We will take the before mentioned epistemological point of view on the physics as a starting point: The physics is the description and interpretation of the external world; this is, physics is an experimental science; and it is based strictly on experimental evidences. If this is so, the way the human beings perceive the external world is fundamental. On these perceptions the physics is based.
We, the human beings, perceive the external world through the senses. Some times we got the help of instruments of observation and measurement; we do it always that we are looking for precise observations. The measuring instruments have the virtue of precisize and quantify our perceptions and observations. The perception of light with the eyes, for instance, is not quantifiable nor precise. Hardly in this way we could manipulated it for useful purposes. Using the instruments we can record it, filter it, decompose it, store it, measure its wave length, measure its frequency, measure its velocity, etc. In this way, the range of the instruments is longer. And human being perceptions are precise.
The instruments show the object and the physical reality of the object. In this form, the
physical object under study, the observed object, exists independently of the observing
person; this is the general consensus; this is a physical postulate. And what we perceive is as it is;
this is another physical postulate. The majority of the physicists follow these postulates. Both
postulates are based on the information available through the measuring instruments. We will
consider an example.
Example:
We perceive the light. We can ask: What the light is? This is a very general question. If we want to answer it from a physical point of view, it is necessary to reformulate it in a way that we can answer it doing experiments. For example:
What is its velocity?, From what is it made of? ,What is the relation light-color?
The physicists experimentally answer these questions and others similar.
Galileo Galilei tried to answer experimentally the first question of the last three. As far as we know, he formulated before than any one this question. His method, even correct, was not sensible enough; the answer that he got was: If its velocity is not infinite, it is extraordinarily high. His method to measure velocities is employed up to our days, as we will see in the last section.
Galileo Galilei invented the method to measure the velocity of light, basing in the definition of velocity. He measured times and distances. And obtained the velocity. The method works to measure the velocity of any mobile. Specially the velocity of light and of particles.
The method is as follows: First, he and his colleague are separated some known distance. Each one uses a lantern and a clepsydra. The agreement is that Galileo uncovers his lantern, and starts his water clock, and when his colleague sees the light, uncovers his lantern. When Galileo sees the light from his colleague lantern stops his clepsydra. The light travels in the time measured by Galileo twice the distance of separation. Second, dividing twice the distance of separation by the measured time, Galileo obtained the velocity.
The method is essentially correct. However, his measuring instruments are not precise and rapid enough. The human reflexes work against. And they are decisive because they are very slow for the purposes they sought. For that, the obtained results. The Galileo conclusion is the most what we can say on the measurements performed. If the light velocity is not infinite, it is very high.
Newton, a generation after, tried to respond the next two questions based on the decomposition of white light into its fundamental colors. He studied the diffraction, the refraction, etc. His answers, based strictly on experimental evidences, are essentially correct. And they will be correct for they are sustained by experimental evidences.
The modern answer to the first question is: The light velocity is finite -300 000 kms per second- and is independent of the state of motion of the emitting source. The modern answer to the second one is: In some circumstances the light behaves like a wave -Young experiment-; in other circumstances, never equal to the before mentioned, it behaves like a corpuscle -Hertz o photoelectric experiment-. The modern answer to the third question is: The light of one color differentiates from the light of another color by its wave length; for example, the blue light has a shorter wave length than the red light. The students can check those experiments, and their basic conclusions, in any book about elementary optics and about modern physics respectively. The answers have technological consequences.
Some of those are: The design and construction of better lenses, of better light filters, of better photo cameras, of better telescopes, of better lenses for ocular implants, of better mirrors, of better photo films, etc. depend on the knowledge that the physicists have on the nature of light and on the nature of the material to make them. The economical repercussions of this knowledge are annually of thousand million dollars in the world market. Additionally, let expand the knowledge borders on the physical word.