in the twentieth century. In the following chapters you will encounter sophisticated adaptations of the double-slit experiment. It is therefore really important for you to understand that interference is a wave phenomenon and how it comes about, which is the reason I have extensively covered this topic here.
A really good way to ensure your understanding of Young's experiment is by trying to explain the double-slit pattern and its conclusion to someone, if necessary an imaginary person! Doing so is an excellent way to find out if you have really got the idea of double-slit interference right.
A DIY double-slit experiment with sound waves..
Ingredients:
A computer (Windows, Apple)
Two loudspeaker boxes
A tone generator program - free online: https://onlinetonegenerator.com/ [7]
Place the boxes approx. 3 feet apart and start the tone generator program. Choose a frequency between 1 and 5 kHz (Kilohertz). Start the tone generator. Stand about 4 feet away from the boxes and move to the left and then to the right, change the height of the tone if necessary. You will clearly discern the maxima and minima.
With this DIY experiment you will have demonstrated the wave character of sound by evoking maxima and minima of sound at certain locations. The maxima and minima of light in the double-slit experiment demonstrate the wave character of light in exactly the same way.
Fields, electromagnetic waves
Nowadays everybody agrees that light is an electromagnetic wave phenomenon. How did we discover this? From ancient times we were already familiar with electrical and magnetic phenomena. De term 'electricity' is said to be derived from electrum, the Latin name for amber. Rubbing amber with a woolen piece of cloth evokes small sparks. We discovered two types of electrical charges. Similarly charged objects repelled each other and dissimilar ones attracted each other. Magnetic compasses were used in navigation.
It turned out in the 18th century that applying the same mathematical methods as with Newton's gravity mechanics, one was able to calculate the dynamic behavior of objects with attracting or repelling electric charges or with magnetic properties. The mathematical tools that Newton had laid down for gravity could be applied in the same way to the electrical attraction between two charged objects. The electrical or magnetic force is, just like gravity, inversely proportional to the square of the mutual distance and directly proportional to the product of the two electrical or magnetic charges. However, when one wanted to calculate with these methods the simultaneous behavior of more than two charged objects, mathematical problems arose. Exact mathematical tools are simply not available for multiple body interactions. In those early days one had to make do with approximative methods and calculations with pen and paper. Nowadays we use computers for more accurate approximations.
To simplify the multiple body calculation problem, the field concept was developed. A good approximation of the behavior of more than two attracting or repelling bodies was found by assigning to every location in the empty space around an electric or magnetic charge a field vector property. A vector is a mathematical object representing a both magnitude and direction. An electric field vector [8] represents the direction and the magnitude of the force that an electric unit charge would experience in that location of space. The same principle applies for a magnetic field vector [9]. Assigning these mathematical vectors to every location in space around a charge, defines thus the electric or magnetic field of that charge. Ask yourself now if you think that such a field is an objective tangible thing.
Figure 3.5: Electric field lines.
This idea of a vector field idea simplified calculations greatly. Figures 3.5 and 3.6 show the graphic presentation of electric and magnetic fields.
Figure 3.6: Magnetic field lines.
S=Southpole N=Northpole
Source: Wikimedia Commons.
The tangent to the field line in every point gives the direction of the field force at that point. The field line density represents the field force. The denser the field lines, the greater the field force. I will not elaborate on the differences between magnetism and electricity, but I hope you will understand that the field concept assigned real properties to something that we consider to be empty space. In this way the abstract mathematical field became gradually a real object.
This is, in my opinion, a striking example of reification, which is the conversion of an abstract idea into an objectively existing object. It will turn out that this reification of the field, starting in the 19th century, will present an obstacle to the better understanding of electric and magnetic effects.
However, field mathematics, whether applied to gravity, electricity or magnetism, resulted in a significant progress in the results of physics research. In 1831 Michael Faraday (1791 - 1867) discovered how an electrical current generated a magnetic field and how, vice versa, a changing magnetic field generated an electrical current. The first dynamos were built. However, a complete mathematical description of electricity and magnetism was still missing.
This task was taken on by James Clerk Maxwell (1831 - 1879), a Scottish mathematician and physicist. In order to build his equations, he extended the field concept in such a way that field lines became almost tangible objects traversing empty space. In 1861 he published a set of twenty equations which described the dynamic behavior of electromagnetic fields completely. His equations said that alternating electrical fields generated alternating magnetic fields that generated electric fields again and so on. Heaviside (1850-1925) and Gibbs (1839-1903) merged and simplified these twenty equations into just four, which are now known as the Maxwell equations.
These four equations (see Wikipedia: Maxwell's equations [10]) describe mathematically how a changing electric field creates a similarly changing magnetic field, and vice versa. The interaction of these fields generates a self-propagating electromagnetic phenomenon behaving like a wave in empty space. Propagation of such a wave takes place in a direction perpendicular to those alternating electromagnetic fields. This wave phenomenon is called an electromagnetic wave [11] or EM wave. See figure 3.7.
Figure 3.7: Vertically polarized electromagnetic wave.
E: Electric field, B: Magnetic field. λ: wavelength.
Source: Wikimedia Commons.
The entire classical electromagnetic theory has its origin in these four basic equations. Note here that classical physics was actually concerned with matter, energy and the
