Did you ever hear the proverb, “the eyes are the windows to the soul”? While it’s true that our eyes often express what we are thinking or feeling, they are windows in another sense as well. Like windows made of glass, the eyes let light in and allow us to see.
How We See
Imagine you are in a room with no windows. You are looking at the chair across the room when suddenly the lights go out. Now you can't see the chair anymore. Why? Objects that do not produce their own light are not visible in the dark.
Except for objects that give off their own light (natural light), we don’t see things just because light strikes them. We see things because light strikes them and then reflects, or bounces back, from their surface. What we see is the reflected light.
The ability to see is called vision. This ability depends on more than healthy eyes. It also depends on certain parts of the brain, because the brain and eyes work together to allow us to see. The eyes collect and focus visible light. The lens and other structures of the eye work together to focus an image on the retina. The image is upside-down and reduced in size, as you can see in the Figure below. Cells in the retina change the image to electrical signals that travel to the brain through the optic nerve. The brain interprets the electrical signals as shape, color, and brightness. It also interprets the image as though it were right-side up. The brain does this automatically, so what we see always appears right-side up.
The Light We See
Imagine scuba diving and coming upon a jellyfish. Would you be startled if the jellyfish suddenly started to glow with bright yellow light? 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. 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. Visible light can be produced naturally or created artificially.
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.
Natural Luminous Light Sources
Luminous objects give off light on their own. This can be produced naturally or created artificially. The sun and other stars are examples of naturally produced 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. 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, like the jelly-fish mentioned above.
Artificial Light Sources
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.
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.
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. The light from the sun is reflecting off the moon. Everything you can see that doesn’t produce its own light is illuminated by light from some other source.
The moon reflects the light from the sun.
- Visible light includes all the wavelengths of light that the human eye can detect.
- We see because light is reflected to the eye and then interpreted by the brain.
- Luminous objects produce their own light. The sun is luminous and produces most of the visible light on Earth. Some minerals, gases, and animals can also produce their own light.
- Types of light bulbs include incandescent, fluorescent, neon, and LED bulbs. They produce light in different ways and have different uses.
- All objects you can see that do not produce their own light are reflecting light from another source. This is known as illumination.
- 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.
- What is visible light?
- What are some examples of luminous objects?
- What is the difference between natural and artificial light?
- On a sunny day, white sand on a beach seems to glow with light. What process explains why the sand is so bright?
- Explain the meaning of the following sentence: The eyes sense light but the brain sees.