Light Energy

I know, you’re thinking, “What is light anyway?” If not, you can just pretend…

I think the easiest way to think about light is to imagine it as tiny particles that are shooting through space. They’re like little light bullets.

Examine this incredibly artistic diagram… Examine it, I tell you!

These “light bullets” are called photons. Here’s how they’re created:

1. When atoms get heated up (or receive some other form of energy) the electrons get so energized that they jump to orbits farther away from the nucleus.

2. The protons pull on the electrons (you know how those two are attracted to each other…) and the electrons return to their original orbits.

Colors of the Visible SpectrumYou can remember the colors in order by remembering the name ROYGBIV. These letters stand for the colorsRed wavelength = about 650 nanometersOrange wavelength = about 590 nanometersYellow wavelength = about 570 nanometersGreen wavelength = about 510 nanometersBlue wavelength = about 475 nanometersIndigo wavelength = about 445 nanometersViolet wavelength = about 400 nanometers

(one nanometer = one billionth of a meter - that’s a millionth of a millimeter! So don’t expect me to draw one on this page, ok?)You don’t need to memorize these wavelengths - just know they’re in order by size and frequency - and all of the wavelengths are very tiny!

Hazel’s shorts are reflecting wavelengths that make purple, but the monster on her shirt is reflecting red wavelengths.

Visible LightMost light energy is invisible to the human eye. Just like our ears can only hear a range of sound frequencies, our eyes and brain can only detect certain frequencies of light. The range of light that we can see is called the visible spectrum.

Light Energy

3. When the do, they release a blast on energy called a photon - that’s light!

Let’s think of that in real life. Imagine a light bulb. Go ahead! Imagine it. That’s right.

Now we can proceed. When you push electrons (electricity) through a tiny wire in a bulb, the atoms in the wire get energized (they get hot) and their electrons jump out to higher orbits. When the are pulled back down, they release photons. The wire glows! Every time you see anything, photons are hitting your eyes.

Can photons shoot through anything?

That depends on their wavelength. Visible wavelengths (the ones we can see - ROYGBIV) can’t. We have a way to classify objects by how much visible light they block.

Transparent objects let in almost all visible light. We can see right through them because the light shoots right through. They’re clear. Most windows are transparent.

Translucent objects let in some light. We can see a glow through them, but they’re obviously not clear. An example would be a piece of notebook paper, most lamp shades, and most curtains - some light shines through.

Opaque objects block all visible light. If I shine a spot light on your back, we’re not going to see it glowing through your belly. You’re opaque.

Opacity is how opaque something is. The thicker something is, the more light it blocks. If you think about it, water is transparent when it’s not too thick. You can easily see the fish in the aquarium as the light reflects from his body. As we’ll learn later though, lots of water is opaque. When a sub goes deep into the ocean, it’s completely dark. The water is thick enough to block out all light.What do lenses do?Lenses bend light. Light goes into one side, and as it goes through it’s path is slightly bent. The bending of light is called refraction.

Think of lenses that you have seen. Where do we find them? List some examples here:

Light Energy

Convex and Concave Lenses

Pretty much everyone has used a magnifying glass. If you have one, take a look at it closely.

Draw a side view of your lens. How is it shaped?

Now look at something on your desk. What do you observe?

(You should see that objects look larger…)

What happens when you move the lens closer to your eye or closer to the object? Be clear in your observations!____________________________________________________________________________________________________________________________________________________________________________________________________________________________________(Moving it closer and farther away makes the object get blurry and change size.)

Now look at something across the room. What do you observe?________________________________________________________________________________________________________________________________________________________(Everything looks upside down!)

What could be a possible explanation for what you see?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________(I know that what I see is light reflecting off the object. Somehow, the lens changes the way the

light behaves. I know the object is not actually larger, or upside down!)

If you don’t have one, they look like this…

Light Energy

The Convex Lens

Things we can do with a convex lens…

Because it can focus incoming light to a single point, we can capture light energy and put it all on one spot. You can probably imagine what will happen then…

Because a convex lens bends light in, we can use it to make objects look larger.

The Concave LensThis one’s caved in. Take a look…

Notice there’s no focal point, but the photon paths are bent (refracted).

focal point

light (photons)

light (photons)

Light Energy

Here’s the coolest thing. You have a convex lens in the front part of your eyeball. There are muscles in your eye which can squeeze the lens to make it more or less convex. This allows you to focus on things at different distances.

Try this:Hold your hand out in front of you face. While focusing on your hand, what do you notice about your view of things far away?

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Now focus on something far away. What do you notice about your view of your hand?

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Light Energy

The PrismThink about times in your life when you have seen “rainbow” colors. Maybe it was a rainbow. Maybe you saw them on the surface of a puddle, or on the wall as sunlight passed through a piece of glass.

See if you can figure out what’s happening. Take a prism - a piece of glass with angled edges - and experiment with it in the sunlight. Record your observations below, then we’ll meet as a class and discuss what we’ve discovered.

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Prism Diagram

White light

prism

Light Energy

Other Frequencies of LightAs we learned before, most light is not visible. Now it’s time for us to learn about some of those other wavelengths. Take a look at the chart below:

• As you can see, there’s a lot more to light than what we can see.

• We use radio waves and microwaves to communicate.

• We can cook with many types of light, but we usually use microwaves and infrared light.

• Ultraviolet light is a dangerous part of sunlight. It gives us sunburn and causes skin cancer.

• X-rays are so powerful that they can shoot through our bodies, but our bones can block lower levels.

• Gamma rays are radioactive rays. The truth is that wavelengths smaller than visible light are harmful to people. Scientist who worked with radioactive elements often died because of exposure to gamma rays. X-rays are dangerous too. That’s why they cover you with a lead apron when you get one, so that you’re only exposed to a small amount.

The Transverse WaveLight doesn’t travel be vibrating molecules, the way sound does, so light is not a compression wave. Instead, light is a transverse wave. Its energy pushes in a perpendicular direction from its path. (I’ll explain…) As you can see though, we use the same type of diagram to illustrate the properties of the wave.

Amplitude = intensity of the light (for visible light, that would be how bright the light is.)

Wavelength shows what type of light we’re dealing with. Different types of light, even different colors, have different wavelengths.

Radio Microwaves Infrared Visible Ultraviolet X-ray Gamma >1m 1mm - 1m

large wavelength small wavelength