## Due before class Wednesday, Jul 25th

Physics 106, Summer 2012

Reading: Chapters 25, All but Sec. 3

Did you complete the reading assignment?

Yes
No

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?
Yes
No

## KEY CONCEPTS:

1) Diffraction is the bending of a wave around an object.

2) Diffraction patterns only occur when the slit is about the same size as the wavelength of light going through it.

3) Interference patterns occur when more than one slit is present.

Exercise 1

Diffraction is the bending of a wave around an object. In optics, we are generally most interested in the bending of light through small slits in a material, rather than around the edge of an object. One example of diffraction through a slit that is very easy to see is in this picture of the Panama Canal. In this picture, it is easy to see the water waves bending around the gate opening.

Which way is the water flowing in this picture?

Left to right
Right to left
Top to bottom
Bottom to top

How do you know this?

With light, it can be harder to see this pattern. The following applet will help you understand how light behaves when it travels through a single slit:

The applet is very easy to use. You can change the wavelength of the light and the width of the slit using the scroll bars and you can freeze and start the simulation by pushing the respective buttons.

Which way is the light traveling in this picture?

Left to right
Right to left
Top to bottom
Bottom to top

How do you know this?

Does the light bend all the way around the slit (so the bending pattern touches the walls around the slit) or just partially around the slit (not touching the walls)?

All the way
Partially

Increase the wavelength to its maximum size and decrease the slit width to its minimum size. Then compare this to when the wavelength is at a minimum and slit width is at a maximum. What happens when the slit width is much larger than the wavelength of the light traveling through it?

Why do we not see diffraction patterns when light goes through openings every day (like through a doorway)?

Exercise 2

When there are two or more slits the diffraction pattern you see changes a bit. Go to: http://www.falstad.com/ripple/ to see this.

To make this easiest to see, and to keep the applet from causing a seizure, I suggest changing the simulation speed all the way to the left and the resolution all the way to the right. Keep the brightness near the middle, but you can move it back and forth until you get what you want to see. If you move it too far to the right or left you won't be able to see the diffraction pattern. Also change the 'Setup' scroll box to 'Setup: Double Slit'.

How has the pattern changed?

What is the cause of this change?

Light only refracts partially when there are multiple slits.
The light waves are interfering with each other.
The wall between the two slits blocks certain areas from being able to have light.
There is an error in the simulation.

Now click on the '3-D View' box. You can click and drag the 3-D graph to get different views of what is happening. The higher the amplitude of the light waves, the more intense, or brighter, the light is at that position. Where is the light the brightest on the diffraction patterns?

The center
The first fringe on either side of the center
The edges (near the wall)

According to the 3-D view, what will you see at the points that had no color on the 2-D view?

Exercise 3

I focused on conceptual pieces of the one and two slit experiments in the previous activities. If you would like to look at things more quantitatively, these to links will help you put numbers with the concepts.

Single slit:  http://www.walter-fendt.de/ph14e/singleslit.htm

Double slit: http://www.walter-fendt.de/ph14e/doubleslit.htm

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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?