The Nature of Light
Page 3: Transmission
Light is transmitted as packets of color (energy).
The speed of light in a vacuum is defined exactly as
The speed of light in materials (not a vacuum) is slower than that speed, because particles in the transmitting medium slow down the light.
In a transparent medium (referred to as a dielectric material), light is absorbed by atoms and then emitted with the same frequency in the same direction the previously absorbed photon was traveling. This occurs with electrons in the atom jumping to a different energy level when absorbing a photon and then jumping back when emitting the photon (Fig. 3.1).
Figure 3.1: Photon absorption and emission in an atom.
Atomic absorption and re-emitting slows down the light. The denser the medium, the more slowing of the speed of light in the medium.
The ratio, between the speed of light in a vacuum and the speed of light in a dielectric material, is called the refractive index of the material. Following are refractive indexes of various transparent materials:
For practical purposes, air is often considered to be like a vacuum in many optics applications, since it does not slow down the speed of light substantially.
Materials like air are referred to as rarefied (or rarer media) compared to more dense materials. For example, air is rarefied compared to water.
When light strikes an interface between different media (that have different refractive indexes) at an angle (not straight on), the direction of the light changes at the interface. The light beam direction becomes less acute (relative to the interface) in the denser medium.
Due to the principle of reversibility (see K. K. Sharma, Optics, p. 286), light can travel in either direction to produce the same result.
In the left side of Fig. 3.2, light in the rarer media approaches the interface
Likewise, due to the principle of reversibility, light traveling in the opposite direction would produce the same angles (right side of Fig. 3.2). Light traveling from the denser medium into the rarer medium will change direction to be a smaller angle relative to the interface in the rarer medium.
[+] Show Refraction Animation
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Fig. 3.3: Refraction animation. [OlegAlexandrov]
Consider the right side of Fig. 3.2 again.
Light in the dense optical medium approaches the
interface at an angle
Now suppose the approach angle
Even more interesting, if the approaching angle
Due to the principle of reversibility, the light rays can be
reversed to achieve the same result: after the approach angle
Figure 3.4: Skindiver on the ocean surface viewed from below. [NOAA]
The ocean in Figure 3.4 is a dense optical medium, and the atmosphere above the ocean is a rare optical medium. The ocean surface, is viewed from within the dense optical medium, and is the interface between the dense and rare optical media.
The atmosphere in this example has blue sky and clouds, which can be seen in patches of the interface (ocean surface) in the foreground (top) of photograph. Those patches of ocean surface are facing the viewer (camera) more head-on (less acute) than the critical angle.
The rest of the ocean surface viewed from below is reflecting the ocean bottom instead of refracting the blue sky and clouds, because the viewing directions from the camera to those patches of ocean are more acute than the critical angle.
The patches that are reflecting the ocean bottom (not refracting the sky) are said to be experiencing total internal reflection in this example.