Rods and Cones

 

Why can we see faint things at night from the corner of eye better than directly at it?

It is because of the differences in our eyes between the two types of light receptors in our eyes called rods and cones and where they are.

Rods, which can only detect differences between light and dark, are much more sensitive to light than the cones, which detect colour. When you look directly at something light from that object falls on an area predominately made up of the less sensitive cones. If you look slightly to one side of something faint, it can be seen much better because the light from the object falls on areas of the retina that have many more rods

The Eye

If you look at the diagram opposite and the structure of the eye, you will see that at the back of eye there is a layer called the retina and in the middle of this is a small area called the Fovea Centralis. Effectively this area is where light will strike if you are looking directly ahead. Any light coming from anything you do not look at directly or is seen out of the corner of your eye will fall on to the retina either side of the Forvea

The Retina

The retina itself is made up of several different types of cells each of which has a specific function, but itís the receptors (rods and cones) at the back of the retina which respond to light. If enough light hits them the receptors create an electrical pulse which is transmitted via the optic nerve to the brain, where it is translated into the image we 'see'.

Cones are able to detect colours and give us our colour vision.

Rods can only detect differences between light and dark, but are much more sensitive to light than cones.

The graphs opposite show the distribution of cones and rods in the retina and where the retina it is most sensitive to light (blue graph).

Although in good light we see things much more clearly and in colour when looking directly at something. Anything which is faint (such as a small star at night) will not give off enough light to for the cones to 'see' it, but if we look to one side of it so that its light falls on the retina on the edge of the forvea where the rods are most closely clustered we can see it because the stars light is strong enough to stimulate the rods and effectively allow us to 'see' it.