Stroboscope

Physics, Biology:	Color Mixing, Temporal Aliasing
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 light and how our eyes work. This demonstration uses a strobe light, so be sure to ask beforehand about anyone being at risk for epileptic seizures.

Materials

• Stroboscope Motor
• Stroboscope discs
• Flathead screwdriver

Safety Precautions

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

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 demonstration.

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, then plug in the stroboscope itself.

Presentation

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, our eyes can capture per second (about 60 fps)
3. Turn on the stroboscope, and point out the blur we now see instead of an image. Ask the audience what they might see when we turn on the strobe light.
4. Hold the strobe light and turn it on at the low flash rate, keeping it aimed at the spinning wheel. 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 and 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 moving extremely fast, 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 is too fast for our eyes to follow. Our brains then receive the blurred image, and try 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

Aliasing is a distortion of a frequency or a signal, resulting in mis-identification. Temporal Aliasing is when we have a distortion caused by the length of time given for the frequency or signal. This effect is common, but it isn't an effect that we think about much because of how often we see it. The wheel, when spinning, is moving much too fast for our eyes to catch it. No matter what image is on the wheel, if we don't use the strobe light we cannot see the image that is on it because it is moving too quickly. This is why we will see circles of color on the wheel instead of the original image, same as how fan blades will become a blur while spinning. If something is moving fast enough, it can look to have a motion that we know isn't actually there, like how a car's tires might look like they are spinning backwards while driving.

We see this blur of motion because our eyes have a speed limit. Frame Rate refers to how many images (frames) something can take per second. Our eyes have a frame rate of about 60 frames per second, or fps, which is fast enough to catch the motion of most things we see. Current animated shows usually have 30-40 fps, and live-action shows will often use 50 fps. Live-action shows use enough frames so to give a natural look to the movement of the actors on-screen, but animated shows are slow enough that our eyes can catch the small "jumps" between frames. If an object is moving too quickly for our eyes to catch it, however, then our brains will try to find patterns in the movement. When a pattern is found, our brains will focus on it, which results in the illusions of movement we see.

By using the strobe light, we can manipulate the motion we "see". Every time the light flashes onto the wheel, the flash is interpreted as a single frame, which affects the frame rate that we are picking up. By having the light flash faster, we can find a point where every flash matches up with the same point on the wheel, making the image look frozen. We can even adjust how fast the wheel appears to move, making it into a dynamic image with spinning balls, rotating gears, or even pumping pistons! This type of aliasing is also known as Strobic Aliasing, since it is induced by the use of a strobe light.

The Color Wheel is unique, for it shows the ways that the colors of light mix together. We know that the primary colors of pigments are red, blue, and yellow. We can mix these colors together to get all other colors, and by mixing them in equal amounts we can get black. However, we know that we cannot mix them together to make white. To get white, we have to remove these colors, because white is the absence of color for pigment.

Color Mixing

Additive (Light)		Subtractive (Paint)
Primary Colors	Red, Blue, Green	Primary Colors	Red, Blue, Yellow
Red + Blue	Magenta	Red + Blue	Purple
Red + Green	Yellow	Red + Yellow	Orange
Blue + Green	Teal	Blue + Yellow	Green
Red + Blue + Green	White	Red + Blue + Yellow	Black

The color wheel has red, blue, green, and white on it. This might seem odd at first, since green is not one of the primary colors for pigment. This is true, and we can tell that when the wheel starts spinning; When the wheel is spinning fast, we see a dark purple blur instead of black. This is because we are looking at pigments mixing, so we can identify the blue/red as purple, the blue/green as a dark teal color, and the green/red as a murky purple/brown color. These colors are blurring together, with them being slightly brighter due to the little bit of white on the wheel.

The strobe light makes a big difference, and when it is flashing fast enough we see some new colors show up on the wheel! The colors we see are because of the light from the strobe light bouncing off the wheel to our eyes, which results in the colors we see not blending on the wheel, like before, but in our eyes. Another thing about these new colors is that they are all lighter shades than the colors before them, which contrasts how we see pigment colors mix. This happens because of the difference in ways that light and pigment mix.

Light is additive, meaning that light can give out all the colors that are mixed into it. The additive property of light is why we use white lights to see. White light is a mix of all colors of light. This means that when white light hits a surface, there will always be at least one color of the light that reflects back, except black. The additive property of light is also why we cannot have a true "black" light. Black would indicate a lack of all colors of light, so a "black light" would just be darkness. Instead, the black lights we use are actually violet lights, which have a lot of energy to them and can cause other materials to glow in the dark!

Pigment is subtractive, meaning that it will absorb all colors of light except for the one it reflects. This is why pigments become darker shades as you add more colors to them; The additional colors absorb some of the other colors already mixed in, so less light is being reflected as other colors are being added. This is also why we cannot get a perfect "white" paint. White means that there is no color being absorbed into the surface, so a perfect white paint would actually be a perfect mirror. The white pigments we use are shades of white, but they still are absorbing some color, which is why we see them clearly instead of seeing just a reflection!

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.