Читать книгу Quantum Physics is NOT Weird - Paul J. van Leeuwen - Страница 32

Diffraction gratings or diffraction grids work along exactly the same principle as Young’s double-slit. The only practical difference is that diffraction grid results show considerably sharper lines (fringes) of a higher intensity. A diffraction grid consists of a great number of parallel slits with very small and precisely equal mutual distances. The earliest form of a diffraction grid was a smoke blacked glass plate where a large number of parallel scratches was carefully applied to the soot. When lit with parallel traveling waves of monochromatic light, coming from a distant source, each slit will function as a Huygens wave source vibrating synchronously with the other slits, all slits producing then synchronous circular extending waves. A positive lens focuses these waves on a screen. In very sharply defined directions the arriving waves will just differ exactly an integer number of wavelengths, thus creating constructive interference in the focal plane of the lens.

Figure 4.11: A diffraction grid with slits. Source: Wikimedia Commons.

At slightly different other angles destructive interference occurs, because of the out of phase contributions to the interference from all the waves originating from this multitude of slits. So, much sharper defined interference fringes will be obtained with a diffraction grid than with a simple double-slit. This is not restricted to the visible spectrum. With such diffraction grids the wavelengths of all kinds of EM-waves, can be measured with great precision. The geometric principle, where the angle that the light rays make with the slit holder determines the path length difference, is completely identical to the double-slit principle. See figure 4.11 for a representation of the path length differences that arise with a diffraction grid.

In figure 4.11, g is the distance between the slits, φ is the angle at which the light is projected, and d is the path length difference between the waves originating from adjacent slits. The relationship between g, d and φ can be used to calculate the wavelength. Just for completeness, the equation is: sin (φ) = n.λ/g where n is the order number of the intensity maximum, the fringe, counted from the primary maximum in the middle.

But to understand what quantum physics wants to tell us, it isn’t necessary for you to memorize or even understand those equations. Forget them if you will. It’s not relevant in understanding the message of this book. Just remember that every diffraction grid or grating will produce interference effects in very distinct directions by causing synchronous wave sources departing from the slits.

Figure 4.12: Reflection on a diffraction grid with grooves such as on a DVD.

Source: Wikimedia Commons.

Reflection of light on parallel grooves, such as with light falling on the tracks of a DVD, will produce the same diffraction effects. See figure 4.12. The principle as depicted in figure 4.11 is the same for reflection, only the incoming beam arrives in this case from the right and synchronous waves will travel back to the right.

This explains the rainbow-like effects that you observe when white light falls on a DVD. White light contains a range of different wavelengths (colors) which produce constructive and destructive interference at different angles so you will see colored fringes. In this case the lenses in our eyes are the focusing devices and the retinae in each eye act as the photosensitive screens.

Diffraction experiment ^[23]: You can try this for yourself with a DVD or CD. Use light from different sources, an incandescent lamp, a fluorescent lamp, and an LED lamp. What is the difference? Can you explain the difference? Can you roughly estimate the groove distance from what you observe if you know that yellow light has a wavelength of around 575 nanometers?