Figure 11 Obtaining coherent radiation the hard way. Filters and pinhole block all but a small amount of the original radiation.
Incoherent Filters Coherent in time Pinhole Coherent in time and space
We would then have monochromatic (one color) light, which is temporally coherent radiation, but it would still be spatially incoherent. In our diagram, we show three monochromatic waves. If we then passed this light through a tiny pinhole as shown, most of these few remaining waves would be blocked; the ones that got through would be pretty much in step. (In a similar manner, a true point source of light would produce spatially coherent radiation; but, as in the process described here, there wouldn’t be very much of it.)
We have, finally, obtained coherent light.
The important thing about the laser is that, by its very nature, it produces coherent light automatically.
Now....
WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?
So desirable are the qualities of coherent light that the complicated filtering process described above has actually been used. For example, one British experimenter, Dennis Gabor, used it in the 1940s in an attempt to make a better microscope. But so great was the effort, and so meager the resulting light, that this project was abandoned.
In the course of Dr. Gabor’s experiments, however, he did manage to make a special kind of picture, using coherent light, which he called a hologram. He derived the name from two Greek words meaning a whole picture. We shall see why in a moment.
Ordinary black and white photographs merely record darks and lights, or the intensity of the illumination, thereby providing a scale of grays, nothing more. But because waves of coherent light consistently maintain their relative spacing, they can be used to record additional information, namely the distance from objects.