16.1 The Nature of Light

Astronomy is the oldest of all the sciences and light has fascinated people since very ancient times. The ancient Greeks thought we saw objects by emitting rays from our eyes which then struck the object and came back to our eyes. This was like what is still pictured in comic books and movies with Superman's X-ray vision. We see some objects-like the stars or our Sun or a candle flame-because of light that comes directly from them; that light enters our eyes and we see the object. We see most other objects because light that originates from some other source strikes these objects and then enters our eyes and we see the objects.

Figure 16.1 Some objects may be seen by the light that they give off.

Figure 16.2 Most objects are seen by light that is reflected by these objects but which originated from some other source.

What is light? Unless or until we make extremely careful observations, it seems clear that light travels in straight lines. We can hear a band around the corner of a building but we can not see it. Sound waves bend around obstacles but light does not. All of our surveying-or something as common as reaching for a doorknob-relies on the idea that light travels in straight lines.

Sir Isaac Newton described light as tiny particles which he called corpuscles. This particle nature of light is consistent with light traveling in straight lines. The reflection of light from a mirror, then, can be described like a tennis ball bouncing or reflecting from a wall. Newton went on to explain the bending or refraction of light as it goes from air into water or glass. To explain this bending of light, Newton required that the speed of light in water or glass was greater than the speed of light in air (this is incorrect but the experimental evidence was not available in Newton's time).

Christian Huygens, a contemporary of Newton's, thought that light was a wave. Huygens explained the propagation of light by saying that each point on a wavefront of light acted as a source for a new wavefront. With no experimental evidence to decide between the particle model of light or the wave theory of light, most scientists followed Newton's ideas and considered light a stream of particles. Remember that Newton was a very great and famous scientist.

However, in 1801, the British scientist Thomas Young reported experiments on the interference of light passing through a double slit which unequivocally proved light had to be a wave. We will get to the details of interference later but now we can simply say that waves interfere and particles do not.

Later in the nineteenth century, James Clerk Maxwell, the first professor of experimental Physics at Cambridge University, developed detailed equations about electricity and magnetism. These equations-now known as Maxwell's equations-summarized and explained electricity and magnetism as fundamentally as Newton's laws of motion had summarized and explained motion two hundred years earlier. Maxwell's Equations predicted an electromagnetic wave with a speed very close to the measured speed of light. Of course, this was further evidence of the wave nature of light. Later experiments confirmed that light is, indeed, an electromagnetic wave.

Figure 16.A James Clerk Maxwell was the first professor of Experimental Physics at Cambridge University. He did fundamental work on the nature of Electricity and Magnetism and found that there could be an electromagnetic wave. The speed of this wave was very close to the measured speed of light. Light is, indeed, an example of Maxwell's electromagnetic wave.

In 1905 Albert Einstein explained details of the photoelectric effect and was later awarded the Nobel Prize for this work (not for his work on Relativity!). To explain the photoelectric effect, Einstein required that light be a collection of particles or lumps. We can call these lumps of light "photons" or "quanta". In this brief introduction to Optics, we are seeing the dual nature of light which we will pursue in greater depth in the section on Modern Physics or in Quantum Physics.

So, is light a stream of particles or is it a wave? For our initial introduction to Optics, it really does not matter. Light travels in straight lines. That would be true of particles and of waves. If we look very closely, as with Young's double slit experiment, we find that light does bend around corners and it does interfere. So it must be a wave. If we look very closely, as with the photoelectric effect, we find that light must come in individual, discrete lumps or photons or quanta. So it must be a particle.

Color is an important characteristic of the nature of light. Long before the time of Sir Isaac Newton, it was well known that when white light passes through a prism-just a piece of glass whose sides are not parallel-a spectrum of colored light emerges as in Figure 16.3. Many earlier observers had thought the colors of the emerging spectrum were caused by the glass. Newton contended that the colors were present in the initial white light. By placing a second prism to intercept the colors that emerged from the first prism, Newton recombined these colors to produce white light, as sketched in Figure 16.6. White light is the presence of all colors. We will talk more about color and color vision and color mixing in the next chapter.

Figure 16.3 After passing through a prism, white light is spread out into a spectrum of colors.

Figure 16.4 Colored light of a spectrum produced by one prism may be recombined by a second prism to produce white light.

Q: Place a screen in front of the spectrum of colored light coming from a prism. Cut a slit in the screen and position it so that only red light passes through the slit. Now let that red light pass through another prism. What colors will you expect to see after this light has passed through the second prism?

A: All the colors of the spectrum were present in the initial white light. The prism spread the different colors out at different angles but did not create them. Only red light goes into the second prism so only red light will come out of the second prism.

Figure 16.B Since only red light enters the second prism, only red light will come out of the second prism.