9.6: Interaction of Light with Matter
- Page ID
- 472630
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Determine if light is absorbed, transmitted, or emitted in a given interaction with matter.
What happens when an electromagnetic wave impinges on a material? There are a variety of possibilities depending on the characteristics of that material as well as the frequency of the electromagnetic wave. As we will see, the material might not interact at all, it might completely absorb the light of the wave, or it might interact in a way that is somewhere between those two extremes.
Most of the world around us is opaque to visible light. We know because we can see it! When a material is opaque that means it absorbs the light. This means that the energy stored in that object has increased by the amount of the energy of that form of radiation. None of the absorbed radiation leaves the material after impinging upon it. Materials can be opaque to other forms of radiation as well. For example, our atmosphere is opaque to most forms of ultraviolet light. (If it were not we would have even greater concerns about sunburns than we already do.) Matter can also be opaque to some forms of visible light but not others, resulting in colors, as we will see.
Materials that allow electromagnetic radiation to pass through them with little or no absorption are said to be transparent. Examples of some common materials transparent to visible light include most molecules in the atmosphere, glass, and water. Just as with absorption, materials can be transparent to any form of electromagnetic radiation, but are hardly ever transparent to all forms of radiation. For example, even though water is transparent to visible light, it is not transparent to microwaves or infrared light. When something is transparent, the radiation that enters the material will eventually exit the material with the same frequency that it entered. In the meantime, it might interact with the material in ways we will discuss later, but it eventually exits with the same energy that it entered with. When radiation impinges on matter that is transparent to it, we say that the radiation is transmitted.
Sometimes materials will scatter the light that they transmit. Almost all transmitted light is slowed down somewhat when passing through a material, and this can result in bending the direction of the light. For transparent materials this bending is relatively small and uniform. This kind of bending is referred to as refraction. But some materials will result in a greater change in direction for the light. This is referred to scattering of light. This scattering can either be diffuse scattering when it occurs in many directions, or reflection when it occurs in a specific direction. We will explore each of these three possibilities in more detail in the following subsections.
Finally, we should mention the process by which a material releases energy in the form of radiation. This is referred to as emission. We will explore some types of emission in more detail later in this text, but it is worth discussing a little bit here so that we have this knowledge when it comes to our discussion of color. If an object is emitting visible light, we would see it as glowing. We could even identify the color that it is glowing and measure the specific wavelength of this light with some simple devices. This is very different than the color that we see for an object because it is absorbing light. When an object absorbs light, we don't see the absorbed light we see the transmitted light. The object is absorbing the colors we do not see and transmitting the colors that we do see. When an object emits light, the light we see is the color that is emitted. We will discuss color in more detail later in this text.
Section Summary
- Matter that is opaque to a form of radiation absorbs that radiation.
- Matter that is transparent to a form of radiation trasmits that form of radiation.
- Matter that releases energy in the form of radiation emits that radiation.
Glossary
- opaque
- a material that absorbs a particular type of radiation
- transparent
- a material that allows a form of radiation to pass through it
- transmit
- when radiation impinges on matter that is transparent to it and the matter passes along radiation of the same frequency.
- emission
- when matter releases energy in the form of radiation.