When a beam of light hits the surfa

When a beam of light hits the surface of a material with a refractive index of n, such as vertical incident, a portion of light enters and leaves the refractive medium is reflected. So when we look at the reflection of a light source on the film, we see the superposition of two columns of waves. If the film thickness is small enough, the two waves will interfere, either for phase length or for phase elimination, depending on the phase difference between the two waves. For example, for the enhanced reflection of red light, for blue light (which has different wavelengths), perhaps the reflection of phase interference, so we see bright red reflected light. If you change the thickness of the film, that is, if you look at another thicker place, the situation may be reversed, the red light will interfere, while the blue light does not interfere. It is therefore bright blue, but also green, yellow or anything else called a name color.