A photo pigment is a molecule in a photoreceptor cell (a rod or cone) that can trap the energy in visible light and convert it into a signal that can trigger a nerve cell. When the nerve cell is triggered, your nervous system knows that light is hitting that particular photoreceptor.

The way pigments usually work is by having a 3-dimensional structure with a special arrangement of chemical bonds. When light of the correct wavelength hits the pigment, the electrons in the bonds are moved around, and this changes the 3-D structure of the pigment. When the pigment changes shape, it causes the photoreceptor cell to send a nerve impulse to the brain. The pigment molecule then has to return to its original 3-D shape to be able to be stimulated again.

Different pigments absorb light of different wavelengths. The cone photoreceptors in our eyes have 3 different pigments that absorb 3 different wavelengths, so the range of wavelengths that our cones can 'see' is quite broad -- ranging from blue to red with a peak at green (remember -- the spectrum goes Red-Orange-Yellow-Green-Blue-Indigo-Violet -- ROY G BIV). This range of the visible spectrum that the cones can absorb is called the photopic response. The range of the spectrum that the rods can absorb is called the scotopic response, and it is much narrower -- really just in the blue-violet range. It is much narrower because rods only have one pigment, and it absorbs blue-violet light. Rods are much more sensitive in low-light conditions than cones are.

So at nighttime when there is very little light, we mostly see with our rods. But because the scotopic response is very narrow, we can only see blues, not the entire spectrum of visible light. This is why everything has a blueish tinge when you are trying to see in very dark conditions.

The photo pigment in rods is called RHODOPSIN. The function of this photosensitive molecule is to convert light into electrochemical energy, which is neural activity. When a photon (basic unit of light) strikes a rhodopsin molecule and is absorbed by it, the molecule changes shape in a way that alters the flow of electric current in and around the pigment molecule.

From your question, I am guessing that you might be a little confused about the actual number of photo pigments in the rods (rhodopsin molecules). It is true that there is only ONE TYPE of photo pigment in the rods. We have about 120 million rods so it is safe to assume there is more than one rhodopsin molecule.

This large number of rhodopsin molecules helps us dark adapt in scotopic conditions. Dark adaptation is the process of becoming LESS sensitive to dark, which means we are MORE sensitive to light. Dark adaptation happens when we are in dark places over a long period of time.