Module 2 - Interaction of Light with Materials

Module 2 - Interaction of Light with Materials

Silvia Kolchens

Faculty for Chemistry, Pima Community College

Silvia Kolchens is instructional faculty for chemistry at Pima Community College, Tucson, Arizona since 1995. Her regular teaching responsibilities include general chemistry, organic chemistry and more recently solid-state chemistry. She received her PhD in physical chemistry from the University of Cologne, Germany, and did postdoctoral work at the University of Arizona. Her professional interests include the development of case studies in environmental chemistry and computer applications in chemistry to engage students actively in learning through inquiry based instruction and visual representations of complex phenomena. Outside the classroom she enjoys exploring the world through hiking, photography, and motorcycling. Email: skolchens@pima.edu

1 Introduction

Do you ever wonder what happens to light when it hits a material? Why do some objects appear transparent or opaque, clear or colored? What happens to an electromagnetic wave when it interacts with a particle? Why is this important and why should we care? We care because it is the interaction of light and materials that enables us to see light and guide it. Understanding the principles of light and material interactions enables us to observe and manipulate our environment.

Figure 1 Sunset - Wasson Peak, Tucson, Arizona; Photo by S. Kolchens

1 The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072

1.1 The Field of Optics

Optics is a science that deals with the genesis and propagation of light in the in all ranges of the electromagnetic spectrum and is an engineering discipline that uses materials and optical principles to build optical instruments. Optics is a very old discipline and there is evidence that Mesopotamian scribes may have used lenses to produce breathtakingly miniscule texts on clay tablets more that 4,000 years ago. As a very modern science today's way of life would not be possible without applications utilizing the interaction of light and matter. These applications include DVD players, TV remote controls, and fiber optic cables used for high-speed communications. Many applications not only guide light but also generate or receive an optical response (i.e. remote control or digital camera). Other applications include medical diagnostic and environmental sensing applications. Future applications will include optical switches to build super fast computers and even faster communication systems.

1.2 A Road Map of Light and Matter

When describing a complex field, like the interaction of light and matter, we gain a better understanding when we create a "road map"; a bird eyes view of the subject.

Figure 2 Interactions of Light and Material

2 The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072

1.3 List of Topics

In this module we will discuss: 1) the propagation of light through different medium; 2) the absorption of light, which is responsible for the colors of materials; 3) the scattering of light by particles, in comparison to the size to the wavelength of light; 4) the diffraction of light, through ordered structures to create interference. Diffraction of light occurs in highly ordered structures with a long-range order in the range of the visible light to create interference patterns. This is typically accomplished through diffraction gratings, but on the atomic or molecular scale structures as crystalline structures are too small to cause any diffraction of the visible light and show only diffraction of X-ray electromagnetic radiation.

2 Discussion

2.1 When Light Hits a Material

2.1.1 Seeing Light The ability to see everyday objects is something that we often taken for granted. Do you realize that no matter how much light is around you if light could not enter your eyes, you would not be able to see anything? A familiar observation is that of the moon at night. Being on the dark side of the Earth you cannot see the Sun. But we know that sunlight is projected into space. The Sun's light remains invisible to us unless it hits an object, such as the Moon or a satellite, and light reflects back to Earth's surface. Stars at night are visible to us because they are direct sources of light. In other words: You cannot see a beam of light unless it is directed at you. This statement may seem paradoxical without a simple experiment.

Figure 3 Sunlight directed towards observer at different locations on Earth

3 The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072

Activity 1 Shine a red laser pointer onto a wall.

WARNING: Never point a laser at the eye, doing so may cause damage to the eye. You will notice the red dot on the wall but you won't actually see the laser beam itself. Now take a little piece of white paper and place it into the beam: observe the paper from the side while moving it back and forth. You will notice that the red beam is reflected off the paper and hence you can see it. Now let's take our experiment a little further. Darken the room and continue to shine the red laser onto a wall. Now take a spray bottle filled with water and spray some water mist into the beam. You will observe that the beam is now partially visible due to the laser beam reflecting off the little water droplets. In general, you will find that a laser in vacuum, or a very clean room without dust particles, is invisible to us; only when dust particles are visible is the laser visible too. This leads to our first observation:

Observation: Light must be directed at the observer for the observer to see the light.

2.1.2 Scatter of Light Now we will examine what we are seeing. Look around a room and observe an object, you will find that no matter where you stand, your eyes can see that object. When light falls on an object, light is redirected in all direction, this is called scattering. You can see an object only when light is scattered in the direction of the observer.

Observation: An object is only visible when it scatters light.

Try this: look straight at a very clean window which does not scatter light much and you will notice that the window is difficult to see. An example of this is when birds fly into windows because they cannot see them.

2.1.3 White Light The Sun's light is what is called white light, because it contains all colors of visible light or the colors of the rainbow. Looking around the room you will notice that objects have different colors. Since we are mostly surrounded by white light, we can identify individual colors such as red, green, etc. Let's try a thought experiment: if we were to take a white light (like the sun) and shine it on a leaf. The leaf appears green because it reflects the green wavelength of visible light. If we then took the light that is reflected off a green surface such as a leaf, and then pass it through a prism, which separates the wavelengths of light into its components. W could measure the intensity of each wavelength and obtain an intensity distribution curve that favors green colors. Likewise, if white light is transmitted through a green leaf, the overall color perception is green.

4 The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072

Figure 4 Intensity distribution curve of light reflected off a green leaf

2.1.4 Absorption of Light You will notice that only the green portion appears in high intensity and portions of the electromagnetic spectrum are missing. So, what happened to the rest of the white light? It appears to be absorbed by the material. But what happens if you hold one object under different colors of light? For example: a yellow light ("bug light") or fluorescent light ("black light")? What we perceive, as the color of the object, changes as we change the wavelength of the incoming light. So, where does the color go? Doing a similar experiment, using perfectly white or black objects, we will find that the white objects reflect all wavelengths of the incoming light source, while the black objects absorb it.

Observation: An object can absorb the light's energy.

2.1.5 Different Types of Light Interactions and Observations So far, we have dealt with objects that either absorb or scatter light; but what if you shine your beam of light onto a clear window or a glass filled with water? One observation is that light can also be transmitted through a material. Other observations may include that light can be dispersed, or that interference patterns form -through the diffraction of light. We may notice that light may: 1) be reflected off a shiny surface, such as metal; 2) be transmitted and refracted by a glass filled with water; and 3) be scattered, dispersed, polarized or absorbed by small particles, such as the air or fog on a cold winter morning. How we perceive these interactions depends not only on the interaction between the material and light itself, but also on our vision and how light interacts with our eyes to make vision possible.

Observation: Objects can reflect, refract, transmit, absorb, disperse, and scatter incoming light.

2.1.6 Quantum Electrodynamics (QED) When light hits matter it is absorbed, reflected or refracted. The correct way to approach this problem is called quantum electrodynamics (QED).

5 The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072

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