To determine the value of n1 based on the given context, let's go through each option:
Option A) refractive index of the less optically dense medium - This option is correct. According to the equation c = sin(n1/n2), n1 represents the refractive index of the less optically dense medium. The critical angle (c) is determined by the ratio of the refractive indices of the two mediums involved in the light propagation.
Option B) refractive index of the more optically dense medium - This option is incorrect. The refractive index of the more optically dense medium is represented by n2, not n1.
Option C) refractive index of vacuum - This option is incorrect. The refractive index of vacuum is usually denoted by "n0" or simply as 1, as it is the medium with the lowest refractive index. It is not represented by n1 in the given equation.
Option D) angle between the incident ray and normal - This option is incorrect. The angle between the incident ray and the normal is denoted by "i" in the given equation, not n1.
Therefore, the correct answer is option A) refractive index of the less optically dense medium, as n1 represents the refractive index of the less optically dense medium in the equation c = sin(n1/n2).