Ben Franklin and Light
1783-1784 Signed Peace Treaty with Great Britain. Invented bifocal eyeglasses.
"I had...two pair of spectacles which I shifted occasionally as in traveling I sometimes read, and often wanted to regard the prospect [look at the view]. Finding this change troublesome, and not always sufficiently ready, I had the glasses cut, and half of each kind associated in the same circle, this: By this means, as I wear my spectacles constantly, I have only to move my eyes up or down, as I want to see distinctly far or near, the proper glasses being always ready."-Ben Franklin
Seeing Different Wavelengths of Light
Use a prism to refract light and project the visible spectrum of colors. Glass is more dense than air, so the light bends as it passes through the prism. Each of the colors in the spectrum bends a different amount so you can see the different colors.
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| Unfortunately the rainbow was so pale it does not show up well in the photo. |
The angle of reflection equals the angle of incidence.
We taped a circle of black construction paper with just a small slot cut out over a flashlight so that we could get one solid straight line of light. When we placed a mirror against the edge of the table and shined the light on it, one could trace the beam as it traveled from the flashlight and reflected off the mirror. Changing the position of the flashlight, and therefore the angle of the incoming beam, made the outgoing beam but you could see that the angle of the reflection was always the same as the angle that the light came in on, or the angle of incidence.
Refraction of Light
When light (or sound) waves reflect off an obstacle, the law of reflection allows us to determine where the reflected waves will go.
We taped a circle of black construction paper with just a small slot cut out over a flashlight so that we could get one solid straight line of light. When we placed a mirror against the edge of the table and shined the light on it, one could trace the beam as it traveled from the flashlight and reflected off the mirror. Changing the position of the flashlight, and therefore the angle of the incoming beam, made the outgoing beam but you could see that the angle of the reflection was always the same as the angle that the light came in on, or the angle of incidence.
When light encounters a transparent obstacle, some of the light will pass through the obstacle.
For this one, we needed a line draw at a 45 degree angle across the page. We used a 13 x 9 inch baking dish, filled it with water and added a tiny bit of milk (1/2 teaspoon). We laid the flashlight on the table, letting the light beam follow the line. When the beam hit the water, part of the light bent away from the line because light travels faster in air than in water.
Light that travels from one substance to another is bent according to the relative speed of light in each substance.
By putting a butter knife in a glass of water, a common illusion results from this effect.
By putting a butter knife in a glass of water, a common illusion results from this effect.
How the Eye Detects Color
Eyes are some of the most complex structures of the body. They use light and electrical impulses to allow our brains to see our surroundings. Human eyesight is our strongest of the senses. Our eyes have a lens that focuses light onto the back of the eye, a curved screen of light sensing nerve cells. When the image hits the back of the eye it is upside down and our brain turns it back right side up. We can see color because we have specialized cones (those nerve cells) that allow us to see color. We also have rods, the black and white vision cells. Many animals, like dogs, have only rods, no cones and so they cannot see color.
Using the masters from this book, you can make models of the eye.
The outer eye is, of course, what we are used to seeing and makes what we see eye-shaped.
We used plastic wrap for the cornea, or the whites of the eye, as it is a protective layer.
The iris changes size as it expands and contracts, letting more or less light in depending on the amount of light in the area.
The lens bends light so that we can focus our eyes to see things near to us and far from us.
"The retina is made up of rod cells that help us to see in dim light and distinguish white, black and gray and of cone cells that help us see in bright light and in color."The boys had fun drawing in cone and rod shapes.
"The nerves pass through a layer of blood vessels that deliver to eye cells nutrients and oxygen in the blood."
All the nerves join at the back of the eye in the optic nerve, which carries the electrical signals from the eye to the brain.
Sources:
Exploring Creation with Physical Science, Jay Wile
The Body Book, Donald Silver
The Ben Franklin Book of Easy and Incredible Experiments, A Franklin Institute Science Museum Book
