Stroboscope

Physics, Biology:	Light, Colors, The Eye
Grade Range:	Elementary School, Middle School, High School
Format:	Stage

The Stroboscope demonstration uses a spinning wheel and a strobe light to introduce some interesting properties of the eye: Temporal Aliasing and Strobic Aliasing. It is also used to show the difference between Color Addition (how the colors of light mix) and Color Subtraction (how paints and dye mix).

Materials

• Stroboscope Motor
• Stroboscope discs
• Flathead screwdriver

Safety Precautions

This demonstration uses a strobe light, so ensure that nobody in the audience nor any of the presenters suffer from epilepsy prior to presenting. Read the Physical Demonstration section of the Demonstration Safety Page before performing this demonstration.

Demonstration

*NOTE*: Ask the contact or the audience before presenting this demonstration if anyone in attendance is prone to epileptic seizures. If so, they should leave the room before this demonstration, and have a co-presenter go with them to talk about the demonstration using the Temporal Tops demo.

Preparation: loosen the screws holding the brass washer/mount, and remove it from the front of the stroboscope. remove the front aluminum disc. Place your desired stroboscope disc on the rear aluminum disc, and return the front disc to hold it in place. Slide the brass washer/mount back on, and push it tightly against the other discs as you tighten the screws into place. Plug in the strobe light to the back of the stroboscope.

1. Point to the disc and ask the audience to identify the image you are using (Gears, Beach Ball, Dots, Spiral, Pistons, Red/Green/Blue Boxes)
2. Spin the disc with your hand, and explain how our eyes have a "speed limit", or a limit to the number of images, or frames, we can see per second (about 60 fps)
3. Turn on the stroboscope, and point out the blur we now see instead of an image
4. Hold the strobe light and turn it on, keeping it at a low flash rate. Slowly adjust the flash rate until you get a still image on the wheel. Explain how the strobe light is "setting the speed limit" for our eyes, giving us a certain number of frames per second.
5. Adjust the flash rate until you have a dynamic image; Picture discs are "moving" or the color wheel starts blending colors. Ask for answers on why the images are moving.
6. Adjust to a high flash rate, so you have a rapid dynamic image; picture discs are blurring, color wheel has a variety of colors on it. Explain the effect they are seeing.
7. Turn the strobe light down to the lowest flash rate, then turn it off. Turn off the stroboscope. As it slows down, and if time permits, you can pick out a second wheel to show them!

Why This Works

Short Explanation

This demonstration is focusing on the effect known as Temporal Aliasing. Temporal Aliasing is when an object is moving quickly, or an image is changing rapidly, and the motion being too fast for our eyes to follow. Our brains then receive the blurred image, and tries to process it as a non-blurry image. The result is that we "see" a pattern or motion that isn't actually there. This effect, when used on images, is how we developed movies and videos! a video camera will take a bunch of pictures every second (usually between 30 and 40 fps), and by flashing the images at our eyes really quickly, we "see" the movement in the video happen! Animated shows, such as cartoons, usually use a fewer number of images per second, so with an animated show we can actually see the tiny "jumps" in-between the pictures.

For the Color Wheel, we are using this effect to see how the colors of light mix together. The wheel has only four colors on it: Red, Green, Blue, and a little bit of White. When it is spinning really fast, and we don't have the strobe light on, we only see a purple-pink blur. This is because the colors are blurring past each other as we look at it, so our brains, rather than try to separate each color, will blend them together. When we flash the light at a medium speed, we see the colors start to blend, giving us Cyan (Blue-Green), Magenta (Red-Blue) and Yellow (Red-Green). When the light is flashing really fast, we start to see other colors as well, but especially more white! This is because colors of light are additive, meaning they give more colors as they blend. Paints and dyes, however, are subtractive, meaning they absorb more colors as they blend. White light gives out all colors, and black paint absorbs all colors. White paint reflects all colors, and black light would just be no light at all. The "black lights" we use are actually violet!

Full Explanation

COMING SOON

Tips and Tricks

• This demonstration is a part of the Electromagnetism Show.
• Be aware that some students may feel nauseated after watching this demonstration. This demonstration can trigger symptoms of motion sickness.
• This demonstration pairs well with the Temporal Tops, Color Mixing, and could be a useful addition to a show on the senses.