Discovery Science: Humans – The eye

Earth Science: Biology – Humans – Sensory Organs

Our sensory organs provide us with information about our environment. The five main senses are sight, hearing, smell, taste, and touch.

Nowadays, it is also common to include the receptors that are located inside an organism or inside organs (proprioceptors) on the list of senses. The proprioceptors relay stimulation from within the body.

Earth Science: Biology – Humans – The eye

The human eye is an especially important organ, as it uses light information to provide us with essential details about our environment. Our eyes register the shape and color of objects and detect movement.

The human eye consists of a hollow spherical cavity with three segments. It is surrounded by an outer protective layer, the sclera, which turns into the strongly refractive cornea at the front of the eye ball, and covers the retina and vascular choroid coat. The front section of the eye contains a liquid-filled, and therefore refractive, anterior chamber; the iris and elastic pupil sit behind.

The iris focuses light waves reflected from an object onto the sensory cells of the retina as a true, but smaller and upside down, picture. Here, the light waves pass through the large vitreous chamber filled with a gel called vitreous humor that, together with the sclera, gives shape to the eye.

Light hits the retina, where sensory cells turn it into nerve impulses that are then channeled to the visual center of the brain through the optical nerve. The eye gets its color from pigments that are embedded in the iris.

Accommodation and adaptation

The focus of the human eye can change by adjusting the curvature of the lens. The lens becomes flatter during distant viewing and thicker and more curved during close- up viewing. This is made possible because the lens is elastic. The ability to adjust the eye to be able to view objects at various distances is called accommodation.

Moreover, the eye can also adjust to various levels of brightness. This process is called adaptation. In bright conditions, the pupil becomes smaller due to contractions of the iris muscles; in the dark, the pupil enlarges to allow more light to hit the retina.

Spatial vision is facilitated by both eyes working together with the visual cortex of the brain. The projections in each eye are slightly offset due to the distance between the eyes. Therefore, a different picture is projected in each eye.

Both projections are simultaneously passed on to the visual cortex where a spatial picture with depth is produced, which takes into account the positioning of the eye and accommodation.


The retina contains two different photo receptors: rods and cones. The very sensitive rods are responsible for distinguishing different levels of bright- ness while the cones are responsible for color vision.

The cones are divided into three types with different ranges of absorption for green, red, and blue light; all other colors are calculated in the brain.

The cones do not work in low light and with our rods we can only distinguish levels of gray The human eye can see light radiation of wavelengths between 380 and 780 nanometers.