18-08-2012, 04:35 PM
THE HUMAN EYE AND VISION
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Quick Review of Waves
As we said earlier, light is an electromagnetic wave. Because it is an electromagnetic wave, light is self-propagating – it does not need a medium. A basic wave can be seen below. The amplitude (A) and wavelength (λ) of light are both very important characteristics – they are illustrated on the simple drawing below. The wavelength of light is what determines the colour we see, and the amplitude determines how bright the light is.
Modeling the Eye as an Optical System
In this section, we will look at ways to create models of the eye. As with many models in science, we will start with a relatively simple one and then build upon it to match reality. Along the way, we will find that some animals in nature have evolved to have eyes that are similar to each of the models. These eyes can be very different, but have one thing in common: eyes in all animals bring light into an object that is approximately a sphere, and the light falls on a detector which converts the light into signals which are sent to the brain to be interpreted.
To model these eyes, we will use a hollow Styrofoam ball as a foundation. On one side of the ball, we cut an opening and cover it with semi-transparent paper or plastic. This area represents the retina, or the place where the image falls and is converted into electrical signals in the real eye. For our purposes, we will just observe images on this “retina” to see how they would appear before being converted.
The Simplest Eye
♦ For the first model, cut a small hole about 1cm in diameter on the side directly opposite the retina. We will call this hole the pupil.
? Describe the light you see on the retina.
As you can see, the light which reaches the retina in this case is not an image but just a spot of light.
♦? To create the first image, use a piece of black paper with a very small hole in it. Place this paper over the pupil and describe what you see.