Energy can take one of two forms: matter and light (called electromagnetic radiation). Light is energy that can travel through space. When you feel the warmth of the sun on your arm, that’s energy from the sun that traveled through space as infrared radiation (heat). When you see a tree or a bird, that’s light from the sun that traveled as visible light (red, orange… the whole rainbow) reflecting and bouncing off objects to get to your eye. Light can travel through objects sometimes… like the glass in a window.
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Key Concepts
Imagine tossing a rock into a still pond and watching the circles of ripples form and spread out into rings. Now look at the ripples in the water – notice how they spread out. What makes the ripples move outward is energy , and there are different kinds of energy, such as electrical (like the stuff from your wall socket), mechanical (a bicycle), chemical (a campfire) and others.
The ripples are like light. Notice the waves are not really moving the water from one side of the pond to the other, but rather move energy across the surface of the water. To put it another way, energy travels across the pond in a wave. Light works the same way – light travels as energy waves. Only light doesn’t need water to travel through the way the water waves do – it can travel through a vacuum (like outer space).
Light can change speed the same way sound vibrations change speed. (Think of how your voice changes when you inhale helium and then try to talk.) The fastest light can go is 186,282 miles per second – that’s fast enough to circle the Earth seven times every second, but that’s also inside a vacuum. You can get light going slower by aiming it through different gases. In our own atmosphere, light travels slower than it does in space.
Your eyeballs are photon detectors. These photons move at the speed of light and can have all different wavelengths, which correspond to the colors we see. Red light has a longer wavelength (lower energy and lower frequency) that blue light.
What’s Going On?
When a beam of light hits a different substance (like a window pane or a lens), the speed that the light travels at changes. (Sound waves do this, too!) In some cases, this change turns into a change in the direction of the beam.
For example, if you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,000 miles per second), called optical density, and the result is bent light beams and broken pencils.
You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil. When light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refractions do not occur.
However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil?
Depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts. Glass is optically denser than water, which is denser than air.
Not only can you change the shape of objects by bending light (broken pencil or whole?), but you can also change the size. Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes.
Questions to Ask
- Can light change speeds?
- Can you see ALL light with your eyes?
- Give three examples of a light source.
- Why does the pencil appear bent? Is it always bent? Does the temperature of the water affect how bent the pencil looks? What if you put two pencils in there?
- What if you use oil instead of water for bending a pencil?
- How does a microscope work?
- What’s the difference between a microscope and a telescope?
Click here to go to next lesson on Light Reflection
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We talk about light years in the Astrophysics class, but here’s something you can try.
It’s totally understandable how it’s confusing, since the word “year” is contained in “light year” but is a measure of distance. Ask him to measure things he’s familiar with first – use a ruler to measure hands, feet, fingers, etc. Then ask him to measure larger things, like the driveway and give him a yardstick, showing him where the smaller increments (cm or inches) are on the yard or meter stick.
Now ask him how he’d measure from your town to the next – would he use a ruler with inches on it? Yardstick? Tape measure?
No – he’d want to try a bigger ruler, something with feet or yards on it. One measurement increment can be found on the other the larger you go.
Now try this if he’s ready… light travels at 186,000 miles per second. So how far does light travel in one second? (If that’s too big of a jump, back up and try it with car speeds while you’re driving – use kph or mph to talk about distance per unit time… our car is traveling 65 miles every hour, etc).
Go back to light – light travels 186,000 miles per second. How far does it go in one second? Two seconds? (2×186,000 miles) Ten seconds? (1,860,000 miles) 100 seconds? Etc…
How about a day? Week?
A year? Light travels 5,865,696,000,000 miles in one year. That DISTANCE is a light year. Instead of saying “5,865,696,000,000 miles” we say “one light year”. So if a star is one light year away, it’s really 5,865,696,000,000 miles away.
And last, give it time. Just because it doesn’t appear to make sense today doesn’t mean it won’t tomorrow. I know I have needed time to really understand things like this myself. 🙂
I have an 11-yr-old Asperger’s son. I can’t get him to understand “light years.” No matter how my husband or I explain, he’s just not getting it. And he’s extremely intelligent. Help?