Demos

Index of refraction - bending light

Learning Objective: In different materials, light travels at different speeds determined by the index of refraction of the material.

Description:

Light travels fastest in vacuum, with a speed of about c = 300,000 kilometers per second (186,000 miles per second). All materials have a specific property called index of refraction which determines the speed at which light will propagate in the material. The index of refraction is determined by dividing the speed of light in vacuum by the speed of light in a material. The index of refraction of vacuum is then defined to be 1.

A larger index of refraction means light travels slower, and a smaller index of refraction means light travels faster. Different wavelengths of light "see" different indices of refraction. This is what causes white light (a combination of all visible wavelengths/colors) to spread into different colors as it passes through a glass prism.

When light encounters a boundary between two materials with different indices of refraction, it bends as it is changing speed. A special case can occur if light is traveling from a material with higher index of refraction to a material with lower index of refraction. If the light's angle of incidence with respect to the boundary exceed a material specific critical angle, total internal reflection (TIR) occurs. This can easily be seen with a laser pointer and a water-air interface.

This principle of TIR is what allows light to be trapped and guided by optical fibers. The fibers are made of out two materials, a glass core of higher index and an outer cladding of lower index. Light travels through the core of the fiber and is totally internally reflected every time it hits the cladding. Optical fibers are used in a number of different applications including long distance communication, medicine, radio and TV broadcasting, and computer networks using the internet

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Polarization

Learning Objective: Light is in general unpolarized, but has polarized components.

Description:

Light waves are produced by vibrating electric charges. Polarization is a property of light that describes the direction of the wave's vibration. Much of the light we encounter on a daily basis is unpolarized, for example light from the sun, a lamp, or flashlight. The unpolarized light waves are vibrating in many different directions. It is possible to transform unpolarized light into polarized light by passing it through a polarizer. The polarization of the light will depending on the orientation of the polarizer.

This property of light it utilized in glare reducing or polarized sunglasses. Again, light from the sun is unpolarized, but when it is reflected off of surfaces like water or the road, it becomes linearly polarized, or polarized along one direction. The sunglasses are a polarizer that blocks the polarization of the reflected light.

In contrast to sunlight, the light we see from computer screen or cell phones is polarized. If you hold up a polarizer to the screen, you will see that it appears dimmer because a component of the polarization is being blocked. If you rotate the polarizer, eventually you will reach a point where all of the light will be blocked and the screen will appear completely dark. You can try this at home by using polarized sunglasses as the polarizer! This is also an easy way to check if your sunglasses are polarized.

We can also use polarizers to show the internal stress of plastic objects like plastic utensils or cellophane. If we shine polarized light through the plastic and then through a polarizer, we can see all the colors of the rainbow appear in different places in the plastic. When the plastic pieces are formed, different areas of the material get stretched or compressed which causes the polarization of different wavelength/colors of light to change. This effect is called photoelasticity birefringence.

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diffraction

Learning Objective: Light behaves as a wave and interference causes different diffraction patterns to appear.

Description:

Diffraction is the bending of the light waves around objects. An analogy we can think about is ripples in a pond. When the ripples of water are travelling across the surface and hit a rock, they bend around the rock and continues travelling. The waves interfere with themselves and makes the different patterns around the rock. This is the same thing that happens when light diffracts.

If we shine a laser pointer at a mirror, what happens? It is perfectly reflected and maintain the same beam shape. If we shine the laser at the back of a CD, we will an intricate pattern of rectangle like features is reflected. Even though a CD looks very similar to a mirror, there are many grooves with tiny squares that are used to store information. The laser is diffracted when it hits the surface of the CD because the small grooves are close to the size of the wavelength of light.

Different wavelengths/colors of light will be diffracted different amounts around the same object. So white light, made up of all of the visible wavelengths of light, will spread out into different colors after it passes through a diffraction grating.

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Laser Communication

Learning Objective: Information can be sent over long distances using laser light.

Description:

Have you ever wondered how we are able to send information across long distances to satellites or over oceans to other continents? Laser communication systems encode information in the laser light by modulating its intensity. When the light hits the receiver, the changes in intensity are converted into an electrical signal and the information is recovered. The principle is used in Bluetooth, the internet, and cellphone communication.

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UV Light - Making UV bead Bracelets

Learning Objective: The structure of a material on the nano-scale can change the macroscopic properties of a material.

Description:

There are many different wavelengths of light - the electromagnetic spectrum of light is large and there many wavelengths that we cannot see. The wavelengths or colors of light that we can see are in the visible spectrum. The light from the sun is made up of many shorter wavelengths in the ultra-violet (UV) spectrum which we cannot see. Each wavelength of light has an energy associated with it, and UV light has higher energy than visible light.

These UV Beads change color when exposed to UV light or sunlight. The beads are made up of polymers which change the orientation of their bonds when exposed to high energy UV light. This change causes the bead to appear a different color for a period of time. If we were to shine visible light, say from a flashlight, on the beads they would not change color because the light doesn't have high enough energy to change the bond orientation in the bead material. Since sunlight contains many wavelengths of higher energy, the beads will change color outside on a sunny day. This is also the reason we need to wear sunscreen to protect our skin from the high energy rays. Sunscreen should also stop the beads from changing color.

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Colorful Chemistry -

pH and Fingerpainting

Learning Objective: Colors can tell us different things about different properties of materials.

Description:

Plants have amazing properties and chemicals. Many fruits and vegetables contain anthocyanins, healthy substances that are fun to play with because they can instantly change color. The juice from red cabbage can be red, purple, blue or green and then be made to change color by changing its chemical environment, something that you can do with things found in the kitchen. The interaction of red cabbage juice with light tells us important information about different substances and allows you to make unique and surprising art.

Chop up red cabbage and simmer in a little water for about 20 minuets. Strain the juice, often red or purple, and let the juice cool before use. Anthocyanins from the cabbage have dissolved in the water and produce an intense color. The juice is red if the juice is slightly acidic or blue to green if the juice is alkaline, also called basic. Diluted vinegar, carbonated soft drinks and citrus juices are used to turn the juice acidic while baking soda in water is used to turn the juice basic.

For beautiful natural colors and color change: Dilute your juice extract with tap water to a pleasant color. Add a few drops of water with baking soda and gently mix. The baking soda water acts as a base and should turn the red or purple juice water to blue or green. Then add a little weak vinegar solution to the blue juice solution until it changes back to red or purple. With a little experimentation you can produce almost any color you like and can even reverse the colors. You can use the pH color scale to find the pH of other things.

You can also make color-changing Litmus paper. Soak a sheet of paper in your juice extract to pick up color. The pH of the paper may cause the extract to change color as its dries so you may want to try several different types of paper. Cut the paper into strips and use them to determine what is acidic or alkaline by simply dipping the paper into the liquid or food. Make a color chart to compare foods. Fold a Litmus paper sheet symmetrically into a compact square or triangle and dip corners into acidic and alkaline solutions to get different colors. Unfold and let dry to reveal a masterpiece.

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