Change of Wavelength with Media


This demonstration shows that the wavelength changes as light passes from air into water since the diffraction angle is sensitive to wavelength as opposed to frequency. A laser beam passes horizontally through a diffraction grating suspended at one end of an empty tank and strikes an opal glass screen located at the far end of the tank. Location of the first diffraction maximum is at I1; the central un-diffracted beam is at I0. When the tank is filled with water, the first diffraction maximum moves to a new location I2. The positions of I1 and I2 with respect to I0 together with the known length of the tank enables one to calculate the wavelength of the light in air and in water. The index of refraction of water is readily ascertained to within +2%.

Special Notes: Use the normal precautions when demonstrating with a LASER. Best results are obtained with a grating ruled approximately 7500 lines per inch.