Sinopsis
The behavior of light when it travels through space and when it interacts with matter plays a central role in the two main paradigms of twentieth-century physics: relativity and quantum physics. As we shall see throughout this book, it is also important for an understanding of the behavior and functioning of organisms.
The strange particle and wave properties of light are well demonstrated by a modification of Young’s double slit experiment. In Young’s original experiment (1801), a beam of light impinged on an opaque screen with two parallel, narrow slits. Light passing through the slits was allowed to hit a second screen. Young did not obtain two light strips (corresponding to the two slits) on the second screen, but instead a complicated pattern of several light and dark strips. The pattern obtained can be quantitatively explained by assuming that the light behaves as waves during its passage through the system.
It is easy to calculate where the maxima and minima in illumination of the last screen will occur. We can get some idea of the phenomenon of interference by just overlaying two sets of semicircular waves spreading from the two slits (Fig. 1.1), but this does not give a completely correct picture.
For the experiment to work, it is necessary for the incident light waves to be in step, i.e., the light must be spatially coherent. One way of achieving this is to let the light from a well-illuminated small hole (in one more screen) hit the screen with the slits. The pattern produced (Fig. 1.2) is a so-called interference pattern or, to be more exact, a pattern produced by a combination of diffraction (see the next section) in each slit and interference between the lights from the two slits. It is difficult to see it if white light is used, since each wavelength component produces a different pattern. Therefore, at least a colored filter should be used to limit the light to a narrower waveband. The easiest way today (which Young could not enjoy) is to use a laser (a simple laser pointer works well), giving at the same time very parallel and very monochromatic light, which is also sufficiently strong to be seen well.
In a direction forming the angle with the normal to the slitted screen (i.e., to the original direction of the light), waves from the two slits will enhance each other maximally if the difference in distance to the two slits is an integer multiple of the wavelength, i.e., d.sin = n. , where d is the distance between the slits, the wavelength, and n a positive integer (0, 1, 2, …). The waves will cancel each other completely when the difference in distance is half a wavelength, i.e., d.sin = (n + 1/2). . To compute the pattern is somewhat more tedious, and we need not go through the details. The outcome depends on the width of each slit, the distance between the slits, and the wavelength of light. An example of a result is shown in Fig. 1.2.
So far so good—light behaves as waves when it travels. But we also know that it behaves as particles when it leaves or arrives (see later). The most direct demonstration of this is that we can count the photons reaching a sensitive photocell (photomultiplier).
But the exciting and puzzling properties of light stand out most clearly when we combine the original version of Young’s experiment with the photon counter. Instead of the visible diffraction pattern of light on the screen, we could dim the light and trace out the pattern as a varying frequency of counts (or, if we so wish, as a varying frequency of clicks as in a classical Geiger counter) as we move the photon counter along the projection screen (Fig. 1.3a). Since we count single photons, we can dim the light considerably and still be able to register the light. In fact, we can dim the light so much that it is very, very unlikely that more than one photon at a time will be in flight between our light source and the photon counter. This type of experiment has actually been performed, and it has been found that a diffraction pattern is still formed under these conditions. We can do the experiment also with an image forming device such as a photographic film or a charge coupled diode (CCD) array as the receiver and get a picture of where the photons hit. A computer simulation of the outcome of such an experiment is shown in Fig.
Content
- The Nature of Light and Its Interaction with Matter
- Principles and Nomenclature for the Quantification of Light
- Generation and Control of Light
- The Measurement of Light
- Light as a Tool for Biologists: Recent Developments
- Terrestrial Daylight
- Underwater Light
- Action Spectroscopy in Biology
- Spectral Tuning in Biology
- Photochemical Reactions in Biological Light Perception and Regulation
- The Diversity of Eye Optics
- The Evolution of Photosynthesis and Its Environmental Impact
- Photosynthetic Light Harvesting, Charge Separation, and Photoprotection: The Primary Steps
- The Biological Clock and Its Resetting by Light
- Photoperiodism in Insects and Other Animals
- Photomorphogenesis and Photoperiodism in Plants
- The Light-Dependent Magnetic Compass
- Phototoxicity
- Ozone Depletion and the Effects of Ultraviolet Radiation
- Vitamin D: Photobiological and Ecological Aspects
- The Photobiology of Human Skin
- Light Treatment in Medicine
- Bioluminescence
- Hints for Teaching Experiments and Demonstrations
- The Amateur Scientist’s Spectrophotometer
