Radar

 

The determination of speed with radar is a common application of the ‘Doppler’ shift. An electromagnetic wave is generated and aimed in the direction of a target object, which ‘reflects’ the wave back to the sender. The direction and extent of the shift of the wavelength from that of the original wave can be used to calculate the speed of the target with respect to the sender.

 

With reference to the previous section on the Doppler shift, the emission-detection situation may be different with electromagnetic energy levels associated with wavelengths longer than those in the infrared region. Microwave and longer wavelength electromagnetic radiation is usually generated continuously from an appropriate emitter[1]. It is not clear whether this process generates continuous waves or discrete energy packets. Unless electromagnetic radiation has two different forms, it is likely that the process produces discrete packets of energy that can appear to be and act like continuous waves, at least when detected. However, unlike the uncontrolled emission of light from molecules, atoms, etc., these lower energy emissions can be produced in a controlled manner using an oscillator with a fixed frequency of oscillation. If the result of this were a series of energy quanta rather than continuous waves, then the quanta (photons?) would presumably be equally spaced during their travel. This spacing could possibly be equal to the actual wavelength (or half wavelength) of the quanta. Thus a detector that detected the arrival of each quantum might indicate a wavelength that would be equivalent to the wavelength of the individual quantum.

 

This raises the question of exactly what is happening when a receiver detects this electromagnetic energy. Assuming that quanta are produced, is the detected wavelength a result of the periodic arrival of the individual quanta, or a result of the wavelength associated with each individual quanta. These may or may not be the same depending upon how the time gradients affect each. If it is the periodic arrival that is detected, then it seems reasonable that any wavelength shift would occur in a similar manner to the wavelength shifts associated with sound or other continuously generated waves. This would also be the case if the emitter were generating continuous waves rather than quanta.

 

On the other hand, if the shift is dependent on the relative speed of the quanta in the emitting device and the target, why does a reflected quanta come back showing a different wavelength? The wave (quanta) would undergo the same changes of speed in both directions through the various time gradients (in reverse order on the rebound) and therefore might be expected to return with the same wavelength regardless of the velocity of the target. Since it does change as a function of the target speed, the returning wave must not have the same properties as the original wave.

 

Reflection of electromagnetic radiation (EMR) is not just the EMR bouncing off of a target and returning. The incoming EMR interacts with the target in a manner such that oscillations are set up that correspond to the energy transferred by the EMR. These oscillations then emit EMR corresponding to the transferred energy in a direction that depends upon the angle of the incoming EMR. When the target is stationary there is no reason for the reflected EMR not to be similar to the incoming EMR. If the target were moving away from the emitter, it seems reasonable to expect that the impact of the incoming EMR on the target would transfer less energy to the target than it would on the stationary target. If the target were moving toward the emitter, then the impact would involve the transfer of more energy to the target. The returned EMR would then have less or more energy than the incoming EMR, which would result in the observed wavelength shift.

 

Radar is commonly used in pulse mode, where short bursts of radiation are emitted. The time it takes for a pulse to travel to a target and return can be used to determine the distance of the target. If this time continuously changes, then the speed of the target could possible be determined by the rate of change of the return time between pulses. However, the frequency change of the pulse is apparently more commonly used to determine speed with pulsed radar. The above considerations would then be applicable in that case.

 

The following animated illustration shows an oscillator producing electromagnetic waves that can behave like particles (energy quanta), or is it particles that it produces that can act like waves? Or is it something else that behaves like both particles and waves? Saying that a quantum of energy (photon) acts like a particle is probably equivalent to saying that it transfers its energy to whatever it impacts just as a moving particle may impart its kinetic energy to whatever it impacts. Saying that a quantum of energy acts like a wave implies that it behaves similar to waves from a vibrating string or waves produced in water. This is probably not quite true. Waves produced by a vibrating string, or other vibration, are simple waves that depend upon some medium to carry them. Waves that are associated with light are what might be called double waves (or double component waves) since both an electric and a magnetic component are present that presumably exist in perpendicular planes and act upon each other, and possibly do not require any medium to carry them. They are pictured as being similar to simple waves, but that is because we do not know how else to picture them. But it cannot be assumed that simple and ‘double’ waves are equivalent even though the same formulas may to apply to both. It might also be noted that particles, such as electrons, can show wave-like behavior such as diffraction, but without being perceived as traveling waves. The illustration here should be viewed with this in mind.

 

Illustration 1

 

Infrared radiation (IR) presents an interesting situation that straddles the considerations in both the previous section and this section. IR is associated with the movement of the atoms within a molecule. This is usually viewed in terms of specific vibration modes of a molecule that are sometimes modeled (physically and virtually) with the use of springs and weights. This implies a continuous oscillation that results in the emission of IR. This possibly forms an inaccurate picture of what is actually happening. IR radiation has specific energy levels that are associated with specific vibration modes within a radiating molecule (i.e. it is quantized). The associated frequency is actually that of the detected photon, and does not necessarily imply that the transition generating the photon is actually a half cycle[2] of an oscillation.  This raises the question as to whether or not these ‘vibrations’ within a molecule are continuous oscillations or transitions that may or may not be regularly spaced. If they are actually oscillations, the question is what form the radiation from these oscillations take as was discussed above. If they are actually transitions that may or may not be regularly spaced, then each transition would produce an energy quanta, or photon. In that case, the situation is similar to that discussed in the previous section on the ‘Doppler’ shift.