In this paper, a simple method is described for visually demonstrating that the wavelength of light reduces when entering a medium of higher refractive index. When a violet laser (405 nm) is reflected off the surface of a Blu-ray disc (track spacing 320 nm) diffraction cannot occur since the wavelength is greater than the track spacing. However, in water (refractive index 1.33) the wavelength of violet light reduces to 304.5 nm and therefore first-order diffraction can occur when a violet laser is reflected off a Blu-ray disc submerged in water. A violet laser was reflected off a DVD in air, and one diffraction order was seen, but two orders were observed when the disc was immersed in water. Red, green, and violet lasers were reflected off CD, DVD and Blu-ray discs and the number of observed diffraction orders compared with theory. When a violet laser was reflected off a CD in air, three diffraction orders were seen, however five orders were observed in water. The use of tonic water in the experiments enabled the diffraction orders to be seen more easily because violet light causes the quinine in tonic water to fluoresce.