Lesson 2: How Do We Know Light Behaves as a Wave?

Mingyu Li

Wavelike Behaviors of Light

An age-old debate which has persisted among scientists is related to the question, "Is light a wave or a stream of particles?" Very noteworthy and distinguished physicists have taken up each side of the argument, providing a wealth of evidence for each side. The fact is that light exhibits behaviors which are characteristic of both waves and particles.
Light exhibits certain behaviors which are characteristic of any wave and would be difficult to explain with a purely particle-view. Light reflects in the same manner that any wave would reflect. Light refracts in the same manner that any wave would refract. Light diffracts in the same manner that any wave would diffract. Light behaves in a way that is consistent with our conceptual and mathematical understanding of waves. Since light behaves like a wave, one would have good reason to believe that it might be a wave. In this lesson , we will investigate the variety of behaviors, properties and characteristics of light which seem to support the wave model of light. On this page, we will focus on three specific behaviors - reflection, refraction and diffraction.
A wave doesn't just stop when it reaches the end of the medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium. Specifically, there will be some reflection off the boundary and some transmission into the new medium. The transmitted wave undergoes refraction (or bending) if it approaches the boundary at an angle. If the boundary is merely an obstacle implanted within the medium, and if the dimensions of the obstacle are smaller than the wavelength of the wave, then there will be very noticeable diffraction of the wave around the object. Each one of these behaviors - reflection, refraction and diffraction - is characterized by specific conceptual principles and mathematical equations. Now we will see how light waves demonstrate their wave nature by reflection, refraction and diffraction.

Reflection of Light Waves
All waves are known to undergo reflection or the bouncing off of an obstacle. Most people are very accustomed to the fact that light waves also undergo reflection. The reflection of light waves off of a mirrored surface results in the formation of an image. One characteristic of wave reflection is that the angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface. This characteristic is observed for water waves and sound waves. It is also observed for light waves. Light, like any wave, follows the law of reflection when bouncing off surfaces.

Refraction of Light Waves
All waves are known to undergo refraction when they pass from one medium to another medium. That is, when a wave front crosses the boundary between two media, the direction that the wave front is moving undergoes a sudden change; the path is "bent." This behavior of wave refraction can be described by both conceptual and mathematical principles. First, the direction of "bending" is dependent upon the relative speed of the two media. A wave will bend one way when it passes from a medium in which it travels slow into a medium in which it travels fast; and if moving from a fast medium to a slow medium, the wave front will bend in the opposite direction. Second, the amount of bending is dependent upon the actual speeds of the two media on each side of the boundary. The amount of bending is a measurable behavior which follows distinct mathematical equations. These equations are based upon the speeds of the wave in the two media and the angles at which the wave approaches and departs from the boundary. Light, like any wave, is known to refract as it passes from one medium into another medium. In fact, a study of the refraction of light reveals that its refractive behavior follows the same conceptual and mathematical rules which govern the refractive behavior of other waves such as water waves and sound waves.

Diffraction of Light Waves
Reflection involves a change in direction of waves when they bounce off a barrier. Refraction of waves involves a change in the direction of waves as they pass from one medium to another. And diffraction involves a change in direction of waves as they pass through an opening or around an obstacle in their path. Water waves have the ability to travel around corners, around obstacles and through openings. Sound waves do the same. But what about light? Do light waves bend around obstacles and through openings? If they do, then it would provide still more evidence to support the belief that light behaves as a wave.
When light encounters an obstacle in its path, the obstacle blocks the light and tends to cause the formation of a shadow in the region behind the obstacle. Light does not exhibit a very noticeable ability to bend around the obstacle and fill in the region behind it with light. Nonetheless, light does diffract around obstacles. In fact, if you observe a shadow carefully, you will notice that its edges are extremely fuzzy. Interference effects occur due to the diffraction of light around different sides of the object, causing the shadow of the object to be fuzzy. This is often demonstrated in a Physics classroom with a laser light and penny demonstration. Light diffracting around the right edge of a penny can constructively and destructively interfere with light diffracting around the left edge of the penny. The result is that an interference pattern is created; the pattern consists of alternating rings of light and darkness. Such a pattern is only noticeable if a narrow beam of monochromatic light (i.e., single wavelength light) is passed directed at the penny. The photograph at the right shows an interference pattern created in this manner. Since, light waves are diffracting around the edges of the penny, the waves are broken up into different wave fronts which converge at a point on a screen to produce the interference pattern shown in the photograph. Can you explain this phenomenon with a strictly particle-view of light? This amazing penny diffraction demonstration provides another reason why believing that light has a wavelike nature makes cents (I mean "sense").
Light behaves as a wave - it undergoes reflection, refraction, and diffraction just like any wave would. Yet there is still more reason to believe in the wavelike nature of light.

When discussing the properties of light and wave formation you should try and include objects that your students can easily relate to such as; a barcode scanner used by the cashier at a local retail store; or a flashlight and mirror demonstrating how light travels, absorbs, and reflects; or how a photocopier works. There is also a website called www.howstuffworks.com, this site has a lot of visual demonstrations to assist with explaining these topics.
Barry Spainer

This is a comprehensive, but high-level piece. Which grade(s) would this be appropriate for? Do you have any recommendations for "leveling" this down to lower grades? --Jason Feliciano

In my last semester of teaching 'technology' to 11th graders, the intended curriculum and materials were not received on a timely basis. The plan was to teach communications technology and we did in fact get to build telegraph devices in class. I then turned to exploring electricity and power plants as a direct result. For a telegraph to operate, the clicking is caused by connecting and interrupting an electromagnetic field. To create this field, we wrapped copper coil around a magnet. This is the same process used by power plants to generate electricity. Well, that then led me to present a unit about the electromagnetic spectrum, and then on to light. I shamelessly used videos and the black line guides I found at United Streaming (since replaced with PowerMedia Plus). The argument that Mingyu discusses about is brought up in these vidoes. More importantly, to answer Jason's question, this material, at this level, is not all that complicated, especially when accompanied by age-appropriate videos. Light is, after all, a visible phenomenon. While one might not get to highly complex arguments about whether light needs to be categorized as just waves or a particle stream, the concepts of reflection, refraction and diffraction or easy to observe and understand, and one can understand that It is a interesting discussion.

Visible light is only a small part of the entire electromagnetic spectrum (and somewhere in the 'middle'), which could be described as waves oscillating at different frequencies. Hopefully my students learned that vocabularly (if they studied this topic in the past, it wasn't evident) and it gave them a taste of physics, which some of them no doubt took in the following year. I also felt that some foundational knowledge about light is very important to technology. Lasers and optics are significant fields. Without those technologies we wouldn't have CD's & DVD's. I found a few interesting videos that showed researchers working with twisting light, and how fiber optics carry data on light waves. As these technologies make their way into the mainstream, a new level of data transfer and the ability to keep up with the exponential origination and delivery of communication will make their mark. [Lynne Bailey]
There is an interesting interactive activity with light refraction, reflection ect at http://www.sciencejoywagon.com/physicszone/lesson/09waves.htm
This site will allow the students to play around with a hands on activity that may help them understand this difficult concept a little more easily.
Tim Sullivan