# Transmission of Light: Definition & Overview

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• 0:02 Transmission of Light Defined
• 0:58 Transmittance
• 2:10 Transmitted vs. Refracted
• 2:55 Lesson Summary

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what is meant by 'transmission of light,' discuss the ways that light can be transmitted through a medium, and also explore how light is measured.

## Transmission of Light Defined

Visible light is the reason we are able to see anything at all. Light moves as a wave, bouncing off objects so we can see them. Without it, we'd be in complete darkness. But, in physics, light can refer to any kind of electromagnetic wave: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, or gamma rays.

When you shine light on an object, a number of things can happen. Reflection is when light bounces off of a surface. Specular reflection is when light reflects off of a shiny surface like a mirror. Diffuse reflection, however, is when light illuminates a dull object.

Another thing it can do is move through the material, and depending on exactly how it does this, we might call it transmission, refraction, or absorption. We will discuss each in more detail in this lesson, but transmission of light is when light waves move all the way through a material without being absorbed.

## Transmittance

When light moves through a transparent (or semi-transparent) material, it can be transmitted, absorbed, or reflected. The transmittance of a material is the proportion of the incident (approaching) light that moves all the way through to the other side.

For example, let's say you're shining a flashlight on a semi-transparent glass block. You start off with 100% of your incident light. The first thing that happens is that 30% of that light is reflected off the outer surface of the glass. This leaves you with 70% to continue through the glass block. Another 50% of the light is absorbed by the molecules inside the glass block itself. That leaves you with 20% that emerges from the opposite side. So you could say that the glass block has a transmittance of 20%.

The transmittance of a material depends on its thickness, but it also depends on the type of light (or electromagnetic waves) you are using. A material might have a different transmittance for visible light than it does for infrared, or X-rays. This is why hospital X-rays go through your skin until they reach the bones, even though visible light does not.

## Transmitted Vs. Refracted

If your flashlight in the previous example was shined at 90 degrees to the surface, the light that is travelling through the material is always called transmitted light. But, if you shine it at an angle, there's another term people might use.

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