Physics 106, Summer 2012
25, All but Sec. 3
Did you complete the reading assignment?
Did you complete the entire quiz or spend at least 15
minutes working on this quiz and the attached links?
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.
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
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?
The water waves are bending around the bottom side, so the source needs to be coming from the top side.
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?
The light waves are bending around the top side, so the source needs to be coming from the bottom side.
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
All the way
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?
It does not diffract in the center, only around the very edges of the slit, and the diffraction pattern is not really observed.
Why do we not see diffraction patterns when light goes through openings every
day (like through a doorway)?
Because these openings, like doorways, are WAY bigger than the wavelength of visible light which is around 500 nm (or 500 billionths of a meter).
When there are two or more slits the diffraction pattern you see changes a bit.
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?
There are bright spots seperated by lines with no light.
What is the cause of this change?
Light only refracts partially when there are
The light waves are interfering with each
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 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?
You will not see any light at these points, it will be darkness.
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
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Was there anything that you didn't understand in the reading assignment?
What was confusing to you?