Light Notes

Light is a form of energy produced by the change in motion of a charged particle. Light does not need a medium (solid, liquid or gas) in order to travel. Electrons moving back and forth will cause light. When the electrons inside of an atom absorb energy they jump to a different energy level. When these electrons fall back down to their original energy level they give off a little packet of energy in the form of light. This packet of light energy is called a photon. Light can either travel as a wave or as a particle. We will be studying how light behaves as a transverse wave.

Some objects produce their own light will other objects reflect light. These sources of light are called luminated objects. Sources of light include: the sun, a light bulb, a match and a candle. Bioluminescent organisms are living things that can produce their own light. A firefly is an example of a bioluminescent organism. Objects that reflect certain amounts of light are called illuminated objects. Objects that reflect light include: a mirror, the moon and a piece of paper.
What is so amazing about light is the speed at which it travels. Light travels 186,000miles per second or 299,798 kilometers per second. That means light can travel a distance of 186,000 miles in one second! It takes eight minutes for light from the sun to reach earth. This is why you hear lightening before you see thunder. Lightening and thunder happen at the same time, yet light travels faster than sound so you see the lightening then a few seconds later you hear the thunder.

Have you ever used a microwave? Listened to the radio? Sat at the beach to get a tan? If you have done anyone of these things you have used light. Light is much more than we can see. Light waves produce both an electric and magnetic fields, some of which we can see others we cannot. Light is an electromagnetic wave, as a wave light may have different wavelengths and frequencies. The total range of all electromagnetic waves is the electromagnetic (EM) spectrum. The electromagnetic spectrum is arranged according to the waves wavelength and frequency. The picture to the left shows the electromagnetic waves (top to bottom) going from long wavelength/low frequency to short wavelength/high frequency.

Electromagnetic Spectrum: Waves

These waves are listed inorder of increasing energy and decreasing wavelength.

Radio Waves

When you listen to the radio or watch T.V. you are using a part of the electromagnetic spectrum with lowest energy and a long wavelength. When you turn the dial on your radio you are adjusting to the same frequency as the radio station. Radio stations are either amplitude modulation (AM) or frequency modulation (FM). Your favorite T.V. shows use a combination of both, AM for the sound and FM for the picture.

Microwaves

These waves carry more energy than radio waves. These waves are responsible for heating your food. They are also responsible for communications (satellites, cell phones etc…) and radar.

Infrared Waves

This is an image of the infrared waves felt as heat from a human body. These waves are heat waves. You can feel the warmth from the sun as infrared waves. Restaurants use infrared waves to keep food warm after it has been cooked. These waves have a higher frequency and shorter wavelength than microwaves. These waves are called infrared because they have a frequency slightly longer than that of visible light.

This is the only portion of the visible spectrum that you are able to see. The colors of the spectrum include: Red Orange Yellow Green Blue Indigo Violet.
Red having the least amount of energy and the longest wavelength and violet having the most energy and the shortest wavelength. All the colors you see are from this little portion of the electromagnetic spectrum.

These Rays are listed inorder of increasing energy and decreasing wavelength.

Ultraviolet Rays
These rays are responsible for giving us a tan or a sunburn when we sit outside. Long time exposure to these rays can cause skin cancer.

X-Rays
If you have ever broken a bone you have had an x-ray done to see what the break looks like. Can you make out the image to the left? It is a left hand with a ring on the finger. An X-ray camera works like any other camera except it gives off invisible energy rays called X-rays. These X-rays travel through your skin. An X-ray camera won't work unless it finds something hard to stop the rays. The X-rays bounce off of your bone and back to the camera. The x-ray picks up the dense parts of the bone. The denser the bone, the more rays that were bounced back, the lighter the bone appears on the "film." (The picture to the left is reversed.)

Gamma Rays

Gamma rays have the smallest wavelength and the most energy out of the entire electromagnetic spectrum. These rays are very powerfull and can kill living tissue. Gamma rays are used in therapy to kill cancer cells.

When you wake up in the morning and look at yourself in ther mirror you are seeing a reflection of yourself. You see your self by the light waves bouncing off of the mirror. Reflection is the bouncing back of a wave. As we see in the diagram to the left. Light waves hit a smooth surface and bounce off with the same angle in which they hit the surface. This is the Law of Reflection: the angle of incidence is equal to the angel of reflection. Since both angles are equal the image appears to be the same. This will happen when we reflect light off of a flat smooth surface.

What happens when the surface is not smooth? When light bounces off of a rough surface difuse reflection is seen. Objects appear blurred, like the reflection of the setting sun on the water. We do not get a clear picture of the sun as we would if the light was being reflected off of a mirror.

Surfaces can also be curved. A satilite dish is a perfect example of a concave surface. The dish is curved inward as to direct all of the light waves in toward the center receiver and then through a cable into your house.

A security mirror is curved outward (convex surface) as to see the enitre store. If you go into 7-11 you will see a curved mirror in the corner. This mirror is designed to spread out the light waves so the clerk can see everything going on in the store.

White light contains all colors of the spectrum. The image of the prism to the left shows white light entering. The light waves change speed and direction as they pass through the prism. Each of the different colors of light then become seperated.
The colors we see are just reflections of light off of objects. If an object reflects all wavelenghts of light then it will appear white. If an onbject absorbs all wavelengths of light it will appear black. An object is the color of light it reflects. All other wavelengths of light are absorbed.
Forexample, a red object reflects red light and absorbs orange, yellow, green, blue, indigo and violet. A green object reflects green light and absorbs red, orange, yellow, blue, indigo and violet.