Behavior of Light Reflection/Refraction Atmospheric Optics
Light
We talked in last week's lectures about electromagnetic radiation. We developed the idea that a wave is a means of transporting energy. Every wave has a wavelength, amplitude, and frequency. We then introduced the idea that ligh t is actually a wave of sorts.
Visible light, light that triggers vision in humans, occupies an extremely narrow portion of the electromagnetic spectrum between roughly .3 micrometers and .7 micrometers. There is a chemical in the cones of our eyes called Retinol. When ligh t of energy .3 micrometers to .7 micrometers strikes the back of our eyes, it triggers a chemical reaction. The energy released in this chemical reaction is carried to our brain as an electrical impulse. That is what starts the vision process.
The behavior of light had long been a very poorly understood science. It had been discovered long ago that light travels in the form of waves. However, early in the 20th Century, the Quantum theory developed. This essentially proposed the ide a that light may travel as individual particles called photons. Most scientists subscribe to the idea that light is essentially particles traveling in the form of waves. This is the basis for which Quantum Physics is built upon - comparing and co ntrasting the wave model of light to the particle model and joining them to find the most accurate analysis of what exactly is taking place.
It is hard to conceptualize that light and radiation are one in the same. Light simply refers to radiation of specific wavelengths (from .3 micrometers to .7 micrometers) that triggers a response in the eye.
As we have learned, all objects that have a temperature above absolute zero emits energy as a function of the fourth power of their temperatures. But at what wavelengths is this energy emitted? Just one wavelength? Or a variety of wavelengths?
The Wein displacement law allows us to determine, based on an object's temperature, at what wavelength the majority of the radiated energy will be emitted at. A good example of this would be to consider the sun. Obviously, the sun is emitting visible light. Of course it is - that is why we can see during the day. But we know that is must be emitting radiation of other wavelenghts as well. Otherwise, why would we need to purchase sunscreen to protect us from harmful ultraviolet radiation? It turns out that the sun emits radiation that covers a very large portion of the electromagnetic spectrum - including microwaves and radio waves.
As an object heats, the Wein displacement law will tell us that the wavelength where the majority of energy is released will increase as well. So consider that next time you look at a colored object. A red ball actually is emitting radiation of many colors other than red. It just happens that based on its chemical properties and its temperature, the majority of the visible radiation being released is at the red portion of the electromagnetic spectrum.
As an object heats up, its wavelength of maximum energy emittance will decrease. and the frequency thus the energy, of the maximum emitted energy will increase. Since violet is actually higher energy light than red, an object will turn red, then orang e, yellow, green, blue, indigo, then finally violet upon being heated.
Speed limit of the universe - the speed of light
It has been mathematically proven that nothing in the universe can exceed the speed of light. The speed of light is 3.0 * 10^8 m/s or about 186,000 miles/second. That is the speed of light in a vacuum. A vacuum refers to a medium that is void of any matter; gas, liquid, or solid.
The speed of light in a vacuum is often represented by the symbol, c. E=mc^2 is the equation of relativity and the famous c here is the speed of light. This is Einstein's Theory of Relativity that relates the amount of energy an object posesses to it s mass.
To give you an idea of the large distances astronomer's deal with, even though light travels at an extremely brisk 186,000 miles/second, it takes light 8 1/2 minutes to reach Earth from the sun. If the sun were to explode right now, you wouldn't know it for 8 1/2 minutes. When you look out at the stars at night, some of the light that you are seeing is hundreds of millions of years old. That is how long it may have taken light from some of those stars to reach Earth. Many of the stars in the night sky may actually have burned out millions of years ago. Pretty amazing? I think so too.
Light slows down or speeds up when it encounters different mediums. This is actually the case with all energy that travels in the form of waves such as acoustic (sound) waves.
Reflection
When light encounters an object or a barrier, a certain amount of energy will be absorbed, the rest will be reflected. Chemical and physical properties, such as color, texture, shape, and electronic configuration determine an object's re flectivity. The whiter and smoother an object, the more likely it will be a good reflector whereas darker coarse objects often absorb more light energy.
Reflection is an important property in considering the Global Energy Balance. A significant portion of energy that arrives at Earth from the sun is reflected back into space. This is called the Earth's albedo. Recent estimates placed Earth's albedo at about .3. That means approximately 30 % of radiation that arrives at Earth from the sun is reflected back into space.
Clouds and snow cover are two instances of situations that could increase an area's albedo.
Angle of reflection
The angle from perpindicular that light strikes a surface is called the incidence angle. The angle from perpendicular that light reflects off of a surface is called the angle of reflection. The angle of reflection is equal to the incide nce angle.
Absorption
Just as matter of different temperatures and chemical and physical properties emits different wavelength energy, they absorb different wavelength energy as well. Absorptivity of an object is defined as the amount of energy that is incident upon it that is absorbed. It is often denoted as (1 - albedo). The theory here is that any energy that is not reflected is absorbed.
When an object absorbs light, it is acquiring energy. Therefore, the atoms and molecules that make up that object will move about faster as motion and energy are related. The temperature will rise with the increase in velocity of the atoms and molecu les. This is how light can actually heat objects. The higher the absorptivity of a substance, the more likely it will be heated by light energy.
Scattering
Our atmosphere, as we will learn in the coming weeks, is full of particles too small for the eye to see. Haze is a meteorological condition that occurs when there is a high concentration of particles in the atmosphere. When light comes into co ntact with particles that are about the same size in diameter as the lenght of the light's wavelength, scattering occurs. This phenomena is often referred to as Rayleigh Scattering.
Refraction
When light enters from one medium to another, it will change its velocity. This will cause the light to bend. This is an example of Snell's Law. The best way to visualize why this occurs is to picture two parallel lines of soldiers marching s ide by side on smooth concrete. Suppose they encounter a muddy field. They will not be able to march as fast. As a result, the faster moving soldiers still on the concrete will crash into the slower moving soldiers encountering the mud. As a result, t he path of the soldiers will bend. This will be illustrated in lecture (hopefully I can do a better job of explaining it then).
The speed of light in a substance is determined by that substance's Index of Refraction. Glass for instance, has an index of refraction of approximately 1.33.
To find the speed of light in a particular substance, you take the speed of light in a vacuum divided by the Index of Refraction, or velocity = c/n, where n is the symbol used for the Index of Refraction.
When light enters from one medium into another, refraction occurs. When this occurs, a percentage of the light is refracted, and a percentage is reflected. If the angle of incidence is too shallow, total internal reflection occurs. Thi s will be illustrated in class and it is the basis for the operation of fiber optic cable.
Colors
White light contains all colors of what is called the monochromatic spectrum. White light can be separated into the basic colors of the spectrum using an object called a prism. The geometric configuration of a prism separates light into the monochromatic spectrum.
Atmospheric Optics
The atmosphere is full of colorful and interesting visual phenomena that occur as a result of the properties of light.
A rainbow occurs when sunlight is refracted through raindrops.
A halo occurs around the sun or moon when ice crystals in the clouds create a cumulative prism effect. There is a meteorological folklore that reads, "A ring around the sun or moon brings rain or snow upon you soon." This addage will be explai ned in class in full detail and there is actually some truth to it.