Meteorology: Understanding the Atmosphere Ackerman and Knox
Why we can't see well under water.
Objects are not in focus when we open our eyes under water. To understand this we have to describe the eye and how it focuses light. The structure of the human eye consists of three parts: the outer, middle and inner eye. The outer layer consists of the sclera (the white portion we see) that protects the eye and the cornea which helps focus light at the back of the eyeball. The cornea absorbs UV light and therefore limits our visual sensitivity to light with these wavelengths. Overexposure of UV sunburns the cornea and is the cause of snowblindness. The middle layer consists of the choroid, lens, aqueous body, and the vitreous body. The lens is transparent and focuses light onto the retina. The aqueous body surrounds the lens and the vitreous body fills the chamber that helps support the lens and transmits light to the back of the eye. The choroid keeps light from straying inside the eye, it consists of the iris, pupil and pigmented tissue. The iris is the colored portion of the eye that lies beneath the cornea and adjusts the amount of light entering the eye. The center of the iris is the pupil, a covered hole in the eye which allows light to enter the eye. The inner layer of the eye includes a light sensitive tissue called the retina and the fovea. The inner layer also includes one end of the optic nerve, which carries visual stimulus to the brain for interpretation. The optic nerve spreads throughout the retina. The point of exit of the optic nerve results in a blind spot.
Cells sensitive to light cover the inner eye. The human eye has two types of these photoreceptor cells, rods and cones . Cones, located in the fovea, respond to bright light and color. Pigments vary in the cones making different cones sensitive to the varying wavelengths of light. Rods are abundant in the retina and are sensitive to dim light. Rods also detect changes in the intensity of light across the field of vision and thus aid in the perception of movement. Rods cannot distinguish color while cones are insensitive in dim light. This is why it is difficult to see colors in dimly lit rooms!
With the help of the lens, the cornea collects and focuses light on the retina. The ciliary muscles, aid in focusing objects at different distances by varying the curvature of the lens. When the ciliary muscle is relaxed the lens is in a flat configuration and objects at infinity are focused on the retina. When viewing objects close to the eye the ciliary muscle contracts causing the lens to become more curved, bending the light rays greater according to Snellís Law. Have you noticed that you cannot focus on two different objects that are at a different distance at the same time?
Focusing depends on the curvature of the lens and the difference between the index of refection of the eye and the surrounding medium. Air has an index of refraction of 1, the cornea 1.376 and the aqueous humor 1.336. The index of refraction of water is about 1.33. The amount of curvature the lens needs in order to focus light on the retina is determined by Snellís Law. Because of the small difference in the index of refraction of water and the eye, the lens must become very curved to focus light on the retina. Living on land, there was no evolutionary need for the lens to become this curved, so objects are out of focus when we open our eyes under water.