To start understanding the Physics behind the diagnostic machines, it is essential to quickly recap the kinds of wave motions one finds taking place around us, the key terms one will encounter, and principles or laws which govern the wave motion. In addition to that, the difference between the types of wave motion forms the foundation of why and how Sound and Electricity-Magnetism (EM) have wave motion as per Physics, yet have fundamental differences between them.
Now as soon as we hear the term “waves” instantly the image of ocean waves flash across our eyes and mind, even if we are not standing on a beach. The key thing we recall is the up & down motion of the water we notice. And if say a duck or seagull is swimming there, then the wave continues to move towards the shore (and returns also) but the duck or seagull retains its position unless of course, it flies off.
In Physics the waves can be either from Sound or Electricity-Magnetism. Foremost, in Sound, the wave motion is longitudinal meaning the compression and rarefaction movement of particles in the environment/air takes place in the direction of the movement of the wave itself e.g., the words of the person with whom we may be talking reaching our ears and vise versa. This we experience daily while hearing different sounds occurring around us. So, if on the water it is the water molecules that experienced a wave motion, in case of Sound it is the “particles in the air” that experience “compression and rarefaction” as the Sound wave propagates in the same direction of the movement of air particles. Below graphics explains this kind of wave motion.
Hence, two conclusions can be drawn by this. In waves seen on a seashore, the motion is up and down of the duck/seagull but the wave in specific moves perpendicular to the movement of the floating birds towards the shore or away from the shore when the wave returns to the sea. In Physics the top position when the birds acquire is called Crest. And when they dip as the wave moves away from them, then it is called Trough. Such a wave motion is called a transverse motion in Physics.
In the figure above, the dark black horizontal line represents the direction of the Sound’s “longitudinal waves motion” but the blue lines represent “transverse wave motion!”. It is actually the zig zag black lines which represent the movement of air particles as the sound wave moves in that medium. The Sound waves are mechanical. And they do not travel in a vacuum.
Unlike Sound waves, a moving charged particle generates an electric field perpendicular to the direction of its motion. And it simultaneously also generates a magnetic field that is also perpendicular but to the other two axes. Such a motion of Electro-Magnetic waves can be depicted as in the following sketch.
Another factor that makes a fundamental difference between the Sound waves is they require a medium to propagate, but the EM waves can move both in a medium and vacuum. The light rays under which we see things are just a small part of EM waves.
The case of Sound waves on interaction with the human body gives rise to images of internal organs as a result of reflection and absorption of these waves by different organs in side the body. The sound waves are returned to the sensing device that first sends Sound waves inside the human body. While the EM waves interact with the fluids, minerals present in proteins, fat, biomolecules, DNA, carbohydrates, water, etc.; made out of carbon, hydrogen, oxygen, nitrogen, potassium, calcium, iron, etc as ions and thereby generating an EM field due to their movement. This perturbed EM field (refer to blog PHSK-6 to understand field) is then captured to create the images of inner parts of the human body for analysis by the physicians.
So, the diagnostic machines have at their back end either of the two types of wave motions which on interaction with the human body, creates images enabling the physicians to diagnose the disease. How this is accomplished we will deal in forthcoming blogs as we discuss each type of diagnostic machine’s functioning?
April 09, 2021