There are two types of sunglasses. The first regular type is just colour-tinted, and the other type looks like colour-tinted mirrors.

1. The first type of glasses seem to have the same colour on both sides of them. This type is like an absorbent filter; it transmits light of a certain colour, e.g. green, and absorb other colours.

2. The other type uses a completely different phenomenon - it reflects a given colour. As a result, a pair of glasses which look red may reflect red colour so that you don't see red when you wear them. The world observed through such glasses lacks red, and there is much more blue in it.

They are so called reflecting filters. The front surface of the glass is covered with a thin selective reflection layer. A similar thin layer covers your computer screen.

Selective reflection of one colour is possible due to the light-wave interference. A reflection layer consists of several alternately arranged layers with alternating high and low reflection index. The thickness of a single layer is l/2, where l stands for the length of wave which is selectively reflected. Light covers the distance of l - (forwards and then backwards) in each layer. Individual reflected waves enhance the same phase, i.e. they undergo enhancement.

The graph here shows an exemplary transmission of the light spectrum (in relative units) for mirrored coating glasses (the orange curve) and, to make a comparison possible, for an interference blue waves transmitting filter glasses, which is also shown on it.

Another graph illustrates, an exemplary transmission of the light spectrum for 'green' (absorptive) sunglasses. As you see, it is the red light spectrum which has the greatest (percentage) share of the light allowed through such glasses. Why do they look 'green' when we look at them at an angle, why does everything look 'greenish' when we look through such glasses?

The secret is hidden in our eye. The colour sensitivity of our eye is different for different colours. In the daytime our eye cone cells are most sensitive to ca. 550 nm light waves, which is green colour spectrum. They are tenfold less sensitive to ca. 650 nm light waves (red colour spectrum) and to those of ca. 470 nm.

In order to describe how we see colours through glasses the sensitivity of human eye to light spectrum should be taken into consideration. The graph shows spectrum range transmission of the light after passing through the glasses multiplied by relative sensitivity to light spectrum, which seems to explain 'green' light perception. The sensitivity factor has been obtained as a mean from numerous measurements taken for individual observers and with disregard for various kinds of eye cone and rod cells.