Introduction: What Is Light? Wave or Particle? Examining the Wave-particle-dualism...


I want to start with a question. What is light? When you go around in nature or somewhere else you will get in contact with typical waves like in a lake or with particles like hail. But what is light?

The so called wave-particle-duality, a special statement of the quantum physics, says that the things around us are both, particle and wave.

What would you expect, when you think they are typical particles? You may say, that they can be detected individually at a choosen place. A particle can hit you or passes by, depending where you are.

Contrary to that a typical wave isn't at just one place. A wave is in an area at the same time. And what is further typical for a wave? Interference. With waves you can often see interference patterns. When a wave hits a grid, than you'll see patterns of maxima and minima behind on the screen. With particles you will never see that.

In the following pages I'll show you, that f.e. light is both, particle and wave...

Step 1: The Parts You Will Need

First of all you'll need a light-tight housing. I bought a large one from amazon. Unfortunately I found out, that the housing itself let light into. When I held a flashlight to a side of the housing, the counting rates of the photomultiplier increased dramatically :-(... That's why I had to stick it with black tape

To lead out the micrometer of the linear stage was not easy too. I used two-components glue.

Step 2: The Heart of the Experiment, the Photomultiplier

A photomultiplier is a extremly light-sensitive device. It can even count single photons. Usually a photomultiplier needs high voltage to accelerate the electrons between the dynodes. At the output you get a short voltage-pulse (nearly) everytime a photon hit the photo-cathode at the front of the photomultiplier.

I used the Hamamatsu 5773. The advantage of this special type is, that you don't need a separate high voltage power supply. The high voltage is created inside just with the 12V input. And it's small too. A further benefit.

Step 3: The Electronics

For the pulses coming from the photomultiplier you need a simple amplifier and a monoflop. The amplifiere turns the pulses of sufficient heights just around (gain = -1) and the monoflop creates longer pulses for the counter when a photon was detected. To avoid that the counter reacts to noise, the circuit consists of a comparator. The level, above the pulses are counted, can be chosen with a potentiometer.

Step 4: The Source of the Photons

For the photons I bought a 5mW dpss-laser from ebay for 10 USD. The wavelength is 532nm. But you can't put the laser inside the housing and turn it on. The photomultiplier would be destroyed because of the huge amount of photons. A 5mW laser emitts about 10 000 000 000 000 000 photons per second! Therefore you have to weaken the laserbeam. A so called neutral-density filter is the part you'll need. I tried several combinations of filters with the optical density 4.0 and 1.0. At the end I use 2 layers of 4.0 density. This weakens the beam by the factor 10^-8.

Now you could say, that still 10^8 photons hit the photomultiplier per second. This isn't right. Behind the grating the beam is splitted into many many beams. So the photons are splitted too. And another important point is, that the window of the photomultiplier is being covered with tape. Just a very small slit is left and therefore the number of photons detected per second is low enough for the counter (for a intensity-maximum around 12000 photons/minute).

Step 5: The Whole Setup

The whole setup consists of the housing with the photomultiplier and the laser, the amplifier+monoflop and the counter. For the electronis I need a symmetrical +12/-12V power supply. The inside of the case is covered with black foam rubber to minimize the scattered light.

Step 6: The Results

So, the experiment has to come to an end. What do we expect to see/measure? If light consists of particles, they could be detected with the photomultiplier. But particles woun't create diffraction-patterns behind the grating. Only with a wave you will see interference-patterns.

But when I turned on the laser and moved the photomultiplier I could hear the speaker of the monoflop. Every click stands for a registered photon. So I detected single photons, single particles. But the number of particles detected within one minute changed while moving the photomultiplier. I got positions with higher counting rates and positions with lower counting rates. This is typical for a wave.

Conclusion: Light can be detected like single particles, but they carry information of an wave with them. Therefore I got diffraction-patterns and the counting-rate increases and decreases behind a grating. Voil'a, I proofed one statement of the quantum physics, the wave particle duality. Light is particle and wave at the same time....

Maybe you're interested in some of my experiments, here's my youtube-channel:

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