WHAT NATURE DOES NOT MAKE

But Fresnel, as a vigorous supporter of the wave theory of light, also knew that a wave, besides having a certain intensity, has a phase or timing which determines when it crests at a certain point in space. (For more on "phase," see March issue). And Fresnel's general formula of reflection shows not only that at grazing incidence all of the wave is reflected, but that it is reflected in antiphase to the incident wave. Where the incident wave has a crest, the reflected wave has a trough, so when you add the two equally strong waves, the result is zero.

But add the two is exactly what you must do if you want to know the result near the plane that divides the two media (point B). What you have there is the direct wave (AB) from the source A, which would be there even if there were no boundary present, plus the reflected wave (CB), which is equally strong and has the opposite phase. The two will cancel: at grazing incidence, the field near the boundary (at B)is zero.

Can one explain the antiphase without mathematics? Perhaps by the saying natura non fecit saltos. I am not sure whether the ancient Romans said so or whether the Latin is used just for snobbery; but it means "nature does not make jumps." In the reflecting medium there is nothing, because at grazing incidence nothing gets in. Very close to the boundary in that medium there is still nothing. Now cross the boundary: if nature abhors jumps, it will refuse to jump to something and will want to stay at nothing. A smooth transition across the boundary therefore requires the incident wave to be annulled, and that is done by the reflected wave in antiphase.

Now apply all this to a radar antenna. As we know from the March issue, you can phase its beam into any direction. But what happens when you put it on the ground? If it is wet ground or water, it will reflect fairly well; if it is dry sand, it will reflect poorly. But no matter how it reflects at a general angle of incidence, at small grazing angles it will reflect perfectly and in antiphase: therefore near the ground the field of a radar antenna is close to zero.

By sacrificing power and using longer wavelengths, one can depress the main beam closer to the ground, which is what the British did in 1940 when the Germans discovered what it was that enabled all of the tiny RAF to be in the air waiting for the Luft-waffe bombers no matter where they chose to strike. The Germans started measuring frantically with electronic gear and found the hole at low altitudes. The British updated their model CH (Chain Home) to CHL and CHEL (Low and Extra Low). But the beam could never be made to touch the ground completely.

So when Rust's Cessna or the Cruise Missile or the Stealth bomber fly very low, distant ground radar cannot catch it.

Augustin Jean Fresnel forbids it.