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3.2 Transmission

In this lesson you will learn the basics of light transmission.

3.2 Transmission

In this lesson, you will learn about the basics of light transmission. In transparent objects, you know from your observations that light passes right through. When light passes through an object without being reflected, scattered, or absorbed, it's called transmission. How this actually happens is something that is incredibly complex to explain. And I don't think it's 100% necessary for you to understand exactly what's happening. Plus, consider that if light is transmitted perfectly without being reflected, scattered, or absorbed it's invisible so you can't take a photo of it anyway. Glass and other materials that transmit light often do not do it perfectly. There is almost always some reflection and absorption and refraction that go along with transmission. Refraction is the change in direction due to the change in the transmission medium. You have heard that the speed of light is constant, and it never changes. This is true, but only in a vacuum. The speed of light in air is nearly the same as it is in a vacuum, but when light encounters a transparent material, like glass or water, it gets slowed down significantly. When light is traveling through air and then hits glass, it bends. How much it bends depends on something called the refractive index of the two media. The refractive index also varies with the wavelength of light. This is called dispersion and causes the splitting of white light into the colors of the rainbow in a prism. It also causes chromatic aberration in lenses. Water has a refractive index of around 1.3, and glass is around 1.4, but diamond is 2.4. Because of diamonds high refractive index and the shape of the ideal cut it bends light more and causes the light to disperse, separating into individual colors. Unlike basic transmission, refraction can be photographed. What you have seen so far in this lesson has been direct transmission. This is where light passes through a material in a very predictable direction. Diffuse transmission is different. In diffuse transmission, the light gets scattered in unpredictable directions as it passes through materials like paper, etched glass, white acrylic, and thin white fabric. Often these materials are called translucent to separate them from transparent materials. Sometimes they are called diffusion material or simply diffusion. These types of materials are very important when it comes to modifying light sources. Some materials have a high level of absorption, but still transmit light such as color filters. Color filters are designed to take broadband light sources, like white light, and filter out certain frequencies. This is a subtractive process, which means that the light source you are filtering out has to be full spectrum. You can't take a green light and put it through a red filter. A red filter filters out green and blue frequencies, so green light will be blocked completely, and you will get nothing on the other side. This is something to keep in mind when you are using various light sources. If you are trying to use a purple filter on a 2800K halogen lamp, you will get a lower output than you might expect. This is because a 2800K halogen lamp doesn't produce much purple color, so when you filter out all the other colors, you're left with quite a bit less than what you might expect. If you wanted to filter out white light with a purple filter, you would be better off starting with a light that has more blue and purple to start. Almost any lighting source with a higher color temperature, such as LED, fluorescent, or strobe in the 5 to 6000 Kelvin range will have much more blue and purple in the spectrum. So when you go to filter out all the other colors of light, you'll have more purple at the end. On that subject, glass itself is a filter of light. Certain types of glass do a great job of filtering out UV light while letting visible light and infrared light pass right through. So that UV filter on the front of your camera, yep, that's just a piece of glass. Now that you've learned the basics about absorption, reflection, and transmission, its time to take a more in depth look at reflection, which is coming up next.

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