Last session we explored visible light. There is also light coming from the sun that we can’t see. This sessions explores black light also known as ultraviolet light. Ultraviolet light (UV) is the invisible light that comes from the sun. There are different kinds of UV light rays – some are dangerous like the UV-B rays that cause sunburn (put on sunglasses). We wear sunglasses to help protect our eyes from the harmful UV light rays; we put on sunscreen to protect our skin. Most UV light is absorbed by Earth’s atmosphere but some gets through.
Some animals like bees can see ultraviolet light but humans cannot. A crocus looks very different to a bee. Flowers often have ultraviolet “nectar guides,” which are invisible to humans but aid foraging bees. The compound eyes of bee detect colors such as blue, green and ultraviolet (polarized and unpolarized), whereas human eyes have color receptors for blue, green and red. The nectars are ultraviolet to their eyes, helping bees to identify them easily. Ultraviolet light detection also help the bee to use the sun for navigation.
Bee Printmaking Activity: Use a pencil to carve out a bee design on a piece of foam. Use black ink or black paint to print from the foam plate. Try making a series of prints, printing on colored card stock, then fold into cards. Older children can write a story of their bee on a cards.
Sun Prints Activity: Explore invisible light by making sun prints. Sunprinting is based on the cyanotype process, which has been used by artists since its development in the 19th century. Objects that eclipse the sun’s light will show up in white, with the rest of the paper turning a vibrant Prussian Blue or a gradient between these two ends. Sunprint paper is sensitive to ultra-violet light. Sun Sensitive Paper is coated with light-sensitive chemicals, which react to light waves and particles when exposed to ultraviolet light. When you place objects on the paper, they block the light and turn white while the paper around them remains blue. Water stops the process and fixes your images on the paper. We use the sun to make prints of physical objects.
The colors of light that the human eye is able to see range roughly from red to blue in color. Blue light has a higher frequency than red light. The light which has frequency just lower than red light is called “infra-red”, and the light which has frequency just higher than blue light is called “ultra-violet”. Infra-red light is often used to warm things like heat lamps over food in restaurants
Explore Black Light: in a dark room under black light, have children draw with a highlighter on white paper or paint with florescent paint on black foam board. Try drinking “glow” water (tonic water) in black light or building with glow in the dark playdough and toothpicks. A black light is just a light bulb designed to emit long wave (UV-A) ultraviolet light and not much visible light. If you put a black light in a dark room it really does not brighten it very much…the room remains almost black with a blue hue. These bulbs do emit lots of light however it’s just that we can’t see it.
About Fluorescence: Some materials and animals have the special property that they absorb ultra-violet light and then re-emit the light at lower frequencies that our eyes can see. This is called “fluorescence”. These materials are sometimes found in minerals like rocks, on our t-shirts, jackets or shoes, and when we walk near a black-light they will seem to “glow” since they are translating the invisible ultra-violet light into easy to see colors, most often white.