THE NATURE OF LIGHT

In the seventeenth century, fully in the era of the great renovation of scientific thought, increasing interest arose on the issue of the nature of light. The leading figures of this search were Huygens and Newton, who elaborated two complementary theories, one based on the assumption that light is a wave, the other on the idea that it has a corpuscular nature.

Huygens conceives light as something that spreads uniformly in the surrounding space: he named it a wave in association with the surface waves which are created on water when something is thrown in it, only adding that, in the case of light, the wave spreads three-dimensionally, as a sphere expanding towards its outside, also called the wave front. This can be derived by Huygens’ assumption that light propagation is not instantaneous but takes some time to occur: this indicates that the movement is composed of successive pulses, and therefore light propagates as the sound does, with spherical (longitudinal) waves.

In order to enable light transmission Huygens (but this concept appeals to many others) conceived the Universe as permeated by a substance, the ether, made of invisible, incompressible, ever-moving particles of no mass, all in contact with each other, that would be set into vibration at the passage of light. So light “consists in a movement of the matter which exists between us and the luminous body”, as it sets the constituents of this matter into motion around their original position, being their vibration transmitted to others in the neighborhoods. There is no effective dislocation of these particles, i.e., light is not carried by the motion of this substance but it rather propagates through its vibration. Every particle on the wave front is itself a secondary source of a new spherical wave that starts propagating and what we see is the final result of the superposition of multiple effects: this is the so-called Huygens Principle. In fact every particle does not only transfer motion to the particle that is directly on its way, but also to all the others which are in contact with it and oppose its motion: it is therefore sensible to think that it produces a spherical ‘effect’ around it, which turns out to be a secondary wave. Based on this principle Huygens could demonstrate the rectilinear propagation of light and other properties: for example the laws of reflection and refraction. He explained his theory in its “Treatise on Light”, 1678.

Newton’s theory instead based itself on the opposite assumption: light is made of corpuscles that effectively move and carry it from the source to the observer. These corpuscles have different colors and white light is a mixture of them: they range from red to violet, all the colors of the rainbow. This hypothesis was formulated after he observed the dispersion of white light from a prism: in his theory the prism separates the different components refracting them at different angles. He set up an experiment to prove it: using two prisms in series he fond that, while the first separates the light into different colors, if all the colored beams are stopped but one, the second prism does not separate the beam any further. That, according to Newton, is the proof that red, blue, green, etc. (not white!) were the pure colors of which light is made of.

This theory was published around 1670 and received both praise and criticism: in particular, Robert Hooke was very skeptical and dismissed it as “not convincing”. Also Huygens, while firstly writing that Newton’s theory was “highly ingenious”, ended up criticizing it as Hooke had done.

In 1704 Newton publishes the book Opticks, in which he stated again his theory and presented experimental and theoretical foundations of optics. Basing himself on his particle theory of light and assuming that these particles are subjects to some kind of optical force, he constructed an optical mechanics. So light rays are described as trajectories of particles, reflection and refraction are explained assuming that the optical forces are different according to the medium and diffraction derives from the fact that optical forces are stronger in the presence of an object, so light rays passing near it are more strongly affected than the others.

Later on, Huygens theory will knew more success that Newton's one as Young and Fresnel demonstrated that many properties of light can be easily explained in the framework of the wave theory.

Caterina Umiltà