Light
Harish, Chennai (WHAT INITIAL?)

2015 is the International Year of Light.
From sunsets to rainbows, from the blues and greens of the ocean to the remarkable range of colors of plants and animals, our first experiences of light and color are through what we see in the natural world. But light has changed the way we see our world in many other ways as well. In electronics equipment such as lasers, in medical disgnostics, even in art and culture, such as the way we view paintings, light and light sources form an important ingredient of our every-day lives.

Types of Light
To understand light you have to know that what we call light is what is visible to us. Visible light is the light that humans can see. Other animals can see different types of light. Dogs can see only shades of gray and some insects can see light from the ultraviolet part of the spectrum. The key thing to remember is that our light is what scientists call visible light.  Scientists also call light electromagnetic radiation. Visible light is only one small portion of a family of waves called electromagnetic (EM) radiation. The entire spectrum of these EM waves includes radio waves, which have very long wavelengths and both gamma rays and cosmic rays, which are at the other end of the spectrum and have very small wavelengths. Visible light is near the middle of the spectrum.

It’s all Energy
Light and EM radiation carry energy. The quantum theory suggests that light consists of very small bundles of energy/particles; it’s just that simple. Scientists call those small particles photons, and the wavelength determines the energy and type of EM radiation, and the number of photons tells you how much radiation there is. A lot of photons give a brighter, more intense type of light. Fewer photons give a very dim and less intense light.  When you use the dimmer switch on the wall, you are decreasing the number of photons sent from the light bulb. The type of light is the same while the amount has changed.

Different Speeds of Light?
As far as we know, all types of light move at one speed when in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second. That speed is really fast, but even when you’re travelling that fast, it takes a while to get places in space. It takes about seven minutes for light from the Sun to reach Earth. It takes over four years for the light from our Sun to get to the nearest star. It would take a particle of light over 100,000 years to get from one side of our galaxy to the other side. All of those values are light moving through a vacuum. You can slow light down in other substances such as the atmosphere, water, or a diamond. Light moves at about 124,000,000 meters per second (less than half the speed in a vacuum) in a diamond.

Types of Electromagnetic Radiation
There are waves of energy and light moving all around us in the form of TV and audio transmissions, gamma radiation from space, and heat in the atmosphere. Scientists call them all electromagnetic radiation. The waves of energy are called electromagnetic (EM) because they have oscillatingelectric and magnetic fields. Scientists classify them by their frequency or wavelength, going from high to low frequency (short to long wavelength). For a wave with a high frequency, it has a lot of energy, so it could be a gamma ray or x-ray. If it has low frequency, it has less energy and could be a TV or radio wave.  All EM energy waves travel at the speed of light. No matter what their frequency or wavelength, they always move at the same speed. Some properties of waves, such as diffraction and interference, are also seen in EM radiation.
Scientists have figured out that there are tiny particles in these waves; they are called photons. The photons are specific units, or packets, of energy. Sometimes those particles interact with each other and change the way the light originally behaved.  Listening to the Heavens All types of EM radiation are useful to the world of science. Look at radio waves as an example. Radio stations and ham radio operators of Earth work with radio waves every day. Radio waves are used to carry communications from one point to another.  Radio waves are also extremely important to astronomers. Astronomers are constantly listening to the radio waves of other galaxies to learn more about their stars. Stars give off large amounts of EM radiation across the entire spectrum, and we can study that radiation to learn more about the universe.

Radiation Doesn’t Scare Us... Much
An important idea you should always remember is that sometimes we use the word radiation. When you think of radiation, you probably think about nuclear power plants, bombs, and X-rays. Sure, those are all types of radiation. Nevertheless, more important to physics is the idea that all light is considered radiation. That means that everything from television and radio waves to gamma rays are all types of radiation. Think about the word LASER. The R stands for radiation, while a laser is just a souped-up flashlight. Think about heat. Most heat is actually infrared light being given off by an object. That heat is also radiation.

Seeing the Light
Let’s take a moment to talk about visible light.  As you can tell by the name, visible light is the light that humans can see. More specifically, you see the light that is not absorbed by objects. Green plants are green because they absorb all of the colors of the visible spectrum except the green color (you could also say the green wavelengths). A red wall is red to your eyes because it is not absorbing light from the red wavelengths. Mirrors reflect all of the colors of visible light.  Not Seeing the Light We describe the world the way we see it as humans. Other living things on Earth see the world in different ways. Dogs only see things in black, white and gray. Some insects see colors that none of us can see. When you are learning about visible light you should remember we mean visible to humans. We should also mention that not all humans can see all the colors. There is an eye defect called color-blindness that affects many men.  Color-blind men cannot see certain colors of the spectrum. It has to do with a genetic defect in their eyes.

Visible Light Colors
We now introduce you to Mr. Roy G. Biv. Was he a scientist? No. Did he create great optics and telescopes? No. He is not even a he.  ROY-G-BIV is the acronym that represents all of the colors in the visible spectrum of light. R (red) - O (orange) - Y (yellow) - G (green) - B (blue) - I (indigo) - V (violet). Not only are those the colors we can see as humans, but they are also in the right order. Red has the longest wavelength and violet has the shortest. You could also say that red is the least energetic and violet is the most energetic of the visible spectrum.

Edges of Visibility
Although we can’t see them with our eyes, some wavelengths of light that bookend the visible spectrum are also important. Infrared radiation is next to the red portion of the spectrum. Infrared light is heat. Scientists use infrared light sensing optics when they want to see differences in temperature. Ultraviolet radiation (UV) is just beyond the violet end of the visible spectrum. UV light is given off by the Sun and absorbed by ozone in the atmosphere. Ultraviolet light can also mutate cells in your skin and give you skin cancer.  Particles and Waves During the early 1900’s scientists proved that electromagnetic radiation not only has packs of energy (quanta), but also proved that light moves in a wave pattern. It’s like a stream of individual packets.

Looking at the Waves
All types of light move in wave-like patterns. In each wave pattern are high points and low points. The distance between two high points, or low points, is called the wavelength.  Scientists use the Greek letter lambda to describe that distance. Depending on what type of light you are talking about, each type has a different lambda, or wavelength. All of the wavelengths of light together are called the EM spectrum.

Looking at the Particles
Light not only moves in waves; it also moves with a flow of little particles. Scientists call these particles of light, photons. The packets contain the energy that makes up the energy of the light. Scientists measure something called the relative energy of different types of light. The energy increases as the wavelength decreases.

Looking at the Energy
Compare different types of light. You will see that as you move up the EM spectrum and the wavelengths get smaller, those types of light have more energy. The big idea to remember is that light consists of both waves, and energy (transmitted with particles). EM radiation, like gamma waves and cosmic waves, has huge amounts of energy compared to a radio wave.  When you look at the visible part of the spectrum, you will see that violet light is more energetic than light from the red part of the spectrum.

Light from the very early Universe, emitted just when it was born, is called Primordial light. The oldest light we see around us today, called the cosmic microwave background, harkens back to a time just hundreds of thousands of years after the Universe was created. Even this light has been seen and studied by astronomers and scientists. Literally, then, we cannot imagine a world without light.