Chapter 10: Detecting Radiation from Space

Scientists have puzzled over the nature of light for centuries. Some of the properties are simple. Light carries energy from one place to another and it travels in straight lines. But other properties are quite unexpected. The wavelength of light is a tiny fraction of a millimeter so light's wavelike properties manifest themselves on a scale quite unfamiliar to us. After his pioneering experiments, Newton decided that light behaved more like a particle than a wave. Each of these properties applies to all types of electromagnetic radiation, not only light but radiation from radio waves to X-rays.

Light passing through two slits, showing diffraction of light waves.

Light often acts like a wave. Suppose you stand by a perfectly calm swimming pool in which a cork is floating. You disturb the cork by jiggling it. You notice that this disturbance causes a set of waves to move out across the water. These waves provide a useful analogy, but not a perfect description, of some of the properties of light. For instance, just as a water wave expands from its source, light spreads out in all directions from its source. Both light waves and water waves can bend slightly around corners. If a swimming pool has an inward-protruding corner or wall, you can observe that a wave bends slightly as it goes past the corner. Light behaves similarly. The property whereby light bends its path slightly as it passes an edge is called diffraction. For astronomers, diffraction limits the sharpness that can be achieved in the image formed by a telescope. Light passing through a slit does not form a perfect image of the slit; some of the light is bent to either side.

A water wave carries energy from one point to another at a certain speed. But notice that the entire body of water does not travel in the direction of the wave's motion — the water at any place is only moving up and down. For example, surfers can ride the energy in a wave, but the same volume of water is not carrying them from the beginning to the end of the ride. In just the same way you can transmit energy in a wave by flicking the end of a tightly held rope. Let's take another example. Sound is a wave of air compression that travels from one place to another. Yet the air molecules themselves do not travel, they just vibrate or oscillate in one place. This is analogous to holding one end of a stretched-out slinky and pushing quickly to send a pulse from one end to the other. These examples illustrate an important property of waves: energy is carried by a disturbance traveling in a medium, but the medium itself does not travel. With electromagnetic waves we do not even need the material medium. Light waves travel through the vacuum of space even though nothing carries them!

Waves adding together to constructive and destructive interference.

Light has another property that is characteristic of waves. If we have two sources of waves, the waves can "add" together. Imagine two waves with the same wavelength. If the peaks and troughs of each wave line up, the result is a wave of twice the amplitude (and four times the intensity). Alternatively, the peaks of one wave might line up with the troughs of the other wave, and the final wave having zero amplitude or intensity everywhere. If the waves are shifted by a different amount, the result is an intermediate intensity. You can see interference in water waves when two sources of waves sit side by side.

A representation of Young's double slit experiment, showing the light pattern that appears on the wall.

Young's original double slit experiment sketch.

Thomas Young

Thomas Young demonstrated this wave nature of light over 150 years ago with his discovery of light interference. In his experiment, light arriving at a screen from two small sources side-by-side produced a pattern of alternating bright and dark stripes. You can think of this as wave arithmetic. The amplitude of a wave can be positive, a level above the mean, or negative, a level below the mean. In Young's experiment, there are directions where the positive amplitude from one source combines with the negative amplitude from the other source to give no signal: a dark stripe. There are also directions where the positive amplitudes of both sources combine to give extra signal: a bright stripe.

Albert Einstein in 1904 or 1905

Diagram of the photoelectric effect.

Electromagnetic radiation also has some properties that are not well described in terms of a wave. For example, when light strikes the surface of a metal, some of the light's energy is absorbed. This energy can release electrons. Longer wavelength light releases lower energy electrons. However, when the light has a wavelength longer than a certain value, no electrons are released from the surface of the metal no matter what the intensity of the radiation. Light acts like a stream of tiny bullets, and unless each of the bullets has a certain minimum energy, no electrons can be released. This is called the photoelectric effect. Albert Einstein won the Nobel Prize for this work before his later and better-known work on relativity.

The particle description is more helpful when we think of light traveling through the vacuum of space. If light is a wave, what is doing the "waving?" It is easy to think of light as a stream of tiny bullets streaking through the universe. Scientists use this description all the time; the particles of light are called photons. However, just as there are situations where the wave description seems inappropriate, there are also situations where the particle description seems inappropriate. Let's imagine Young's experiment if light behaves as a stream of particles. The photons would leave the two side-by-side sources and would travel in all directions equally. In this scenario, the illumination should be uniform with no bright and dark stripes. The arithmetic of waves leads to a natural explanation of interference. Put in terms of the arithmetic of particles, there is no way that two particles can add to give no particles!

Which is light: a wave or a particle? Consider a platypus. It has some duck-like and some beaver-like properties, but it is neither. Similarly, light has some wave-like and some particle-like properties, but it is neither a pure wave nor a pure particle. Perhaps the problem is our need to classify the nature of light into categories that we can visualize easily in everyday life. We have to accept that light has "dual" properties. Considered in microscopic detail, light is a phenomenon not wholly familiar in terms of analogs in our everyday world. This truth is often displayed in the microscopic world of the atom, where behavior may be well described by physical laws and mathematics, but not easily grasped by intuition.