1. Module 5

1.30. Page 2

Lesson 6

Module 5—Wave Theory of Light

 

Explore

 

Diffraction

diffraction: the bending and spreading of waves when they interact with obstacles in their path

 

By the early nineteenth century it was well known by scientists that mechanical waves, such as those of water, displayed the unique property of diffraction.

 

A diagram shows diffraction by representing a wave front with a series of vertical lines at left. It moves toward the right, where there is an obstacle with a hole in the centre. The lines are bent, becoming arcs, as they pass through the hole to show diffraction.

Diffraction occurs when any wave front bends or changes direction as it passes by the sharp edge of an obstacle or through a small opening in the obstacle. As illustrated, waves are diffracted as they pass through a small opening in a barrier. The amount of diffraction depends on the wavelength and the size of the opening. A barrier with multiple openings is called a grating. The CD or DVD is an example of a grating; it has many small openings in the form of concentric rings from which light waves may emerge after reflecting off the metal backing of the disc.

 

Watch and Listen

 

To observe diffraction of mechanical and electromagnetic waves, such as visible light, open the Wave Interference simulation. You may select any one of three wave forms: water, sound, and light.

 

 

In each mode, select one slit (); then click the barrier and drag it close to the wave source. Observe the circular shape of the wave front that emerges from the opening. You may add walls to the sound and water sections and explore diffraction around corners as well.

 

Diffraction and Interference

 

A barrier with a single slit acts as a single point source of light producing ever-expanding circular wave fronts. If a second slit is introduced beside the first one, there will be two identical point sources of light, each producing circular wave fronts that will contact one another. Therefore, if light has wave-like characteristics, the two point sources should interfere to produce a distinct interference pattern. Interference patterns were explored in your previous physics course and are reviewed here.

 

Interference refers to the way in which two or more wave forms combine to produce a resultant wave form. There are two different types of interference: constructive interference and destructive interference.

 

constructive interference: occurs when two waves combine to produce a resultant wave larger than either of the original waves

 

destructive interference: occurs when two waves combine to produce a resultant wave smaller than either of the original waves

 

node: a point on a standing wave pattern where there is no displacement and the wave appears to be standing still

 

antinode: opposite of node, a point on a standing wave pattern where there is maximum displacement

If complete destructive interference occurs, both waves cancel each other out and produce no resultant wave. On an interference pattern this is referred to as a node.

 

If complete constructive interference occurs, both waves combine to produce the largest possible resultant wave. On an interference pattern, this situation is referred to as an antinode.

 

Watch and Listen

 

See how two waves interfere constructively and destructively in the video Young’s Double-slit Experiment. Note that in the destructive phase, it’s as if a positive amplitude (above the middle line) is added to a negative amplitude (below the middle line) to produce a zero amplitude.

 

Module 5: Lesson 6 Assignment

 

Remember to submit your answers to A 1 to your teacher as part of your Module 5: Lesson 6 Assignment. 

 

A 1. After viewing the video, draw two waves (one below the other) and the resultant wave to illustrate the following.

  1. complete constructive interference
  2. complete destructive interference

When a wave front encounters two slits, each slit produces a circular wave front, leading to a distinctive interference pattern characterized by repeating regions of destructive and constructive interference. In terms of light, this is observed as repeating dark and bright regions on a screen.

 

Watch and Listen

 

Watch Young’s Double-slit Experiment again. Pay close attention to the production of a two-slit interference pattern.

 

Try This

 

TR 1. Sketch the two-slit interference pattern seen in the previous video clip.

 

Thomas Young’s Double-slit Experiment

 

The two-slit interference pattern was first demonstrated at the turn of the nineteenth century. At the time there was a long-running, heated debate surrounding the nature of light. Was it a stream of particles or was it a wave? The scientific community of the time was split with many scientists supporting Isaac Newton’s theory that light was made of tiny particles, which would be unable to interfere as waves would.

 

In 1801, Thomas Young, a professor of natural philosophy and physics at the Royal Institution in London, performed a key experiment to support the wave characteristics of visible light. He placed a narrow card similar to that of a playing card into a beam of light coming from a nearby window.

 

When the card was held sideways, splitting the beam into two identical light sources, an interesting pattern was observed on a nearby wall, which is illustrated in the image. In the illustration, the light of a single wavelength (colour) is directed at two small slits. After passing through the slits it falls on a distant screen to produce an interference pattern, which has many sequential bright and dark bands.

 

A diagram shows the pattern produced when the light of a single wavelength (colour) is directed at two small slits. After passing through the slits, the light falls on a distant screen, producing an interference pattern that has many sequential bright and dark bands.


 

Watch and Listen

 

You can perform a similar experiment to that of Thomas Young using a simulation. Open the Wave Interference simulation, and select the light wave form.

 

Select () and then click the barrier and drag the barrier close to the wave source. Click on “Show Screen.” Observe the interference pattern produced. The pattern is easier to observe with high amplitude waves. You can adjust the amplitude slider on the light source when observing the interference pattern.

 

Young’s double-slit experiment showed that light, when shone through two slits, was diffracted and produced a definite interference pattern on a screen. This evidence helped to convince the scientific community that the behaviour of light could be explained with a wave model. Light displays properties unique to waves: interference and diffraction.

 

Watch and Listen

 

Watch Young’s Double-slit Experiment for a final time. This time pay close attention to the overview of diffraction, interference, and Young’s Experiment.