31. Modern Physics#
31.1. Flames and colors#
Try a candle, what colors do you see? Try some alcohol, since the pandemic quite abundant. What color is the flame? Now add some salt to the alcohol. What color do you see now (more orange and better visible, sodium!). All elements have their own spectral lines, Sodium has the famous yellow/orang lines. Some other salts available? Just Google on “flame colors” to get many examples.
MODERN PHYSICS VISUALIZATIONS
Physicists like the surprise element in modern physics experiments, but to experience the surprise, one has to have expectations based on prior knowledge of traditional physics. Many students do not have that and take the outcomes of the experiment for granted without surprise or interest. Therefore, the teacher has to prepare the students carefully for the experiment. Following are some visualizations which might help.
31.2. Particle-wave duality#
Before presenting particle-wave duality, the teacher should contrast the classical differences between particles and waves. Take a glass of water or –if available- a basin. Make a wave. Where is the wave in the basin? Everywhere! The teacher talks, where is the sound? Everywhere! We cannot localize a wave, it is everywhere. Now take a piece of chalk or a pen or pencil, or a coin. Where is it? There are sharp boundaries for any object, objects are localized. This is the essential difference between waves and matter/particles. Then proceed to duality. Hopefully you have a beamer with PhET simulations or www.falsted.com.
31.3. Double slit, role play of how it is NOT#
Imagine the situation of shooting microscopic bullets through a double slit. That can be simulated in a roleplay. Make two narrow slits by arranging tables and use the wall of the classroom as screen. Send students through the slits, they are only allowed straight lines and so they end up in two clumps on the screen. They form an image of the slit. If you send them one by one, this image builds up gradually. That is what you expect with particles. However, if the bullets were electrons, or protons, or even bigger molecules like C\(_60\) (buckeyball), then it is NOT like that. While the electron or proton or C\(_60\) is on its way, we do not know where it is. We do not even know through which slit it goes. The particle might be everywhere during its travel, but it does end up at a particular spot on the screen and that spot might not be in a straight line behind the slit. Eventually all these spots together form an interference pattern typical for waves. Electrons, protons and molecules propagate like waves but are absorbed and emitted like particles. Also show some simulations. There are many versions of the double slit experiment, for example with detectors at the slit(s), or even so called delayed choice experiments. Ananthaswamy (2018) wrote a captivating and very readable book about this famous class of experiments.
31.4. Mass numbers of the elements#
The fact that mass numbers of elements are not integers is difficult to understand. The non-integer mass numbers are a result of averaging masses of isotopes and mass deficits due to bonding. Get some students up front, for example, 4 single students (4 protons) and one pair (a deuterium nucleus consisting of a proton and a neutron). If indeed one out of 5 Hydrogen atoms would be Deuterium, then the mass number would be about 1.2. However, the mass number is about 1.008 showing that less than 1 in 100 Hydrogen atoms is Deuterium and the rest single protons. Atomic mass is an average of the masses of many atoms of a single element including the naturally occurring isotopes.
31.5. Rutherford experiment#
Rutherford scattering is difficult to visualize for students. Let one student in the classroom (=the atom) hold up a bag…the Gold nucleus. Then the instructor (or a student) takes some small pieces of chalk and is blindfolded (or closes the eyes). From random positions in front of the class the instructor throws electrons in the general direction of the back of the class. The chances that the chalk hits the bag are relatively small. The smaller the bag, the smaller the number of hits. So the number of chalks being bounced back says something about the size of the bag. In Rutherford’s experiment, very few electrons are bounced back meaning that the nucleus only occupies a very small part of the atom. Actually, Rutherford had expected a large and soft nucleus so all electrons would get kind of stuck inside and none would bounce back. So he commented “as if you shoot bullets at a piece of paper and some bounce back!”
31.6. Big bang and Hubble 1#
Govert Schilling (2017, p167) compares the cosmos with a raisin cake. The raisins form the corners of a system of cubes in which each cube is 1 x 1 x 1 cm. When the dough rises so that after one hour the distance between neighboring raisins is 2 cm, then each cube is 2 x 2 x 2 cm. Suppose you sit on a raisin, then you see the nearest raisin move away from you with a speed of 1 cm/hour. However, the next raising is now at 4 cm instead of the initial 2 cm, so that one moved 2 cm/hour. This could also be done in a roleplay, positioning students in a matrix.
31.7. Big bang and Hubble 2#
A very easy way to show this multiplication of distances is to line up 4 students at 1 meter distance from each other. Then the distance from number 1 to number 2 doubles to two meters. Let students compute what the distances between 2 and 3 and 3 and 4 should become. Numbers 3 and 4 also have to move along with that and do this one meter step. But number 3 also has to take the extra step which #4 has to follow. And then #4 has to take the extra step yet. So now the distances between 1, 2, 3, 4 are 2m, 4m, and 8m. It is very instructive to see this.
31.8. Big bang and Hubble 2#
Often cosmic expansion is shown with a balloon. Blow a balloon a little bit and mark some points. Then blow it up more and the points will get farther away from each other. We should really look only at what happens at the surface of the balloon and not at three dimensions. The pitfall is that students will see this 3-dimensionally as expansion from one particular point.