Chapter 2 explored the idea of light as consisting of fundamental particles, moving along well-defined trajectories. This ‘billiard ball’ model, in which a light beam is a collection of individual,compact, and well-localized bundles of energy, stands in contrast to the alternative view, which is that light is a wave. This conception of light has been pursued in parallel with the particle view of light, although it took many years of discussion and experimentation for the wave picture to become fully accepted.
Unexplained phenomena?
Other observations, such as those made by Francesco Grimaldi in the mid-17th century, gradually accrued evidence that did not fit with the particle model. Grimaldi saw that light rays deviated from straight lines when they passed through small apertures,such as a tiny hole in a screen. He noted that the light was diffused and that the edges of the beam were fringed with colour, especially pronounced for small objects such as a hair or a piece of gauze. He concluded that the striations seen when light was incident on a small or narrow object were evidence that the light had been bent from its original path as it passed the edges of these objects. If light really consisted of particles moving along straight rays, such solid objects would surely just cast a shadow, and not cause light particles to deviate into strange patterns.
Further, the problem well known to Newton and his contemporaries of the bizarre way in which light was refracted through certain materials, notably crystals such as calcite—a naturally occurring mineral—confounded explanation in terms of simple particles. An example of this behaviour is shown in Figure 15. The word LIGHT, written on a sheet of paper, is illuminated by a light bulb. Two pieces of calcite have been placed over each half of the writing. In the left half of Figure15(a), two images of the word appear, displaced with respect to each other. In the right half, the two images are displaced in the opposite direction. The lower image in the left half and the upper in the right are just as one would expect from ordinary refraction of light reflected from the paper, seen through the crystal. But the second set of images appears to arise as if from a different refractive index. By placing a polarizer over the crystal, as shown in Figure 15(b)and 15(c), it is possible to isolate images formed by two different orientations of polarized light. Each polarization experiences a different refractive index. This is the phenomenon of birefringence.