But it is also possible to focus laser light as no light has ever been focused before. At close range a laser beam can be focused down to a circle just a few wavelengths across, concentrating its energy and making it possible to drill holes only 0.0002 inch in diameter. The photo on page 52 shows the exquisite control that can be exercised. Let us see what this focusability means in terms of power. Consider, by way of analogy, a dainty 100-pound lady in a pair of spike-heeled shoes. As she takes a step, her weight will be concentrated on one of those heels. If the area of the heel is, say, one quarter of a square inch (½ × ½ inch), the pressure exerted on the poor tile or carpet rises to 400 pounds per square inch (4 × 100) and if the heel is only ¼ inch on a side, the pressure will be 1600 pounds per square inch! [Illustration: Making and Viewing a Hologram] MAKING A HOLOGRAM Object Object beam Holographic plate Mirror Reference beam Laser VIEWING A HOLOGRAM Hologram Image Eye Coherent light source What we are getting at, of course, is the fact that the coherence of the laser beam permits it to be concentrated into a tiny area. Thus whatever total energy is being sent out by the laser can be concentrated to the point where its effective energy is tremendous. The sun emits some 6500 watts per square centimeter. Laser beams have already reached 500 _million_ watts per square centimeter. But the power of the laser does not derive solely from its ability to be focused. Even an unfocused beam is several times more powerful than the sun’s output (per square centimeter). [Illustration: Figure 13 _The typical hologram, looks like a geometric design, but it contains more information than would an ordinary photograph. The images below, made from a hologram, show the detail, apparent solidity, and parallax effect of the reconstructed light waves. The parallax effect is the ability to see around the objects just as one could if they were really there. (See frontispiece.)_] [Illustration: Model tank] [Illustration: Tank, from another angle] The crucial difference between the sun’s light or any ordinary kind of light and laser light lies in the extent to which the emission of energy can be controlled. In the production of ordinary light the atoms, as we know, emit spontaneously, or in an uncontrolled fashion. But if the atoms could be forced to take in the proper amount of energy, store it, and release it when we wanted them to, we would have _stimulated_, rather than spontaneous, emission. This, however, is practically the same as the amplification principle we discussed earlier. In that case, a small radio signal is jacked up into a large one by stimulating an available power source to release its energy at the same wavelength and in step with the smaller signal. The question is, how can we do this with light? CONTROLLED EMISSION