Day 93

So today we started 2nd Semester, and decided we would look at some of the stuff we understood, or thought we understood, about the Particle Model of Light.

I began by asking 3 questions:

1) If light behaves like a particle, what should we expect light to do if streams of light particles try to pass through each other? (we never went out of our way to look at or address this during 1st semester)

2) Why does light refract? When will it bend towards the normal or away from the normal?

3) If we shine a beam of light particles through a tiny opening, describe and diagram what happens. If we make the opening smaller, what changes, if anything.

Responses to each of the three questions can be summed up as follows:

1) they should bounce off of each other, and cause scattering of particles. We should be able to see these particles. (some kids said the light intensity should be greater too.)

2) Light speeds up or slows down as it passes into a new medium, and it will bend towards the normal when it slows down.

3) The opening will only allow some light to go through, and we will see a tiny dot of light. If the hole is smaller, then less light goes through, and we see a smaller dot of light.

Well, lets test it out!

Demo 1: I took two lasers (since this would be a really high concentration of light particles traveling together) and had them cross though each other. First, we do not see any scattering. If the light particles were hitting and colliding, then they should collide, scatter, and we would see them. Second, even when we spray disco fog into the beams, we do not see the lasers scattering any light. They just pass through each other.

What does that mean? Well, maybe not much. Students brought up the times I said that light is not a physical particle like a ball. They also mentioned that it was simply a model, and not meant to be taken literally (light = mass particle). It just transfers light energy.

Demo 2: So the setup: I put a big white board leaning up to a table like a ramp. That’s it. If you roll a ball along the table top, at an angle to the ramp, once the ball gets tot he ramp it rolls down the ramp, bending inward towards the “normal.”

Students: Well that did what we expected.

Me: Why is that?

S’s: Well it bent in towards the normal.

Me: But that happens when light enters into a medium with higher index of refraction. This was air and air.

S’s: Yeah, but gravity is always going to speed it up, and so it bends in towards the normal.

Me: but with light we said it bends towards the normal when the light slows down. Here it bent inward and sped up!

S’s:….oh.

So now there is a bit of a problem with the model, a particle does not behave like we thought it would, so there is a bit of disconnect between our particle model and how light behaved. Still, students argue that it can’t be compared because of mass and gravity issues. Still, it does introduce a bit of dissonance.

Demo 3: A laser beam is shown through two razer blades. At first the students see the small dot of light that they predicted. That’s when I move them closer together, and wham, you get something like this.

That is NOT what was expected. A single stream of particles has been separated into a series of streams of particles. It is not an illusion either. If you use a bright enough laser in a dark enough room, disco fog can be sprayed and students can see the lines of light travel across the room, separating from each other, and the empty spots in-between.

Now the cognitive dissonance is at a high. What does all this mean? maybe a particle model of light is not sufficient to explain how light behaves. We need something more.

But we cannot completely ignore a lot of what we know already.

What else can transfer energy?

Waves.

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