiber core and that changes gradually with increasing radius. When light enters the fiber it is always bent towards the fiber core and, therefore, is guided by the fiber. The advantage of GRIN fibers over step-index fibers is siginificantly reduced modal dispersion.
