An Understanding to Colours and Refraction

Spectrum:
a) The distribution of energy emitted by a radiant source, as by an incandescent body, arranged in order of wavelengths.
b) The color image presented when white light is resolved into its constituent colors: red, orange, yellow, green, blue, indigo, and violet.

Dispersion:
a) Separation of a complex wave into its component parts according to a given characteristic, such as frequency or wavelength.
b) Separation of visible light into colors by refraction or diffraction.

Recomposition:
a) When the colors of the spectrum may be recombined by means of a lens to form white light (shine a white beam of light through a prism then place a lens where the refracted light is emitted).

White light can be divided into its basic colors/spectral colors (red, orange, yellow, green, blue, indigo and violet). To remember this, just memorize red, yellow, blue and violet. The colors in between are a combination of the 2 beside it. Ex. In between red and yellow is orange because red and yellow makes orange. Between yellow and blue is green because yellow and blue makes green.

Note: The notion of seven basic colors comes from ancient times, but today we only refer to 6 because most people are unable to see indigo as a distinct color.

You can separate white light into its primary colors by allowing a beam of white light to pass through a prism. This is called dispersion.

When you recombine the colors of the spectrum with the use of a lens, this is called recomposition
You can see this if you take the 3 primary colors magenta, cyan and yellow put them on a circle
and spin them really fast (this is only possible because your retina's in your eye can't take in all the information at once so your brain sums up and blends together all the colors, creating white).
This is known as color by addition.

You will notice that when white light passes through a prism, red is always on the top and blue is always on the bottom. This is because the wavelength for the color red is much shorter than that of violet light which is always found on the bottom.
To easily remember this, try to associate the wavelengths with particle movement. The more there is per second, the more energy it has, and thus it is hotter. Hot stuff rises and vice versa for cold stuff.

Note: wavelength IS NOT amplitude. It is the distance between a period.

A rainbow is the sun's spectrum produced by water droplets in the atmosphere. Light enters the spherical rain droplets where it is refracted, dispersed and reflected internally.
The rainbow arc appears at specific points in the sky because only droplets of waer that are located along the arc will reflect the spectrum at the correct angle into the eye of the observer. The angle is approximately 42 degrees to the horizontal sunlight (the sun must also be shining over the shoulder of the observer and only when the sun is shining on a large area of rain droplets, not water vapor).

Color by Subtraction (Subtractive Theory of Color)
The color of an object is created by absorbing all other colors except the color that it is. For example, if you have a red rose, theoretically, all the other colors (orange, yellow, green, blue, indigo and violet) are absorbed creating heat and leaving only red light to be reflected back into the observer's eye.
But that is only theoretical. In practice, most colors are impure and reflect more than just that one color back. These colors are called compound colors. For example, if you were to take a paint and paint a wall yellow. You would be assuming that all other colors besides yellow are being absorbed, but in reality that is not true. The colors adjacent to yellow are not being fully absorbed (orange and green) and are being reflected back as well as yellow.
Colored filters behave in the same fashion. If you were to take a yellow compound filter and pass white light through it, yellow, orange and green light would pass through.
If you were to place two compound filters on top of each other that did not have any common colors, the resulting color would be black. For example, if you were to place a blue compound filter and a red compound filter on top of each other the resulting color would be black because the red filter absorbs yellow, green, blue, indigo and violet while the blue filter absorbs red and orange. Thus leaving you with no color.
The same applies to objects being viewed through a compound filters. For example if you where to try to view a red rose through a blue filter, the rose would look black.

The color scheme goes as follows
Red + Yellow = Orange
Yellow + Blue = Green
Blue + Violet = Indigo
Blue + Yellow + Red = Black.

Color by Addition
Color by addition is when you shine a certain color(s) on an object. The colors created by this are a little weird and they are as follows.

Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Red + Green + Blue = White

Color by addition is used by TVs and lighting crews. It is very important for lighting crews to know their color by addition.
For example, if a group of people are performing on a stage and they're all wearing red suits and the lighting crew decides to shine a blue light on them, then all their suits become black and are very hard to see now. This is because their red suits are absorbing all colors except red and when you shine a blue (really blue, green and indigo/violet) light on them, it is being absorbed and nothing is being reflected back into the eye of the observer.

Infrared and Ultraviolet
Infrared is the common words for the light just beyond the red end of the spectrum. Ultraviolet is the common word for light just beyond the violet end of the spectrum.
Infrared is produced by heat. We emit infrared, fires emit it, the sun emits it, etc. It is used to find heat loss areas in building, finding natural resources, healing damaged muscles, heating food in restaurants, hurrying the drying process of paint.
Ultraviolet is produced by the sun, arc welders, carbon arc lamps and mercury vapor lamps.. Ultraviolet is easily absorbed by the clouds, smoke, and haze. This is the light which can cause sunburn. It also increases the acceleration of vitamin D in the skin. This light must be treated with respect. Excessive amounts of exposure may produce a bad sunburn or cause destruction of areas on the retina resulting in blind spots.

Index of Refraction

When the speed of light is reduced when passing through a medium, it is refereed to as "optically dense". This can be anything from water to diamond.
The ratio of the speed of light in a vacuum (c) to that of a given material (v) is called the Index of Refraction.
N = c/v

The higher the index of refraction, the more the light is slowed down when it travels through a vacuum/air into the substance.

For example:

Light travels through a liquid at 2.25 X 10^8m/s. What is the index of refraction.

3.00X10^8/ 2.25X10^8 =
3/2.25 =
1.33

The index of refraction of an unknown liquid is 1.33. How fast does light travel through this liquid?

V = 3.00X10^8 / 1.33
V = 2.55X10^8

Law of Refraction

When light passes through a medium other than a vacuum and air, it will bend, thus creating an angle of incidence and an angle of refraction. Snell's law states that sin I / Sin r = N medium / N air.
Summary:
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant (n)
- The incident ray and the refracted ray are on opposite sides of the normal at the point of incidence, and all three are in the same plane.

If you were to go from the medium to air, you would have to change the formula to the following:

N air / N medium = sin I / sin R
Hence, 1/N = sin I / sin R

Critical Angle

When light travels from one medium to another where its speed changes, some of the light is reflected and some is refracted. This is called partial reflection/partial refraction.
As the angle of incidence increases, the intensity of a reflected ray becomes progressively stronger and the intensity of a reflected ray progressively weaker.
Also, as the angle of incidence increases, the angle of refraction is increases, eventually reaching a maximum of 90 degrees. Beyond this point refraction ceases and all the incident light is reflected, at the boundary, back into the optically denser medium. This phenomenon is called total internal reflection. When the angle of refraction is 90 degrees, the incident ray forms an angle of incidence that has a unique value for any two materials. This unique angle of incidence is called the critical angle of incidence/critical angle.

To calculate this, use the following formula:

Sin 90 (N of air = 1) = Sin x (N of medium)