Honors Physics - Chapter 16: Lesson 4

Chapter 16: Lesson 4

Everyday Light

Chapter 16: Lesson 4 - Everyday Light

Thin Film Interference

Constructive and destructive interference of light waves is also the reason why thin films, such as soap bubbles, show colorful patterns. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be on the order of the wavelength of light.

Consider the case of a thin film of oil floating on water. Thin-film interference can take place if these two light waves interfere constructively:

the light from the air reflecting off the top surface
the light travels from the air, through the oil, reflecting off the bottom surface, traveling back through the oil and out into the air again.

If a soap film is held vertically, its weight makes it thicker at the bottom then at the top. The thickness varies gradually from top to bottom.

Polarized Light

Ordinary light can vibrate in all directions. Polarized light is light in which electric fields are all in the same plane. A polarizer takes light that is vibrating in all directions and only allows light that is passing in one direction to pass through.

Picked Fence Analogy

Let's pretend that we are neighbors. We have a vertical picked fence between our houses, like the top picture. You are sick, so I am bringing you your homework. I can pass your homework to you vertically, through the fence. If I try to pass your homework horizontally, it will not go through the fence. This is just like light. Vertical light will go through a vertical polarizer, but horizontal light will not go through a vertical polarizer.

You get tired of me bringing home your homework, so you build a horizontal fence next to the vertical fence. I can try to put your homework vertically or horizontally, but it will not fit through the fence. This is the same with polarizers. If we have a vertical and a horizontal polarizer next to each other, no light will get through. Polarizers that have axis perpendicular to each other allow no light to pass.

What is Polarized Light?

Watch this video to see what happens when light hits a polarizer.

Polarized Sunglasses

Sunglasses are polarized. Polarized sunglasses filter out horizontal light waves (glare) and allow the vertical light to pass through. Look at the difference that polarized sunglasses make!

Polarized Light on a Computer Screen

Light that is emitted from a computer screen, cell phone, and some TVs are polarized. Watch the video to see what happens when I use a polarizer by my computer screen.

Malus's Law

Malus's law explains the reduction of light intensity as it passes through a second polarizing filter. If the light intensity after the first polarizing filer is I1, then a second polarizing filter, with its polarizing axis at an angle θ, relative to the polarizing axis of the first, will result in a light intensity, I2, that is equal to or less than I1.

This equation is on your equation sheet, but we will not be doing any math problems with it. This is just to show you how the intensity of light is calculated with polarizers.

Doppler Effect of Light

Just like we can hear the Doppler effect of sound, we can see the Doppler effect of light. When waves are going away from us, we see LONG red waves. When waves are going towards us, we see SHORT blue waves.

Watch the first 3 minutes and 20 minutes of the video to have a great recap of the doppler effect of sound and light. The information after the 3:20 minute mark is interesting, but concepts that we will not be getting into in CP Physics B.

Edwin Hubble

Using the Doppler Effect

In 1929, Edwin Hubble suggested that the universe is expanding. Hubble reaches his conclusion of the expanding universe by analyzing emission spectra from many galaxies, like the video above discussed. Hubble noticed that the spectral lines of familiar elements were at longer wavelengths than expected. The lines were shifted toward the red end of the spectrum. No matter what area of the skies he observed, the galaxies were sending red-shifted light to Earth. Hubble concluded that all galaxies are moving away from Earth.

The expanding universe can be thought of like a balloon with coins stuck to it. As we blow up the balloon, the coins all move farther and farther apart. There is, on the surface of the balloon, no "center" of expansion.

If you apply the same theory as the coins on the balloon to a wave on a balloon, you can see the wavelength increase as the balloon increases in size. Longer wavelengths are closer to the red end of the spectra and shorter wavelengths are more toward the blue end of the spectra.

Check Your Understanding:

Click on the down arrow when you have your answer to check to see if you are correct. Use the information below the answer the six questions.

Imagine that you have taken a camping trip to the Rocky Mountains with five friends. On a quiet evening when there seems to be nothing to do, you decide to perform a seemingly impossible feat. You announce the following to your friends.

"I will climb that mountain five miles to the north taking with me only a pad of paper, a pencil, and a super-sensitive light meter that measures illuminance. Here are five identical flashlights. Beginning at 11:00 P.M. you may walk, jog, or run directly toward or away from me for one hour, or you may stay where you are. At midnight, stand still and shine the flashlights at me for a few seconds. Resume walking, jogging, or standing still for another hour until 1:00 A.M. Flash your lights at me again. Then repeat the whole procedure for one more hour. I only ask that you line up from left to right and keep these relative positions so I can identify you by assigning you numbers from 1-5. I'll come back to camp shortly after 2:00 A.M. and you can ask me questions about your activities between midnight and 2:00 A.M. and I will answer every question correctly.

You head for the mountain, climb to the top, and over the next few hours collect the data shown in the table. What are your answers to your friend's questions? Explain each answer.

At midnight, which of your friends was closest to you?

Friend 5's flashlight provided the greatest illuminance.

2. At midnight, which of your friends was farthest from you?

Friend 4's flashlight provided the last illuminance.

3. From midnight to 2:00 A.M., one of your friends did not appear to move. Which one?

Friend 1 - it was the only friend whose flashlight produced the same illuminance at all times.

4. From midnight to 1:00 A.M., which of your friends traveled the greatest distance?

Friend 5 - based on illuminance, this friend increased the distance from the mountain by a factor of 3.

5. Which of your friends appeared to move continuously between midnight and 2:00 A.M.?

Friend 2 - the illuminance changes with each succeeding hour, which implies continuous motion.

6. Which of your friends appeared to be in continuous motion towards you?

Friend 2 - the illuminance increased steadily from midnight to 2:00 A.M. which implies that the friend was moving closer.

You have finished Lesson 4!

Be sure to head over to google classroom and fill out the exit pass.

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