A common problem with sunglasses is the appearance of glare, which is produced by light shining on the back of the lens and reflected into your eyes. The anti-reflective protective coating serves to reduce this reflection of the lens by light. Like the scratch-resistant protective layer, the anti-reflective coating is a very strong, thin film on the lens. The antireflective coating material has a refractive index between air and glass. This makes the intensity of the reflected light from the inner and outer surfaces of the film almost equal. When the thickness of the film is about a quarter of the wavelength of the light wave, the light reflected from the inner and outer surfaces of the film will cancel each other out by destructive interference, thereby minimizing the glare you see. An anti-reflective coating is also applied to the front surface of the refractive contact lens and some sunglasses to eliminate the "hot spots" created by the lens reflecting the intense light.