Light

Note: Please read the waves page before you read this page!

The Electromagnetic Spectrum

As we learned in E&M, electromagnetic waves are light. They are the combination of electricity waves and perpendicular magnetic waves that travel in the same direction. (look at the picture at the bottom of E&M to help)

All electromagnetic waves move at the same speed. However, they differ in frequency and wavelength. There is a whole scale of this difference, called the electromagnetic spectrum. Here is a picture of the electromagnetic spectrum I stole from this youtube video.

Gamma rays have the highest energy because they have the highest frequency while radio waves have the lowest energy and the lowest frequency. The light we can see is only a tiny part of the full electromagnetic spectrum.

Transparent and Opaque Materials

Light can pass through some materials - these are called transparent materials. Some examples are glass and water. Materials that absorb/reflect light (like bananas and candy) are called opaque materials. 

Transparent materials like glass allow light to excite their electrons, which transfer their energy from one atom to another - in other words, they absorb and re-emit light energy over and over again until the energy exits the glass. This causes a slight time delay, making light travel slower through glass than empty space. When objects are transparent, their color is the same as the color they transmit. For example, red-tinted glass is mixed with red pigments that absorb and transmit selective frequencies of light.

By the way, before I forget to say this, light is able to travel through vacuums, unlike sound. The speed of light in a vacuum is around 3 x 10^8, or 300000000 m/s. That's extremely fast. So fast that light can travel around the Earth seven times in a single second.

However, most materials around us are opaque. For example, in a banana, light hits the atoms in the banana, and turn into thermal energy. Bananas do not absorb and emit light like glass does. But they do reflect light - which brings me to my next point.

Colors

The reason we can see at all is because light reflects off of objects and into our eyes. It is also the reason objects have color - certain objects reflect certain frequencies of light while absorbing the others, creating the color we see. For example, the banana from earlier reflects yellow, which is why it appears yellow. However, it is also true that sometimes objects reflect multiple colors. They appear as the combination of those colors. Yellow daffodils, for example, reflect a mixture of frequencies that allow them to appear yellow.

Things that appear white reflect all wavelengths of light. These wavelengths combine to form white (NOT brown). When an object is black, is absorbs the light hitting it. There is very little reflection. 

However, when an object reflects light, the angle of reflection is equal to the angle of incidence (draw a picture). this is called the law of reflection.

As I mentioned earlier, colors combine to form other colors. For example, if you shine red, blue, and green light at an object the red and blue combine to form magenta, the red and green combine to form yellow, and the blue and green combine to form cyan. Thus, red, green, and blue are called additive primary colors.

When two colors add to form white, they are complementary. In this case, magenta and green form white, cyan and red form white, and yellow and blue form white.

This effect is true with shadows as well. The lights combine, forming different colored shadows.

Magenta, cyan, and yellow are known as subtractive primary colors due to the fact that if you subtract blue from magenta you get red, if you subtract green from cyan, you get blue, and if you subtract red from yellow, you get green.

In my opinion - actually, it's a fact - whenever I combine colors, I get this ugly brown. Clearly physics is broken.

The sky is blue due to high frequency light being scattered by the atmosphere. When it is polluted, low-frequency light is also scattered, making the sky appear a more whitish-blue. Sunsets are red because red, orange and yellow light are the least scattered by the atmosphere. Red light passes through the atmosphere far more than the other colors, so during sunset time, light has to travel through more atmosphere to reach our eyes, and thus more light is scattered - leaving only red for us to see. Clouds are white because water molecules are larger than individual molecules and they behave different. they reflect all colors of light about equally - creating a whitish cloud. At dawn or dusk, the cloud appears more pinkish.

When white light is refracted twice, as in a prism, the separation of light by colors is very noticeable. this effect is called dispersion. Dispersion forms rainbows. when sunlight hits raindrops, the raindrops act as prisms and disperse light, thus creating a rainbow. a rainbow is actually a full circle - pilots have been reported to have seen the full circle rainbow before. We only see the half circle because the rest is interrupted by the ground. Rainbows always face the person who is watching them, and when you move, the rainbow moves with you. That means you can never get to the end of a rainbow, and there no pot of gold. Unless, of course, you are a mary sue protagonist with fairy dust hair and all the powers in the world. Then you're just built different.

Diffuse Reflections and Smoothness

Bumpy objects reflect light worse than smooth objects do. Diffuse reflection occurs when light hits a rough surface and scatters in many directions. If a surface is polished, the distance between elevations on the surface are less than 1/8 of the wavelength of light. Wow... that's smoother than my brain on a Monday.

Of course, smoothness is relative to the type of wave hitting the object. something that is smooth to a radio wave may not necessarily be smooth to a gamma wave.

Reflections in mirrors are known as specular reflections. They appear to be behind the mirror, creating a virtual image. 

Refraction and Mirages

Polarization

We know that light waves are transverse. One way to see this is through polarization. The waves are plane-polarized, which means that they are confined to move in a singular plane. This could be a vertical or horizontal plane, or a plane in any other direction.

Incandescent lamps, however, are not polarized - the electrons that produce them are vibrating in many random directions. The light is not limited to one direction.

Polaroid filters are prime examples of how light polarization works. If you shine unpolarized light through a filter, it will mostly appear the same. However, if you shine polarized light at a polaroid, though, you might get different results. In one direction, you will see the light (yeehaw). However, if you flip the filter over, you will see nothing. Why? This is because the polaroid filters 50% of unpolaroid light - its axis filters the light thats waves are traveling in one direction. Flipping the filter over means that polarized light cannot go through the axis. If this is confusing, look at the image below.

Unfortunately, that's all I have to say on the matter of light. Please enjoy the last page of the physics section! Don't worry, I plan to add biology next - see you in the next one!