having different densities referring to fig2 when the refractive index of a first medium is n 1 the refractive index of a second medium is n 2 the refractive index of a third medium is n 3 and angles between light incident on the individual mediums and perpendiculars with respect to individual interfaces between the mediums are u03b8 1 u03b8 2 and u03b8 3 snell 39 s law is as follows that is when light is incident from the first medium on the second medium reflection and refraction of light occur at the interface at this time when an angle between the perpendicular of the interface and an incident light beam is u03b8 a and a reflection angle and a refraction angle are u03b8 r and u03b8 b the law of reflection and the law of refraction can be obtained as follows according to huygens 39 principle u03b8a u03b8r the reflection angle u03b8r is equal to the incident angle u03b8a this relationship is constantly established regardless of the light wavelength and the types of the two mediums when light is incident on a medium a at an angle of u03b8o in a vacuum if the refractive index of the medium a is na the following expression is established this expression is called the law of refraction or snell 39 s law here if light is incident on a medium b at an angle of u03b8o in a vacuum when the refractive index of the medium b is nb the following expression is established from this.