Final answer:
When two transparent materials are in contact and the one on the left has a lower index of refraction than the one on the right, light reflecting from the medium with a lower index of refraction to the medium with a higher index of refraction will cause a phase change of the wave.
Step-by-step explanation:
When two transparent materials are in contact and the one on the left has a lower index of refraction than the one on the right, light reflecting from the medium with a lower index of refraction to the medium with a higher index of refraction will cause a phase change of the wave. The index of refraction is always greater than or equal to one because it is defined as the speed of light in a vacuum divided by the speed of light in the material. Table 25.1 provides a list of indices of refraction for different substances, and for gases, the index of refraction is generally close to 1 unless high precision is required.