Next, the teacher should focus on color. Selective reflection (or transmission in the case of transparent objects) of light is what causes an object to have color. If light in various parts of the spectrum illuminates an object they will appear to be brown or black except in the part of the spectrum which they reflect. This can be demonstrated in front of the class using different colors of light on an object. Then the teacher should discuss mixing colored light, including additive and subtractive primary colors. This will help in understanding why the sky is blue, why sunsets are red, why clouds are white, and why water is greenish blue.

Reflection, refraction, and Fermat’s principle of least time should be defined in the beginning of the third section. One example of refraction is the apparent mirage that appears ahead of you when driving on a hot, sunny day (due to warmer, less-dense air near the surface of the road refracting sky light). Other causes of refraction should be discussed. The concept of total internal reflection should also be taught. Examples of different lenses and the image they produce should be demonstrated before the class.

Light, like sound, has some of the properties of a wave. Light wave fronts are made up of tinier wave fronts, which is known as Huygens’ Principle. Then the idea of diffraction can be introduced by observing light through a slit and noting the diffracted light on the wall. Similar to the interference observed when listening to sound waves is the interference pattern of overlapping light waves. This can be demonstrated similar to diffraction, but using a double slit instead. To illustrate that light waves are transverse and not longitudinal the teacher must polarize the light by using a polaroid sheet. Using a single polaroid sheet can block one component (horizontal and vertical) of light. Sunglasses block the horizontal component of light, thus reducing glare from reflected light (off of water). The teacher could then discuss colors by transmission through polarizing materials, three dimensional viewing, and holography if desired.

Light emission involves the transition of electrons from higher to lower energy states within an atom, which is called excitation. Emission spectra of different light sources (phosphorescent, fluorescent, and incandescent) can be viewed through a simple spectroscope made in class by purchasing a kit (including a tube with a slit on one side and a hole on the other covered by a diffraction grating. The difference between incoherent and coherent light must be discussed before learning about the laser. Applications of the laser are many, many—a very common one is the bar code.

Contrary to the wave-like nature of light is its particle-like nature of light. Light is unique that it acts like a wave and a particle, which have mutually exclusive properties. We will now explore the quantum theory of light. In the world of quantum physics, energy is quantized which means it comes in discrete bundles, no half or partial bundles. One quanta of light is a photon. Knowing this, the photoelectric effect can be discussed. Quanta, although considered a particle, do exhibit some wave-like traits as well. Electrons, which are known and measurable quantities, also exhibit this duality. The dual nature of quanta led Werner Heisenberg to question our ability to accurately measure quanta-sized particles, which led him to the uncertainty principle. Neil Bohr further explored the dual nature of light and electrons and found that, mutually exclusive properties must also behave as complementarity.