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Transparent and Opaque Materials in Electromagnetic Waves

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  • 0:05 Transparent and Opaque…
  • 0:46 Transparency and Transmission
  • 2:41 Opacity and Reflection
  • 4:04 The Non-Visible Wave Types
  • 6:08 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

What makes an object transparent or opaque? How does light pass through some objects and not others? And what about the other types of electromagnetic waves? We'll investigate all these questions in this lesson about the transparency and opacity of materials.

Transparent and Opaque Materials

What happens to light waves when they encounter objects? Do they go through the objects? Do they bounce off the objects? When a light wave strikes the surface of an object, a variety of things can happen. One of these things is called resonance. When resonance occurs between a light wave and an object, the object absorbs the energy of that light wave. The light energy stays inside the object when resonance occurs, but what about when resonance does not occur? What happens to light waves that are reflected off the object or transmitted through the object? In this lesson, we're going to study light waves a little further in order to understand transparency and opacity.

Transparency and Transmission

An object is said to be transparent when light passes through it without being dispersed, or scattered. Clear glass is transparent, and clean water is transparent. Although light travels through these materials, we know that they also block things like wind, sound waves and the movements of people and animals. For example, you can't walk through glass. So, how can a light wave pass through the glass without being changed at all?

Light waves are absorbed by an object when the frequency of the light wave matches the resonant frequency of the object. Absorption occurs when none of the light waves are transmitted through the object. Transmission, if you weren't sure, just means the passing of electromagnetic waves through a material. In the case of objects that are transparent, all of the light waves pass through. Transparent objects exhibit complete transmission of the light waves through the object.

An object looks transparent because light waves pass through unchanged.
Transmission Light Passes Through

What really happens inside glass materials when a light wave passes through? We know that there aren't any tunnels connecting one side to the other. So, what's going on? When a light wave strikes the surface of the glass, it sets the electrons vibrating at a certain frequency. This frequency is not the resonant frequency of the glass. The vibrations pass from the surface atoms to the neighboring atoms and then on to more atoms through the bulk of the glass. The frequency doesn't change when the vibrations pass from one atom to another. Once this energy gets to the other side of the glass, it is re-emitted from the opposite surface. The light wave effectively passes through the glass unchanged. As a result, we can see straight through the glass, almost as though it isn't even there. So, now you know: transparency occurs because of the transmission of light waves through the bulk of an object.

What about objects that are translucent? What's happening when light is scattered as it passes through an object? The scattering of light is related to a subject called refraction, but we won't be discussing that in this lesson. We're going to talk about opaque materials next.

Opacity and Reflection

So, we've learned that light waves are absorbed when they resonate with an object. They are transmitted when they pass through the atoms of an object. But, what if a light wave does neither of these things? When a light wave is neither absorbed nor transmitted by an object, then it is reflected.

Reflection is a change in the direction of a wave when it strikes a surface. Reflection can be specular (as in glossy or smooth-surfaced objects) or it can be diffuse (as in objects with rough surfaces). In either case, reflection occurs because the vibrations of the surface electrons do not pass their energy down through the material. Instead, they vibrate a little bit and then re-emit the energy back, away from the material. When this happens, the object appears to us as opaque. Opaque materials do not allow transmission of light waves. In other words, we can't see through an opaque object. We can only see the surface because the light waves are re-emitted from the surface back to our eyes. Opacity occurs because of the reflection of light waves off the surface of an object.

When light waves are reflected, an object appears opaque.
Opaque Light Reflected

Now, I bet you're wondering - what about the different colors of opaque materials? Why does my car look red when my friend's car looks blue? Well again, you're going to have to hold that thought. Color is related to the selective absorption of different wavelengths of visible light. For now, just keep your mind on transparency and opacity - whether or not we can see through an object or material.

The Non-Visible Wave Types

Transparency and opacity are easy enough to understand in the context of visible light. We can see the results of it, can't we? We can see when an object is transparent to a light wave. We can tell when something is opaque. But, the principles of transparency and opacity apply to all the electromagnetic waves. Visible light, remember, is just one of those types. So, here's a question: if glass is transparent to visible light waves, does that mean it's transparent to the other EM waves? The answer, surprisingly, is no.

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