Topic 3. Is light a wave or a particle?
a. Light is a particle because:
it travels in straight lines;
it forms sharp shadows;
light beams are localized in space;
it was easier to imagine a stream of particles traveling in a vacuum
reflection and refraction of light were easily understood using particle model, (although the model for refraction turned out to be wrong in detail even though it explained the general observations).
b. Light is a wave because:
two crossing beams do not collide;
light beams travel at only one speed;
Counter examples: tidal waves and surf
Newton (1642-1727) decides light is probably a stream of particles – hard to disagree with Newton
Newton’s mistake: doesn’t know about l/D – the parameter that decides whether the “wave” or “particle” aspects will govern the observation
Thomas Young (1801) and Augustin Jean Fresnel (1816) demonstrate interference and diffraction and settle the question
Maxwell (1860) develops electromagnetic wave theory of light.
He shows that moving charges can generate a wave that travels with exactly the measured speed of light.
But – the question arises again in the 20th century in connection with black body radiation and quantum mechanics. The wave model of Maxwell is found to be inadequate – especially when describing the interaction between light and matter.
Furthermore, classical particles can also exhibit wave-like behavior under some circumstances. The wavelength associated with a particle is usually very small, so that l/D for a particle is almost always much less than 1.
=>Both classical “particles” and classical “waves” share common characteristics and the distinction is not always clear. The parameter l/D still counts.