Laser demo in 4th grade
Last year I borrowed some supplies from Optical Society of Chicago for a hands-on optics demo in DD1's 3rd grade classroom. They liked it so much, that they asked me to do the same for the other class.
Then, they asked me to come back and talk to them about lasers. Uh-oh.
Here is a short overview of what I did in my lunch hour on Wednesday afternoon:
Light bending
The effect of creating a rainbow out of a prism actually results from the chromatic dispersion. That is speed of light inside transparent media is different for different wavelengths. Blue light travels slower.
Then, you take a concave lens and the edge of it is virtually a prism. And thus you get the effect of chromatic aberration - focal length is different for different wavelengths (i.e. different colors will focus at different distance from the lens. So, in simple optical systems you will have color halo effect around the observed object (i.e. a star will have a colored band). To cancel this effect, a chromatic doublet is used - in addition to the convex lens, a second lens, concave, of slightly different index of refraction (speed of light will be different). The two are glued with optical cement.
More on this and other topics here:
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
Now, what about those lasers?
Laser stands for Light Amplification by Stimulated Emission of Radiation. Well, they did remember that L stands for light :) Whenever I give a mouthful term like that, I explain what it means. It's light. Well, it's making more light (i.e. amplification). The Radiation is also light, but one that is created by the material of the laser (as opposed to incident light). Emission of Radiation simply means "making light". And we stimulate the light, we make it shine. So, in lame terms, LASER means "making more light by making light". They still didn't get it, I think.
Then, I explained how that emission of radiation happens. How electrons get excited (careful! kids are not getting excited, right?). But then, do you like working hard? I was surprised how many kids answered the presumed "yes!". But then they regressed to "noooo". Aha! Well, the electrons don't like working either and they jump back to their ground level immediately (the kids already know about the structure of the atom). So, here I tried very hard to stress out that electrons are only permitted on specific energy levels, quantum levels, never in-between. Aha, and when they drop from level 4 to level 3, that extra energy is emitted in form of radiation (light). And since the energy levels are the same, the light is always the same color (and I really should have explained that energy = constant/wavelength, but was nervous and forgot!)
So, after the explanation we made a laser with 144 ping-pong balls, a box and a tray...
The box of ping-pong balls (electrons) is sitting on the floor in "ground" state. One very brave volunteer is holding a tray. Two brave volunteers (careful, she's brave, but she didn't count on being hit!) transfer the balls from the box to the tray in handfuls. The tray is shaking and the electrons transfer from level 2 (bottom of tray) to level 4 (top of tray) and topple down to the ground making sound (that's the light!). Five volunteers serve as amplifiers and clap every time they hear (erm, the floor is carpeted!) the ball fall.
Then we saw a 35sec video with much better representation of ping-pong balls as electrons and with strobe light as the laser.
It was great. The kids enjoyed. They asked questions. And I hope that somewhere, somehow, maybe some of them are a little more interested in science or engineering...
Then, they asked me to come back and talk to them about lasers. Uh-oh.
Here is a short overview of what I did in my lunch hour on Wednesday afternoon:
Light bending
The effect of creating a rainbow out of a prism actually results from the chromatic dispersion. That is speed of light inside transparent media is different for different wavelengths. Blue light travels slower.
Then, you take a concave lens and the edge of it is virtually a prism. And thus you get the effect of chromatic aberration - focal length is different for different wavelengths (i.e. different colors will focus at different distance from the lens. So, in simple optical systems you will have color halo effect around the observed object (i.e. a star will have a colored band). To cancel this effect, a chromatic doublet is used - in addition to the convex lens, a second lens, concave, of slightly different index of refraction (speed of light will be different). The two are glued with optical cement.
More on this and other topics here:
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
Now, what about those lasers?
Laser stands for Light Amplification by Stimulated Emission of Radiation. Well, they did remember that L stands for light :) Whenever I give a mouthful term like that, I explain what it means. It's light. Well, it's making more light (i.e. amplification). The Radiation is also light, but one that is created by the material of the laser (as opposed to incident light). Emission of Radiation simply means "making light". And we stimulate the light, we make it shine. So, in lame terms, LASER means "making more light by making light". They still didn't get it, I think.
Then, I explained how that emission of radiation happens. How electrons get excited (careful! kids are not getting excited, right?). But then, do you like working hard? I was surprised how many kids answered the presumed "yes!". But then they regressed to "noooo". Aha! Well, the electrons don't like working either and they jump back to their ground level immediately (the kids already know about the structure of the atom). So, here I tried very hard to stress out that electrons are only permitted on specific energy levels, quantum levels, never in-between. Aha, and when they drop from level 4 to level 3, that extra energy is emitted in form of radiation (light). And since the energy levels are the same, the light is always the same color (and I really should have explained that energy = constant/wavelength, but was nervous and forgot!)
So, after the explanation we made a laser with 144 ping-pong balls, a box and a tray...
The box of ping-pong balls (electrons) is sitting on the floor in "ground" state. One very brave volunteer is holding a tray. Two brave volunteers (careful, she's brave, but she didn't count on being hit!) transfer the balls from the box to the tray in handfuls. The tray is shaking and the electrons transfer from level 2 (bottom of tray) to level 4 (top of tray) and topple down to the ground making sound (that's the light!). Five volunteers serve as amplifiers and clap every time they hear (erm, the floor is carpeted!) the ball fall.
Then we saw a 35sec video with much better representation of ping-pong balls as electrons and with strobe light as the laser.
It was great. The kids enjoyed. They asked questions. And I hope that somewhere, somehow, maybe some of them are a little more interested in science or engineering...