Light is literally all around us, but not many people stop to ask what this light really is. Newton and Hooke (both amazing scientists) were two of the first who tried to analyse what light was made of and how it propagates. Newton thought light was made out of small particles and Hooke thought light were some kind of waves. For a long time Hooke was thought to have the right answer, untill Albert Einstein found proof supporting Newton with the photo-electric effect. To this day we still believe light has both wave and particle properties. In this demonstration we will show the discovery of Einstein supporting Newton's theory that light is made of small particles.

For an explanation of the physics behind our expermiment you can watch our video:

A project for the course Design Engineering for Physici from the bachelor Applied Physics at Delft University of Technology.

Made by: Zachaar Ivanov, Arun Mulder and Noah Tap

Supplies

CdS photoresistor
Multimeter
Power supply (Arduino)
At least 2 different colours laserpointers of (We used a green and red laserpointer of 5mW)
Electrical wiring (banana cables and jumper cables for arduino)
Cork
Small PVC-tube
Duct tape
Toothpicks

Step 1: Put LDR Through a Piece of Cork

To begin, you will want to make a setup to reduce the ambient light that falls on the LDR, but still allow for the lasers to access it.

Firstly, get the LDR and a small PVC-tube, a cork and something to deform the cork (for example: a utility knife, a small saw and/or a vile).

Cut the cork in half and measure the diameter of the PVC-tube.
Now cut or file the cork so it fits partly inside the PVC-tube.
Now you push the LDR through the cork carefully and fit the whole cork in the PVC.

Step 2: Crafting Stand for the LDR

Next, you will need to secure the setup to keep the LDR from moving.

Grab another cork, some toothpicks/cokctail sticks and some duct-tape.

Carve out a piece of the cork and secure PVC-tube onto it with tape.
Now stab three or four toothpicks in the cork so it can stand stable.
You may need to cut some pieces of the toothpicks for the best result.

Step 3: Building a Stand for the Laserpointers

Now the LDR is stable and in the dark, you need to make a setup that keeps the lasers in place and is able to directly aim onto the LDR surface.

Get a cork and some small and long cocktail sticks.

Cut a cork in half horizontally, and again vertically.
Now carve out a semi-circular piece of the corks so the the laserpointer can rest on it.
(You can put some duct-tape over the cork if necessary.)
Connect two pieces of cork together with cocktails sticks, at least half the length of the laser pointer.
Now use three shorter cocktail sticks to stabalize.
Make sure both sides are at the same heigth and the laserpointer will shine right onto the LDR in the PVC-tube.
(You may need a ruler for this.)

Step 4: Electrical Wiring

Now it is time to take a look at the wiring. At it's core, this is a series circuit connecting the power source, LDR, and multimeter.

In this step you need to connect the LDR to your power source. We will explain how we did this with an Arduino.

We soldered a jumper cable to each of the LDR wires sticking out of the cork.
(You can also use 2 female jumper cables as follows; LDR, female cable, male cable, Arduino. Using female cables requires some extra care as they are prone to fall of.)

One of the soldered wires connects to the 3V or 5V (blue in the picture). The other cable (orange in the picture) connects to the multimeter, which is explained in the next step.

Chosing 3V or 5V does not matter, as long as you keep consistent with your choice.

Step 5: Connecting the Multimeter

After you connected you LDR to you power source, we can now connect it to the multimeter.

Connect one of the cables from you LDR to a banana cable (orange to red in the picture). We did this by slightly bending the end of the jumper cable, and inserting them into the back of the banana connector.

This banana cable will go in to the Ampere input (as shown in the pictures above, red cable).
Grab another banana cable and connect it to the multimeter COM output (black in the picture).
Connect this banana cable to a new jumper cable.

Since banana cable can't connect to Arduino directly, we used another Arduino jumper cable to complete the circuit.

Step 6: Putting Everything Together

We have now created every part of our experiment and can put everything together.

Put the laserpointer on its stand. Put the open end of the PVC-tube in front of the laserpointer. You may need to adjust them slightly so the laser 'point' hits the LDR directly.
Make sure all the wires are connected and activate your power source.
You can now turn on the multimeter to measure the current. You should measure a little current, which depends on the voltage your power source aplies and the light you shine on the LDR.

Everything is now ready and you can test the photo-electric effect for yourself!
All you need to do is shine the different lasers onto the LDR, and see how the current changes.