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Sources of Visible Light

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Imagine scuba diving and coming upon a jellyfish. Would you be startled if the jellyfish suddenly started to glow with bright yellow light, like the jellyfish in the picture above? If so, then the jellyfish would have done its job. Jellyfish give off light to startle possible predators. The light they emit is visible light.

The Light We See

Visible light includes all the wavelengths of light that the human eye can detect.   Most of the visible light on Earth comes from the sun.  It allows us to see objects in the world around us. Without visible light, we would only be able to sense most objects by sound, touch, or smell. Like humans, most other organisms also depend on visible light, either directly or indirectly. Many animals—including predators of jellyfish—use visible light to see. Plants and certain other organisms use visible light to make food in the process of photosynthesis. Without this food, most other organisms would not be able to survive.

Q: Do you think that some animals might be able to see light that isn’t visible to humans?

A: Some animals can see light in the infrared or ultraviolet range of wavelengths. For example, mosquitoes can see infrared light, which is emitted by warm objects. By seeing infrared light, mosquitoes can tell where the warmest, blood-rich areas of the body are located.

Luminous Light Sources

Luminous objects produce their own light.  The sun and other stars produce light because they are so hot.  Lightning and fire also naturally produce light in this way.  

Certain minerals like amethyst, diamonds, and emeralds can produce light by absorbing ultraviolet light and then giving off light in the visible range.  At the following URL, move your mouse over the minerals in the picture to see the magic of fluorescence:  http://www.fluomin.org/uk/ .  

Living things can produce light as a result of chemical reactions. The jellyfish in the opening photo above produces light this way.  So does the firefly (lightning bug) in the  Figure   below . Other organisms that can glow include some insects, worms, fish squid, bacteria, and fungi.  This is a more common property in ocean-dwelling organisms. 

License: CC BY-NC 3.0

The firefly is luminous. Its glowing abdomen helps it attract a mate. [Figure1]

Most artificial lights use electricity for energy and have a light bulb that changes the electrical energy to visible light. The production of visible light can happen in various ways, depending on the type of bulb.

License: CC BY-NC 3.0

The compact fluorescent light bulb (CFL) contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. Phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. [Figure2]

Incandescent lightbulb with glowing filament

Incandescent light bulbs produce artificial light when electric current passes through a wire filament inside an incandescent bulb, the wire gets so hot that it glows.

License: CC BY-NC 3.0

The "OPEN" sign is a long glass tube that contains neon gas. When electricity passes through the gas, it excites electrons of neon atoms, and the electrons jump to a higher energy level. As the excited electrons return to their original energy level, they give off visible light. Neon produces red light. Other gases produce light of different colors. For example, krypton produces violet light, and argon produces blue light. [Figure3]

License: CC BY-NC 3.0

LED stands for “light-emitting diode.” An LED light contains a material called a semi-conductor, which gives off visible light when an electric current flows through it. LED lights are used for traffic lights and also indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable. [Figure4]

Illumination

Many other objects appear to produce their own light, but they actually just reflect light from another source. Being lit by another source is called illumination. The moon in the Figure below is glowing so brightly that you can see shadows under the trees. It appears to glow from its own light, but it’s really just illuminated by light from the sun. Everything you can see that doesn’t produce its own light is illuminated by light from some other source.

Moon light landscape

The moon reflects the light from the sun.

Summary

  • Visible light includes all the wavelengths of light that the human eye can detect. 
  • Luminous objects produce their own light.  The sun is luminous and produces most of the visible light on Earth.  
  • Types of light bulbs include incandescent, fluorescent, neon, and LED bulbs. They produce light in different ways and have different uses.
  • Some minerals, gases, and animals can also produce their own light.
  • All objects you can see that do not produce their own light are reflecting light from another source. This is known as illumination.

Vocabulary

  • illumination: Being lit by and reflecting light from another source.
  • luminous: Objects that produce light.
  • visible light: Range of wavelengths of electromagnetic waves that the human eye can detect.

Review

  1. What is visible light?
  2. What are some examples of luminous objects?
  3. What do most artificial lights need to make visible light?
  4. On a sunny day, white sand on a beach seems to glow with light. What process explains why the sand is so bright?

Missouri Standards

  • 1.2.A.h

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0

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