90. Double slit diffraction and interference of light

Historical burdens on physics
90 Double slit diffraction and interference of light
The single-slit and the double-slit experiment play an important role in the
teaching of physics. The diffraction patterns are discussed extensively. The
double-slit experiment is presented as a proof of the wave character of the
light. Later it is used as a means to demonstrate the nature of so-called
quantum objects.
1. The diffraction of light at the single slit and the double slit is treated in a
detailedness and thoroughness, that exceeds the standards of a general
education. When Young carried the double-slit experiment out for the first
time it played the role of an experimentum crucis. Today we know such a
great number of other proofs of the wave character of the light, that the
double-slit experiment has lost much of its original significance. Moreover,
we know that the light corresponds to only a small fraction of the electromagnetic spectrum. Regarding the other types of electromagnetic radiation,
we care much less for a proof of their wave character. We simply take it for
granted that they are waves – with good cause, since nothing would work if
it was not so.
2. The single- and double-slit diffraction experiments are complicated. They
combine two phenomena, which are not always clearly distinguished: diffraction and interference. Sometimes it is even claimed that there is no difference between them.
3. If we would ask an unbiased student to design an experiment which
shows the interference of two light waves, he or she would definitely not
think of the double-slit arrangement. The manifest idea would be to use two
light sources. Only if the student has understood why such an experiment
does not work, and even not when using two lasers, he or she will accept
that a more sophisticated idea is needed. A well-designed experiment
should give the student the feeling: “This could have been my own idea”.
The double-slit experiment is surely not of this kind.
4. In quantum physics the double-slit experiment is used as the stage for all
kinds of contradictory stories. Light is imagined as consisting of photons,
i.e. tiny bodies, which, in order to get from the light source to the detector
have to pass either through one or through the other slit. In spite of all
warnings, that are pronounced, the idea of the individual tiny particles is
ineradicable. To emphasize the particular character of the photons, they are
now often called quantum objects, instead of particles. But even so, when
speaking about them, the language remains that which is used when
speaking about small individuals. Actually, as soon as the question of
through which slit a photon is going, one has already admitted that one
takes the idea of small bodies for legitimate. Simultaneously, one has made
a statement about the size of the particles: Their lateral extension must be
smaller than the slit width.
Already before Young’s experiments and Fresnel’s theory, there were good
arguments in favor of both, the wave and the particle model of the light.
Naturally one was convinced that only on of them could be “true”: Either the
light is a wave or it consists of particles. With Young’s experiments the verdict seemed to be rendered. In the following decades many more arguments in favor of light as a wave were found, but not only that. With Maxwell’s theory, 70 years after Young’s experiments, the nature of the waves
was understood, or at least it was believed so. Moreover, more and more
waves of this nature were fond on both sides of the spectral region of the
visible light. In spite of all these new evidence for the wave nature of the
light, when teaching we still attribute to Young’s experiments a significance
as if it were the only proof of the wave character of the light.
2. Quantum physics led the double-slit experiment to new heights. We learn
from quantum mechanics that we rejoiced too quickly. The “true nature of
the light” is more intricate.
3. A stable tradition of problems in written exams has developed in which no
final secondary-school examination and university-entrance examination
can be imagined without a problem about the diffraction of a wave at a single or double slit.
1. When the inference of light wave is to be shown experimentally, first discuss thoroughly why the experiment cannot be done with two laser beams.
2. Instead of discussing the double-slit and the single-slit arrangement, use
from the beginning a grating. The results are more convincing.
3. For some purposes the Michelson-Interferometer is more appropriate.
The advantage: no sine function, no diffraction.
4. Discuss the diffraction phenomenon with radiations where the effect is
much greater than with light: electromagnetic microwaves, or sound waves.
Then the interesting question is not why light shows diffraction but on the
contrary, why diffraction of the visible light is such a small effect.
Friedrich Herrmann, Karlsruhe Institute of Technology
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