When you look at something or hear it make a sound, have you thought about how you can tell where it is? How do you determine its direction and how far away it is? Studies of human sight and hearing tell us that two different systems are involved. One system works for sight, and another for sound.
Hold up a finger at arm’s length from your face. Close one eye and look at the finger and what is beyond your finger. Now switch eyes, and you will see that objects beyond your finger appear to move. When you look at a distant object, the brain receives two signals–one from each eye. Based upon how much the background seems to vary, your brain then computes how far away the object is. That’s how you can tell where it is because your brain combines both images to give you a distance perspective.
To locate a sound’s source, the brain gets a signal from each ear. The two signals arrive at slightly different times depending on the width of the skull and the direction of the sound. We cock our heads to take into account the angular location of the source, and the brain creates an auditory spatial map that pinpoints the sound. Your senses handle sound differently from sight because of the difference in speed of the two signals. Light travels at 186,000 miles (300,000 m) per second and sound travels at 1087 feet (331 m) per second. Your brain combines the object’s sound signals received by both ears, and that is how you can tell where it is.
All of this is amazing enough, but researchers at the Max Planck Institute for Biological Cybernetics in Tubingen, Germany, and Queen’s University in Kingston, Ontario wanted to learn more. By using visual tests on a barn owl while monitoring its brain activity, they found that different nerve cells respond to “specific angular differences.” The barn owl used auditory methods with its vision to give it a three-dimensional map of the area. In that way, the owl has an instant picture of where to fly to get the most unobstructed path to its target. The director of the institute said, “We speculate that the brain uses similar algorithms to solve similar problems” such as matching problems.
How is it possible for us to see through objects (like air, water, and windows) and not through others (like wood, steel, and window blinds)?
Light is a form of electromagnetic wave energy oscillating in a particular frequency range and energy level. There are many more frequencies (and energy levels) in the spectrum of electromagnetic waves. X-rays are electromagnetic waves at a higher frequency than light. Radio waves from cell phones, radio, and Bluetooth devices are also electromagnetic waves at a lower frequency than light. We can’t see the waves that are above or below light frequencies because our eyes were not designed to see them.
We say that an object is opaque if we can’t see through it and transparent when we can see through it. When some light passes through an object, we say that it is translucent. Wood is opaque to visible-light frequencies, but it is transparent to electromagnetic waves in other frequency ranges. For that reason, we can listen to the radio or use our cell phones or wi-fi inside our houses. Our bodies are partially transparent to X-rays. That allows doctors to use X-rays to check for broken bones.
If our eyes were sensitive to radio waves and not light frequencies, we would be able to see through most solid objects. Then we would not only lose our car keys, but we would also lose our car–and our house too! The things we need to see would be invisible, and all of the electromagnetic waves around us would fill our vision with confusion.
Electromagnetic waves of different frequencies can pass through some materials but not others because of their wavelengths and the energy levels of the electrons in the atoms of the materials. So X-rays can pass through skin and muscle better than through bones. Radio waves can pass through wood, but not through steel. Light can pass through clear glass, but not wood or steel or cookie dough.
Our ability to see the incredible color in the world around us is amazingly complex. We don’t actually see color with our eyes. We see color with our brains.
Most humans have trichromatic vision. Our eyes only detect red, green and blue. If our eyes detect a lot of red and green but not much blue, our brains decide that we are seeing yellow. When our eyes register equal amounts of red, green, and blue, our brain decides that we are seeing gray. If red and blue are present, but not much green, our brain decides we are seeing purple.
Some of us do not have red or green receptors in our eyes, especially people with XY chromosomes (males). We call it color blindness, but in reality, our eyes just don’t see one particular set of wavelengths. Some of us with XX chromosomes (females) may have tetrachromacy which means we see more than the three primary colors.
In the animal world, color is produced by many different techniques. The wings of the Morpho butterfly appear to be blue or violet depending on how the light strikes them. This is due to light-scattering scales that cover the insect’s wings. Dragonfly wings look similar to the Morpho wings, but the dragonfly’s color comes from waxy crystals that cover layers of the pigment melanin. We call the method of color production in these insects “structural color” because it is produced by the structure of the material rather than by pigments. Cameleons also use structural color using nanocrystals in their skin. They can tune the nanocrystals to reflect different colors. In this way, they can match the color of their environment or their mood.