If you'd like to know more about the essential role that light plays in photography, then you’ll love our course, A Photographer's Guide to Light. In this lesson, you’ll learn the basics of light transmission.
A Photographer's Guide to Light: Transmission
When light hits an object or substance (like a gas) without being fully reflected, scattered, or absorbed, that is called transmission. Glass and other materials that transmit light often don’t do it perfectly. There’s almost always some reflection, 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 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 travelling through air and then hits glass, it bends. How much it bends depends on something called the refractive index of the two media.
Dispersion and Refractive Index
The refractive index also varies with the wavelength of light. This is called dispersion and causes the splitting of white light into the colours of the rainbow in a prism, as in the image below. 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 diamond's high refractive index and the shape of the ideal cut, it bends light more and causes the light to disperse, separating into individual colours. Unlike basic transmission, refraction can be photographed.
So far in this lesson we’ve looked at direct transmission, where light passes through a material in a very predictable direction. Diffuse transmission is different—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; sometimes, they're 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, like colour filters.
Colour 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 because a red filter filters out green and blue frequencies, so green light will be blocked completely, and you won’t see anything on the other side.
Light Sources and Filters
This is something to keep in mind when you’re using various light sources. If you’re trying to use a purple filter on a 2800K halogen lamp, you’ll get a lower output than you might expect because a 2800K halogen lamp doesn't produce much purple colour, so when you filter out all the other colours, 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’d be better off starting with a light that has more blues and purples to start with.
Almost any lighting source with a higher colour temperature like LED, fluorescent, or strobe in the 5–6,000 Kelvin range will have much more blue and purple in the spectrum, so when you filter out all the other colours of light, you’re left with more purple at the end.
It’s worth remembering that 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 through. The UV filter on a lens is just a piece of glass.
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About the Authors
David Bode created the video course that includes this lesson. Dave is an expert on video and audio production, and he lives in the upstate New York area. He works as a camera operator, editor, inventor, motion graphics designer, recording engineer, and studio musician.