9 3.3 Wave Behavior
Behavior of Light
Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, it is either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.
Absorption, Transmission, and Reflection
This video reviews the concepts of absorption, transmission, and reflection of light:
Diffraction, Scattering, and Refraction
Diffraction is the bending and spreading of waves around an obstacle. It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength. In the case of visible light, the separation of wavelengths through diffraction results in a rainbow. The figure below shows the grooves on a CD diffracting visible light and producing iridescent colors.
Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object.
The sky appears blue because of this scattering behavior. Light at shorter wavelengths—blue and violet—is scattered by nitrogen and oxygen as it passes through the atmosphere. Longer wavelengths of light—red and yellow—transmit through the atmosphere. This scattering of light at shorter wavelengths illuminates the skies with light from the blue and violet end of the visible spectrum. Even though violet is scattered more than blue, the sky looks blue to us because our eyes are more sensitive to blue light.
Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles. For example, when the full spectrum of visible light travels through the glass of a prism, or through a raindrop, the wavelengths are separated into the colors of the rainbow.
We will discuss refraction in more detail and introduce polarization of light in the sections below. These concepts are especially important for understanding techniques in optical mineralogy and petrology.
Guided Inquiry
Refractive Index
Electromagnetic energy travels at the speed of light, c, in a vacuum. However, when electromagnetic waves travel through matter, they slow down. The speed at which they travel through a material is specific to the density and composition of the material.
The refractive index of a material characterizes the relationship between the speed of light in a vacuum and the speed of light in that material.
The refractive index of a material is defined as:
where n is the refractive index, c is the speed of light in a vacuum, and v is the observed speed of light in the material. Since the speed of light in matter is always less than c, the index of refraction i