Light plays a very important role in our lives. Without light we would not be able to see. Light from the sun generates heat, and can be used to generate electricity. To do this, light must travel to us.
Seeing the Light
Let’s take a moment to talk about visible light. As you can tell by the name, visible light is the light that humans can see. More specifically, you see the light that is not absorbed by objects. Green plants are green because they absorb all of the colors of the visible spectrum except the green color (you could also say the green wavelengths). A red wall is red to your eyes because it is not absorbing light from the red wavelengths. Mirrors reflect all of the colors of visible light.
We describe the world the way we see it as humans. Other living things on Earth see the world in different ways. Dogs only see things in black, white and gray. Some insects see colors that none of us can see. When you are learning about visible light you should remember we mean visible to humans. We should also mention that not all humans can see all the colors. There is an eye defect called color-blindness that affects many men. Color-blind men cannot see certain colors of the spectrum. It has to do with a genetic defect in their eyes.
Edges of Visibility
Although we can’t see them with our eyes, some wavelengths of light that bookend the visible spectrum are also important. Infrared radiation is next to the red portion of the spectrum. Infrared light is heat. Scientists use infrared light sensing optics when they want to see differences in temperature. Ultraviolet radiation (UV) is just beyond the violet end of the visible spectrum. UV light is given off by the Sun and absorbed by ozone in the atmosphere. Ultraviolet light can also mutate cells in your skin and give you skin cancer.
When a light ray hits an object and bounces off, it is called reflection. When you think of reflection, think about mirrors. They reflect all of the light. That is the reason you can see yourself. Even the ocean reflects light, just not all of it. If you are above the ocean, you can’t see the reflection that well, but when you are at an angle, look closely; you can see a reflection of the sky. So the ocean only has partial reflectivity.
Bending Light with Refraction
Lenses are pieces of glass that bend light. The easiest thing to think about is lenses in eyeglasses. People who do not have 20/20 vision might see things a little out of focus. They wear glasses or contact lenses to make their sight clearer. Those glasses have specially ground lenses that bend the rays of light just enough to focus the image for the person to see properly. All lenses bend and refract rays of light.
In the refraction section we said that light changes speeds when it moves from one medium to another. A medium is a substance like water, air, or glass. When light slows down or speeds up it changes direction a little bit. There are three basic shapes that a lens can have: concave, convex, and planar. A concave shape is bowed inward, like looking into a mixing bowl. Convex is just the opposite, bowed outward. Have you ever seen those mirrors in the grocery stores, where everything is reflected in a spherical way? That’s convex. Planar is just that, a plane. It’s a flat surface. Just think of a planar mirror on your wall.
Telescopes and microscopes are excellent examples of how lenses are used every day. By using different combinations of lenses, light is focused to create an image we could not see with the naked eye. Telescopes are able to see very distant objects that are very small to our sight and magnify them so we can see the details. The larger telescopes offer a greater ability to see objects that are more distant. Microscopes work with a similar idea but are concerned with size, not distance. Microscopes enlarge very small objects that are close to the viewer.
Prisms are a very special type of lens. When refraction is at work in a prism it breaks the beam of visible light into its basic colors. In visible light, the magic colors you can see are red, orange, yellow, green, blue, indigo, and violet. Scientists say ROY-G-BIV. A prism is made up of two planar surfaces at an angle. It uses the slower speed of light in glass to its advantage by refracting the light twice. Because of the different wavelengths of light, each color is refracted a different amount. When the light ray leaves the prism, it speeds up again (entering the air) and refracts a second time. That second dispersal creates the colorful spectrum of colors.
Enjoy these activities and video about light!
http://www.bbc.co.uk/schools/scienceclips/ages/7_8/light_shadows_fs.shtml Exercise about shadows
http://www.primaryresources.co.uk/science/powerpoint/transparent.swf Transparent, translucent and opaque objects
http://www.youtube.com/watch?v=2P3nKJHO2j0 Video about light, reflection and refraction