Unit 1: Introduction into Interferometry

Tallinn University (TLU); Leibniz Universität Hannover (LUH); University of Crete (UoC); Dublin City University (DCU); University of Groningen (RUG)

Unit 1: Introduction into Interferometry

1.1 A short History of Light

Isaac Newton (1643-1727) states that light is composed of tiny particles (corpuscles) of different color properties that are ejected from a light source and propagate in a straight line as long as they do not encounter any obstacles. This theory explains why bodies cast sharp shadows. Refraction is being interpreted as the result of an attraction that the refractive medium exerts on the light particles. However, the prediction that followed from this idea, that the more refractive the medium, the greater the speed of light, was later experimentally disproved.
Christiaan Huygens (1629-1695) compared the processes of propagation, reflection and refraction of light with those of waves on a water surface. Explanation of the Law of Reflection by the principle of elementary waves (Huygens’ principle) Explanation of refraction with the same principle, assuming different velocities of light in different mediums
James Clerk Maxwell (1831-1879) stated that light is an electromagnetic wave. Quantitative explanation of the relationship between electric and magnetic fields Reduction of the speed of light to other elementary natural constants: c = (ε0·μ0)½
Albert Einstein (1831-1879) found out that the energy of a light is contained in localized portions of size h·f, the light quanta or photons. Explanation of the photoelectric effect Explanation of the Compton-effect On the other hand, the light quantum concept is not suitable for describing diffraction and interference phenomena.

Task 1.1

Name the characteristic quantities of a wave.

Task 1.2

Design your own wave by using the following worksheet!

1.2 Light + Light = Darkness?

**Figure 3:** The emitted light of two incandescent lamps does not constructively overlap.

Superposition of the light of two lightbulbs:

Only short wave trains of different wavelengths (colors) are emitted.
In the human eye, many of these wave trains mix and create a “medium” color impression of the lightbulb.
The wave trains are too short to superimpose for a long time.
Wave trains can’t cancel out themselves.

**Figure 4:** The emitted light of two lasers does overlap constructively.

Superposition of the light of two lasers:

Very long wave trains of (nearly) one wavelength are emitted (coherent light).
The wave trains are long enough to superimpose for a long time (Interference).
Wave trains can amplify or cancel out themselves.

Task 1.3

Open the following worksheet and vary the different parameters with the sliders. Observe how the changes affect the superimposed wave.

1.3 Double Slit Experiment (Thomas Young, 1802)

In 1802, Thomas Young conducted experiments to prove the wave nature of light. Young did not yet use the classic double slit design, but instead cardboard cards with which he divided a beam of light. The following figure shows a double-slit setup using a laser as the source. A typical interference pattern can be seen on the right.

**Figure 5:** Basic design of a double slit experiment with a laser as the source of light

Task 1.4

Using the following worksheet, investigate how the two waves from the two columns S1 and S2 superimpose on the screen. The superimposed wave is shown in red. Pay attention to its amplitude.

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STEM Digitalis by Tallinn University (TLU); Leibniz Universität Hannover (LUH); University of Crete (UoC); Dublin City University (DCU); and University of Groningen (RUG) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.