The Sun, the Eye, and Mr. Darwin

It is almost certainly no biological accident that eyes sensitive to visible light have developed in the course of evolution on the Earth. Our Sun emits most of its energy in the visible part of the spectrum, so animals that developed eyes capable of "seeing" at those wavelengths attained a clear survival (and therefore evolutionary) advantage. Not only does our Sun produce mostly visible light; we will see that it produces the most intensity precisely in the yellow-green part of the visible spectrum--the part to which our eyes are most sensitive.

The Limitations of the Human Eye

The human eye is a remarkable biological invention and a shining triumph of the process of evolution. Although it was the detector that started us on mankind's exploration of the Cosmos, it has some shortcomings that ultimately limit that exploration:

1. The eye has limited size and therefore limited light-gathering power.

2. The eye has limited frequency response, since it can only see electromagnetic radiation in the visible wavelengths.

3. The eye distinguishes a new image multiple times a second, so it cannot be used to accumulate light over a long period in order to intensify a faint image.

4. The eye cannot store an image for future reference (like, say a photographic plate or an electronic detector).

Astronomers have developed a variety of instruments and techniques to supplement the human eye and to overcome these limitations.