Topic #17: Light, an Electromagnetic Wave Phenomenon

Topic #17: Light, An Electromagnetic Wave Phenomenon:

1. Light Travels in a Straight Line

2. The Speed of Light

3. Transmission and Absorption of Light

4. Illumination by a Point Source

5. Light - An Electromagnetic Wave

6. Light and Color Vision

7. Polarization of Light

8. Interference in Thin Films

Notes should include:

Light Travels in a Straight Line: Light travels in a straight line. Have you ever noticed light rays being made visible by moisture, dust, or smoke in the air? You can see the rays and the fact that light does travel in a straight line. If it were not for diffraction and the scattering of light, the effects of light traveling in straight lines would be more obvious to us. For example, if light rays were not diffracted around corners and scattered as they reflect off of objects, imagine how dark areas that were in shadow would actually be. The straight lines that are used to represent light waves are called rays. Diagrams that are drawn using rays to represent light waves in order to represent the behavior of light are called ray diagrams and the use of ray diagrams to study light is called ray optics.

The Speed of Light: Light is so fast that people once thought it traveled from one point to another instantaneously. Around 1676, an astronomer by the name of Olaf Roemer observed eclipses of one of Jupiter's moons as it disappeared behind the planet and reappeared a while later. He made a number of careful time measurements of this phenomenon. He made observations over several months and realized that he was getting an increasing error between his measurements nightly and a table he had initially constructed which provided a projection of the data that he expected to collect over time. He concluded that, though the planet Jupiter moved only a little in its large orbit, the Earth had moved significantly further along its path, which took it further and further from Jupiter each day that passed during the observations. This meant that the light reflected off of the moon being observed had to travel a greater distance to get to Earth as each day passed. From his observations, Olaf concluded that light must have some fixed speed.

As the nineteenth century ended and the twentieth began a scientist by the name of Albert A. Michelson came up with a means of getting a reasonable measurement of the speed of light. He set out to measure the time for light to complete a round trip between two mountains that were 35 kilometers apart. That is a total distance of 70 kilometers. Remember that, because light travels quite fast, a relatively long distance is needed to measure its speed. On one mountain he set up a rotating octagonal mirror. He aimed a beam of light towards this rotating mirror on the top of this first mountain. On the second mountain were two mirrors side by side both partially facing one another and both partially facing the first mountain, such that a beam of light coming off of the rotating mirror would bounce (reflect) off one mirror onto the other and then bounce (reflect) off the second mirror right back towards the rotating octagonal mirror on the first mountain. An observer was placed in line with the source of the light beam and the octagonal mirror. The beam of light was only visible to the observer when the mirror was rotated at the proper speed. Any other speed rather than the correct one would result in the beam being reflected in some direction other than the position of the observer. The motor turning the mirror was started from rest and sped up until the light beam was seen by the observer. From the rate of rotation of the mirror the speed of light was computed.

Transmission and Absorption of Light: Materials that allow light to pass through them are said to transmit light. If you can see through these materials they are called transparent. If they allow light to pass through them, but you cannot see objects through them they are called translucent. A luminous object gives off (emits) light waves. If a body reflects light it is said to be an illuminated object. The brightness of a light source is expressed using a measurement called luminous intensity. The unit of measure for luminous intensity is the candela. This is the international unit for luminous intensity measurement. One candela is the brightness of a light source consisting of one square centimeter of a white thorium oxide powder heated to a temperature of 2046 kelvins. The rate at which light is emitted from a source is called the luminous flux. Luminous flux is measured using the unit called the lumen (a unit of power). One lumen is the light energy being emitted from a point source of one candela intensity and flowing outwards through a one square meter area on a transparent sphere surrounding the point source having a diameter of one meter. Light can be absorbed as well as emitted. The rate at which light energy falls on one unit area of surface is called illuminance. Illuminance is measured in the units of lumens per square meter. Because light from a light source spreads out as it travels outwards from a light source the light dims, that is, with distance the illumination decreases. This means that the illuminance of a surface varies inversely as the square of the distance between the light source and the surface. To increase the illumination of a surface, you must either increase the intensity of the light source or decrease the distance between it and the surface it is illuminating.

Light, An Electromagnetic Wave: Visible Light is one kind of a large category of electromagnetic waves. All properties of waves apply to light waves. A major characteristic of electromagnet waves is that unlike mechanical waves, they require no medium through which to travel. They can travel through a vacuum, such as in space. The whole continuous spectrum of light (electromagnetic) waves includes gamma rays, x-rays, ultra violet, visible, infra red, micro, and radio. These waves are listed here from the highest frequency and shortest wavelength electromagnetic waves to the lowest frequency and the longest wavelength electromagnetic waves.

Light and Color Vision: Issac Newton may be among the first to have seen that a glass prism will break white light into its constituent colors of the rainbow (ROY G BIV). This arrangement of colors is called the visible spectrum. White light is a mixture of the colors of the rainbow. If you were to mix the colors back together you would get white light again. What happens when you see color in an object in the presence of white light? What you are seeing is the color that is not absorbed by the material upon which the white light is shining. The color you see is the wavelength(s) of light reflected back to your eye. For example, red cloth contains dyes, which contain chemical pigments that absorb all colors of visible light except red. The red color is seen because the red is the wavelength of visible light not being absorbed. In a pure blue light source, what color would you see when you looked at the cloth that looked red in white light?

Polarization of Light: When a wave is passed along a medium such as a rope, you can imagine that the waves could pass through an imaginary slot if the wave is oriented in the same direction as the slot. Otherwise the rope wave would be caught or stopped by the slot and not allowed to continue beyond the slot. The concept of polarization of light is similar. Imagine that ordinary light rays are like a bunch of ropes generating waves with all sorts of different orientations. Now imagine a polarizing filter to be like a slot. The only waves that will get through are those that are oriented correctly. They must match the slot or in the case of light waves the orientation of the polarizing material, such as is found in sunglasses having polarizing lenses. Instead of calling a polarizing filter a slot it is called a filter and the orientation of the waves that pass through this filter are said to lie along a particular plane. These waves are said to be plane polarized, because they have the proper orientation. If you wear polarized sunglasses, you will probably notice that the light passing through the filtering lenses is dimmer, because on the average only about half of the rays get through.

Interference in Thin Films: If you have every noticed that soap film and films of oily materials often appear to have colors of the rainbow spread across their surface. This occurs because the thin film varies in thickness and the thickness determines which color of light will reflect back to you eyes. For simplicity imagine a very, very thin prism. As you move from the narrow tip downwards towards the base the prism increases in thickness. Anywhere that the thickness of the prism is odd number multiples of one quarter of a wave length of a specific color (1 x 1/4l, 3 x 1/4l, 5 x 1/4l, etc.) you will see a line or band of that color only. This is caused by a combination of two different reflected rays. When a light ray strikes the surface of a transparent material a portion of it is reflected and if the medium is denser than the air through which the light ray was traveling before it reached the material the reflected wave is inverted. A portion of the light ray will enter the material and be reflected off of the other side and return essentially along its original path. As this internally reflected light ray emerges from the material it will experience interference with the externally reflected portion of the original light ray.

If the two reflected waves are in phase they would cancel at this point because one is inverted. However, if the internally reflected wave is one half wavelength out of phase with the externally reflected wave they will reinforce each other and you will see the color having that particular wavelength and not those colors which were canceled by interference. The positions along the surface of the material where a specific color appears as a line or band are the points where the thicknesses are the odd number multiples of one fourth of the wavelength for that color. Since very thin prisms or films vary in thickness, you will see many different lines or bands of color on the surface, because there is likely to be at least one position with the correct thickness for each color somewhere on the film. It is also highly likely that you would see many repeating lines or bands of color, because there are likely to be multiple positions of the same thickness in the film. If the film is actually moving, you are likely to see the lines or bands of colors swirling around as you do in a soap or oil film.

The reason that odd number multiples of one quarter of a color's wavelength is required for the thickness of the film for lines or bands of color to be produced has to do with the length of the internally reflected ray's path. For the interference between the two reflected waves to produce a reinforced wave rather than a diminished or canceled wave the internally reflected wave must be one half of a wavelength out of phase with the externally reflected wave. This requires that the path length of the internally reflected wave be 1/2 of a wavelength more than some multiple of the whole wavelength so the two waves will be out of phase by one half of a wavelength. Because the wave travels first forward through the medium and then backwards through the medium after reflection, the material need only be 1/4 of a wavelength thicker than some multiple of a wavelength for the wave to have traveled the extra 1/2 wavelength. To help you understand this process always remember that, if the thickness of the film were to be simple multiples of a wavelength, the internally reflected wave would be in phase with the externally reflected and inverted wave. This would result in the complete canceling of the two reflected waves because crests and troughs of equal magnitude but opposite in direction would be meeting and canceling one another.

Vocabulary: light, ray model, luminous, illuminated, luminous flux, lumen, illuminance, lux, candela, luminous intensity, transparent, translucent, opaque, spectrum, primary color, secondary color, complementary color, dye, pigment, primary pigment, secondary pigment, thin-film interference, polarized

Skills to be learned:

Solve problems involving the speed of light.

Solve problems involving luminous intensity and illuminance.

Assignments:

Textbook: Read / Study / Learn Chapter 16 about light

WB Exercise(s):

Activities: TBA

References:

This Handout and Overhead and Board Notes discussed in class

Textbook Chapter 16

WB Lessons and Problem Sets

- “Light, an Electromagnetic Phenomena”

 / Light, An Electromagnetic Phenomena