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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 299,792,458 meters per second. That speed is, by definition, always exact, because the meter is defined in terms of light.

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 slowing down is the product of a consistent interplay between the electric and magnetic fields of the penetrating light, the vibrations of the electron cloud, and the light generated by these vibrations.”
Basic Sciences in Ophthalmology, p. 23

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:

Crown Glass
Flint Glass

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.

Figure 3.2:  Refraction of light between different media. Rarer medium is shown white, denser medium gray. Angle labeled alpha-2 is larger than alpha-1.

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 at angle alpha-1 (relative to the interface) and refracts (transmits) into the denser medium at an angle alpha-2 that is larger (less acute) than alpha-1.

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

Internal Reflection

Consider the right side of Fig. 3.2 again. Light in the dense optical medium approaches the interface at an angle alpha-2 and transmits into the rarer medium at a shallower angle alpha-1.

Now suppose the approach angle alpha-2 becomes smaller. In that case, the refracted angle alpha-1 also gets smaller. As alpha-2 becomes smaller, alpha-1 also becomes smaller.

Eventually, when alpha-2 becomes small enough, then alpha-1 vanishes (becomes zero). That angle of alpha-2 is called the critical angle. At the critical angle of an approaching light ray in the dense medium, none of the light ray is refracted into the rare optical medium.

Even more interesting, if the approaching angle alpha-2 gets smaller than the critical angle, the light ray is reflected back into the dense optical medium (and not refracted into the rare medium).

Due to the principle of reversibility, the light rays can be reversed to achieve the same result: after the approach angle alpha-1 from rare optical medium (left side of Fig. 3.2) becomes smaller and vanishes, then light from within the dense medium is reflected into the dense medium in the direction that refraction would have gone, as shown in the following photograph:

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.

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