The refractive index of a material governs how much light bends as the light moves into it. You’ve probably seen this bending effect when looking at the surface of a swimming pool: the bottom of the pool looks closer to the surface than it actually is because light rays bend as they travel from water to air.
But if the refractive index of two materials is the same, as is the case for sunflower oil and Pyrex, then light doesn’t bend at all, and you end up with the nice effect demonstrated below.
To say that this demonstration impressed my pupils would be an understatement.
During the 2012 Olympics, the underwater cameras in the swimming pool have been tweeting regularly. On Sunday, the PoolCam sent out my favourite image of the whole London 2012 Olympic Games: a magnificent demonstration of total internal reflection.
Light refracts as it travels from one medium to another. Total internal reflection occurs when light travels from a medium with a high refractive index to one with a low refractive index at an angle above the critical angle for those two media.
In the photograph above you can see out of the pool at the top of the image because the angle of incidence is less than the critical angle. Beyond the critical angle, light is totally internally reflected and the bottom of the pool is reflected back towards the camera. Because the angle is the same in all directions this creates a semicircle, which is visible at the top of the image.
If the camera is on the bottom of the pool looking up you therefore see a perfect “see-through” circle looking at the roof, surrounded by water reflecting the bottom of the pool. This effect can be seen in a previous @L2012PoolCam photograph: