The long hallways of Leiden’s science faculty are quiet and peaceful when suddenly a young student flashes by. A few moments later, he skims along a girl holding a wooden plate plastered with tin cans, copper wires and duct tape. It’s a scene that would fit perfectly in a sci-fi movie like Back to the Future, but in reality we are witnessing two physics students testing out their own radar gun, developed for the Bachelor course Practicum Physics Experiments. It gives participants of today’s Experience Day a taste of what to expect from a Physics education in Leiden. Leiden University organizes this biannual day for high school students to familiarize themselves with a study of their interest.
Bachelor Physics students Ellen Riefel and Mark Knigge needed to come up with a device to build for their practicum course, when they came across radar gun building instructions from MIT. It seems like an extensive task, but Ellen and Mark decided to take on the challenge and start devising a plan to actually build one. ‘At first, I didn’t expect it to really work,’ says Mark. ‘But in the end we succeeded and the radar gun performs even better than we hoped for.’ The two students taped a set of electronics on nothing more than a wooden plate and turned it into a working radar gun, with an error margin of only a few km/h.
From one side of the plate, a tin can sends out a 3 GHz radio signal, which bounces off a moving object and is collected by a similar can on the other side of the device. A small box transforming the analog signal to a digital one sends the information to a laptop. Ellen and Mark wrote an algorithm so the laptop can calculate the speed from the measured change in frequency induced by the so-called Doppler effect. Mark: ‘We were puzzled for a long time by an unexpected output. In the end we figured out that we had forgotten about a factor 2, because the signal travels back and forth to the car. In a project with so many elements these are the things you typically overlook.’ Ellen: ‘It is frustrating when your code doesn’t work, but once it finally comes together, it is really satisfying.’
Armed with a working radar gun, the team went outside to test their device on the streets. After three seconds aimed at a car, it produces a reliable signal, much to the fear of some drivers. ‘We saw cars braking and switching lanes and back,’ says Ellen. And while the gun cannot be used for actual ticketing, it is very useful in physics outreach. Mark: ‘We use it as an example of how exciting Physics can be. We showed it for example at the Physics Ladies Day a few weeks ago.’