Speed of light in the microwave with Magic Stars

 I took the turntable out of a microwave and put in wooden blocks.

I placed Magic Stars on greaseproof paper on top of a heat proof mat and balanced it on the wooden blocks. I shut the door and turned on the microwave.

If you use your imagination, you can see that in some places the chocolate is more melted than in others. The non-melted bits represent places where the wave has not been going up and down. Technically they are called nodes. We have made a stationary wave - not a GCSE term - so to measure the wavelength, it is not just from one melted bit to the next but you need to go across to the melted point after that. So you are measuring across two sections of non-melted stars. I reckoned it was about 10cm. I read the frequency from the back of the microwave - it was 2450 MHz. I used the equation wave speed = frequency x wavelength = 2450 000 000 x 0.10m = 245 000 000 m/s. The true speed of light is 300 000 000 m/s so I was pleased to be so close with such an inexact experiment.

Slow-mo wave equation on ripple tank

I used a digital camera to film the ripple tank at 240 frames per second with a stopwatch visible. When played at normal speed it takes so long to play all the frames that the effect is slow-motion. Even that wasn't quite slow enough so I resorted to Windows Media Player. Right click and choose Enhancements and then Play Speed Settings.

That gives the control window shown top left. I moved the slider along to a very slow speed. When the film is paused, the forward and back arrow functions will move through the film one frame at a time. I paused the film and wrote down the time shown on the stopwatch. With the film moving slowly, I traced the movement of one wave from the top of the ruler down to the bottom of the ruler. I then paused it again and wrote down the time from the stopwatch. It took 0.91 seconds to move 15 cm. Speed = distance / time = 0.15m / 0.91s = 0.16 m/s. Comparing the wave pattern to the ruler, I think that the wavelength is 12mm. The wave equation is  wave speed = frequency x wavelength. So frequency = wave speed / wavelength = 0.16m/s / 0.012m = 14 Hz. It is harder to measure wavelength precisely because the price to pay for more frames per second is lower resolution on each picture. They are more blurred and less bright.

Putting my foot down gently on High Seat

It was terribly wet everywhere on the D of E practice expedition but especially on the bog before High Seat. I resorted to putting my foot down gently so as not to sink in. This is because the slower my foot is going, the less momentum it has when it hits the ground. That means it has less momentum to lose in a given time, so the rate of change of momentum will be smaller, which means that the impact force is smaller and I don't sink in as easily.

Ripple tank refraction

I put the usual Perspex wedge into the water. It makes the water shallower. That makes the waves travel slower. All the waves have the same frequency because they are made by the same motor. I have highlighted the crests of the waves in red. These are the wavefronts, which was Huygens way of analysing wave motion. I have also added blue arrows to show Newton's rays which show the movement of the waves. Look at how the wavefronts bend when they reach the shallower water. That's because the end in the shallower water slows down and is left behind. The rays are at 90 degrees to wavefronts so the ray changes direction as well. Finally, the waves slow so they get closer together. The wavelength decreases.

Diffraction on my ripple tank

Plane waves come down the screen and pass through a gap about one wavelength wide. They spread out as semi-circular ripples. That's called diffraction. You can see more of it happening in the gap on the right hand side of the screen between the edge of my barrier and the edge of the ripple tank.

Getting the ripple tank working

I woke up in the night last night and figured out how to make the ripple tank work. I added a large blob of Blu-Tack to the motor's load:

This slowed it down enough for the wavelength to be big enough to see when I drove the motor through 3V and controlled the speed with a potentiometer:

I put a ruler into the water so that I could measure the wavelength.

I'd say that the wavelength is 1.5 cm. I used a strobe to freeze the rotation of the blob of Blu-Tack. That said 8 Hz. Using the wave equation, wave speed = frequency x wavelength so wave speed = 8Hz x 1.5cm = 12cm/s. I had hoped to time a wave going down the screen but it would take about a second to cover the distance shown in the picture so the timing would hardly be reliable. I think filming in slow-mo is the next adventure!

Inertia experiment 5

 Push in the springy bolt on the front of a trolley.

 Catch the slot under the front lip so that it stays in place.

Put a marble onto one of the drilled holes on top to hold it in place on the still trolley. I like to keep one finger beside it to remember whereabouts above the bench the marble started.

 Press the trigger release. The bolt pushes the trolley backwards away from the wall.

Notice that the marble has stayed roughly above its initial position on the bench. This is because it has inertia. The external force acts on the trolley not the marble. You experience this if you are standing on a bus or train that moves off quickly. You fall backwards not because there is a force throwing you backwards - quite the opposite. No force acts on you - you are merely trying to stay in one place. It is also why cars have headrests to prevent whiplash injuries, but that's another post!