When light rays strikes a surface, part of the beam passes through the surface and the rest reflects back, like a ball bouncing on the ground. Where it bounces depends on how you throw the ball.
Have you ever looked into a pool of clear, still water and seen your own face? The surface of the water acts like a mirror and you can see your reflection. (In fact, before mirrors were invented, this was the only way people had to look at themselves.) If you were swimming below the surface, you’d still see your own face – the mirror effect works both ways.
Have you ever broken a pencil by sticking it into a glass of water? The pencil isn’t really broken, but it sure looks like it! What’s going on?
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When a beam of light hits a different substance (like the water), the wavelength changes because the speed of the light changes. If you’re thinking that the speed of light is always constant, you’re right… in a vacuum like outer space between two reference frames.
But here on Earth, we can change the speed of light just by shining a light beam through different materials, like water, ice, blue sunglasses, smoke, fog, even our own atmosphere. How much the light speed slows down depends on what the material is made of. Mineral oil and window glass will slow light down more than water, but not as much as diamonds do.
How broken the pencil appears also depends on where you look. In some cases, you’ll see a perfectly intact pencil. Other times, you’ll guess neither piece is touching. This is why not everyone can see a rainbow after it rains. The sun must be at a low angle in the sky, and also behind you for a rainbow to appear. Most times, you aren’t at the right spot to see the entire arc touch the ground at both ends, either.
Lenses work to bend light the way you want them to. The simplest lenses are actually prisms. Prisms unmix light into its different wavelengths. When light hits the prism, most of it passes through (a bit does reflect back) and changes speed. Since the sunlight is made up of many different wavelengths (colors), each color gets bent by different amounts, and you see a rainbow out the other side.
Double Your Money
Here are a few neat activities that experiment with bending light, doubling your money, and breaking objects. Here’s what you do:
Materials:
- glass jar (or water glass)
- penny
- eyeballs
Download Student Worksheet & Exercises
Here’s what you do:
1. Toss one coin into a water glass (pickle jars work great) and fill with an inch of water. Hold the glass up and find where you need to look to see TWO coins. Are the coins both the same size? Which one is the original coin? (Answer at the bottom of this page.)
2. Look through the top of the glass – how many coins are there now? What about when you look from the side?
3. Toss in a second coin – now how many are there?
4. Remove the coins turn out the lights. Shine a flashlight beam through the glass onto a nearby wall. (Hint – if this doesn’t work, try using a square clear container.) Stick a piece of paper on the wall where your light beam is and outline the beam with a pencil.
5. Shine the light at an angle up through the water so that it bounces off the surface of the water from underneath. Trace your new outline and compare… are they both the same shape?
6. Add a teaspoon of milk and stir gently. (No milk? Try sprinkling in a bit of white flour.) Now shine your flashlight through the container as you did in steps 4 and 5 and notice how the beam looks.
7. Use a round container instead of square… what’s the difference?
Answers:
1. The smaller coin is the reflection.
2. One coin when glanced from above, two from the side.
3. Four.
4. Beam is a circle.
5. Beam is an oval.
6. I can see the beam through the water!!
7. The round container distorts the beam, and the square container keeps the light beam straight. Both are fun!
The coin water trick is a neat way for kids to see how refraction works. In optics, refraction happens when light waves travel from one medium with a certain refractive index (air, for example) to another medium which has a different refractive index (like water). At the boundary between the two (where air meets water), the wave changes direction.
The wavelength increases or decreases but the frequency remains constant. When you sine light through a prism, the wavelength changes and you see a rainbow as the prism un-mixes white light into its different colors.The light wave changed direction when it traveled from air to glass, and then back to air again as it leaves the backside of the prism.
Did you try the pencil experiment? Did you notice how if you look at the pencil (placed at a slant) partially in the water, it appears to bend at the water’s surface? The light waves bend as they travel from water to air. To further complicate things, the way the eye received information about the position of the pencil actually makes the pencil to appear higher and the water shallower than they really are! Can you imagine how important this is for trying to spear a fish? The fish might appear to be in a different place, so you need to account for this when you take aim!
Click here for the Disappearing Beaker experiment!
Exercises
- When one coin is in the water, you can actually see two: Are the coins both the same size? Which one is the original coin?
- In step 2 of the experiment: How many coins are there when viewed from the top of the glass? What about when you look from the side?
- What happened when you tossed in a second coin?
- How did your outlines compare?
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