Final answer:
The index of refraction for a liquid is the ratio of the speed of light in vacuum to the speed of light within the liquid. The index values for various substances are provided in reference tables, with gases typically close to 1.0.
Step-by-step explanation:
The index of refraction (n) for a liquid is the ratio of the speed of light in vacuum (c) to that in the liquid. When we talk about refraction, we refer to the change in direction of light as it passes from one medium to another.
The critical angle can be found when light travels from a medium with a higher index of refraction to one with a lower index, like from plastic to air, using the equation c = sin ¹(n₂/n₁), where n₂ represents the index of refraction of the second medium (air) and n₁ represents the index of refraction of the first medium (plastic or liquid in question).
The speed of light in vacuum is approximately c = 3.00 × 10⁸ m/s. To find the index of refraction for a substance, you would use the formula n = c/u, where u is the speed of light in the material and n is the index of refraction. For gases, it is often assumed that n = 1 unless great precision is needed, as indicated in Table 25.1 which provides indices of refraction for various substances.