Lesson 14: Quantum Physics II

Quantum Physics Part II

In the last lesson, we talked about how energy has been quantized. Energy is not smooth and continuous but comes in discreet, indivisible chunks. This fixed the Ultraviolet Catastrophe and showed why the Earth wasn’t just a big ball of flame.

Before we go any farther, we need to make sure you know the difference between a particle and a wave.

We talked about electrons in the last lesson. Electrons are particles.  You can think of a particle as a very small object which has physical properties.  Think of particles as super small billiard balls. If one particle hits another, it will act the same way as the balls act when playing pool.

Consider how particles differ from waves.  Waves have no substance.  They can not be felt.  Waves can move through solid objects.  Think about a sound wave.  You can hear noises in another room because the sound waves are moving through the walls to get to your ears.  A particle could not penetrate a wall.

Now that we know the difference between a particle and a wave, let’s talk about the photoelectric effect.

Scientists of the time were also concerned with what was known as the photoelectric effect.  You could shine light on this metal sheet, and electrons would bounce off. 

It was thought that if you shined a brighter light on the sheet, such as going from a 50-watt to a 100-watt bulb, you would see the electrons shooting off the sheet of metal faster with more energy, like if you hit the pool ball harder.  But all it did was shoot off more electrons with the same energy. 

However, if you changed the frequency, the color of the light (see the chart above), the higher the frequency, the faster the electrons were shot off the metal sheet. Purple light has a lot more energy than red light.

Remember how Plank said the higher frequency, the bigger the chunk?  The higher the frequency, the more energy, so obviously, being hit by a bigger chunk of higher energy, the faster the electron would fly off the sheet in the photoelectric effect.

Einstein took over from where Plank left off and said light was made of photons.  Photons were particles of energy.  Remember we said particles were like really small billiard balls?  Light energy must come in chunks, and the size he discovered could be determined by Plank’s equation, E=hf.  We talked about Plank’s equation last lesson.  The energy in the light chunks is its frequency times h which is known as Plank’s constant, which was a very small number.  

Einstein explained the photoelectric effect. 

Here is a simple way of looking at it.  We used a kid’s toy in the last lesson, so let’s get another kid’s toy, a nerf gun.  If you were to shoot at an object with a nerf bullet, it might just bounce off the object and not do anything.  This would be like a low-frequency photon.  But now, pull out a real gun and shoot the object with a bullet.  The object might not only move but shatter.  This would be like a really high-frequency photon.

But wait, haven’t we been saying throughout this course that light was an electromagnetic wave?  How could an electromagnetic wave also be a particle?  How can you shoot things with light?  How can you play pool with light?

Think about this. Consider radio waves or waves sending information to your cell phone.   We can’t see or feel these waves, but we know they are all around us.  So how can they also be physical particles of energy?  We don’t feel ourselves being bombarded by little pellets or something.  How can something be a wave of energy and have physical properties at the same time?

So, what is light?  Is it a particle or a wave?

Are you ready for the answer?

It is both!  It is both physical and non-physical.

What?  That is not even possible.

Is it?

To discover the answer, let’s talk about the two-slit experiment.  This is the heart of quantum physics.  This is the experiment that changed the world.  This is the experiment that is going to blow your mind and forever change how you perceive reality.  Get excited.  If you haven’t heard of this before, your world is never going to be the same.

Look at the picture above. There is a board with two slits and behind the board is a photographic plate that will detect light.  We are going to shine light through the slits.

If we close the slit on the right and only let the light go through the left slit, we see that the light is detected by the photographic plate on the left directly behind the opening.  Nothing unordinary yet.  This would agree with Einstein.  Just as if we had shot BBs through the slit at the board.  The BBs would line up behind the open slit.  Sounds like particles to me.

If we close the slit on the left and only let light go through the right side, we see that the light is detected by the photographic plate only on the right.  Again just as if the light was made of physical BBs.

So what would you expect if you opened both slits at the same time?  You would expect most of the light to be photographed directly behind the slits, as in the figure above.

But that is not what happens.  When you open both slits, you get a pattern on the photographic plate like the one in the figure above.  You get multiple alternating light and dark areas spreading across the entire plate. This is called an interference pattern.  And this pattern is only made when two waves collide.

The image above is a picture of an actual wave interference pattern.

How can particles produce a wave pattern?

So let’s do this.  Let’s keep both slits open and let’s decrease the intensity of the light so only one photon goes through a slit every few seconds.  Keep in mind just one photon every few seconds.  If you wait long enough, do you know what pattern you will see on the photographic plate?  You will see the wave interference pattern.

What?  How can a single photon, which we know is a physical particle, go through a slit by itself and eventually produce a wave pattern like the one above?

Now close the slit on the right, so the single photon has to go through the left.  Now the wave pattern disappears, and we go back to what we would expect.  The photons line up behind the slit on the right the same as if you were shooting BBs through the slit.

If you are confused about this, don’t worry.  It makes no sense to the natural mind.  How can something go from being a wave to being a particle or from being a particle to being a wave?  Waves can’t be seen or felt.  Particles are physical.

But it gets even worse.  Scientists wanted to know what was going on.  So they used a detector.  They kept both slits opened, and they fired individual photons through the slits and wanted to use the detector to see which slit it was going through.  Something really strange happened. When they used the detector to see which slit the particles went through, you no longer got the wave interference pattern, but the photons lined up behind the slit it went through.  But when they took away the detector, the interference pattern returned.


It was like when we are looking, the photons became real particles, But, when we aren’t looking, they are waves. 

We are going to be spending a lot of time in future lessons talking about what this means.  But we have to stop right now.  This is something you really just need to think about for a while.  It is weird.  It is strange.  Again, remember you can’t make sense of it logically.  Many of the things we will discuss in future lessons make no sense logically, but they are true.

I’m really interested in the drawings that will be made for this lesson. If you would like to email me a picture of your drawing, I would love to share it with the rest of the class.

Don’t forget to teach someone the crazy thing you just learned.