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Some TOK questions:
Is light a wave or a particle? What about an electron?
How does measuring an event change the event?
Expectations:
Qualitatively describing the diffraction pattern formed when plane waves are incident normally on a single-slit
Quantitatively describing double-slit interference intensity patterns
General Resources:
General Resources:
Kognity Textbook Chap 4 - Use you ACS Login
IB Physics Site: Topic 4 - Comprehensive notes
IB Physics Site: Topic 4 - More notes
Topic 4 Flashcards - Vocab Devo.
Flippin' Physics -
The Physics Classroom - Two-point Interference
openstax - Diffraction
Questions that May Arise...
Questions that May Arise...
Are electrons waves or particles?
1. Diffraction through a single slit
1. Diffraction through a single slit
Single slit
On the slides that are used here the width of the slit is labelled b, as shown below.
Insert the single slit slide with slit width’s of 0.075 mm, 0.15 mm and 0.4 mm into the slide holder. Pass the laser light through one of the slits and observe the pattern on the screen.
Investigate the effect on the diffraction pattern of changing the slit width b, by using the movable rider to move the slide across the laser beam. You will want to include a brief description of the effect of the width of b.
Investigate the effect on the diffraction pattern of changing the distance to the screen D by moving the screen closer to or further away from the slide.
Investigate the effect on the diffraction pattern of changing the wavelength λ of the light by using both the red (longer wavelength) and green laser (shorter wavelength).
SL - QUALITATIVE Description
2. Diffraction around an Object
2. Diffraction around an Object
Use the slide with the thin object and observe the pattern when the laser light shines around the object.
Describe what you see.
Predict the effect of increasing the width of the object.
3. Double Slit Interference
3. Double Slit Interference
On the slides that are used here the width of the slit is labelled g, as shown below. The equation used in this topic uses the symbol d for slit separation.
SL/HL - Quantitatively describing double-slit interference intensity patterns
Intensity at central maxima:
If you recall, the intensity is proportional to the square of the amplitude. Therefore if the amplitude is doubled the intensity will be quadrupled.
YDSE Note Sheet
YDSE Problem Set - COPY HERE
Effect of slit separation - value of d in the YDSE eq.
Use the double slit slide with slit width b = 0.15 mm and slit separation g = 0.25 mm, 0.50 mm, 0.75 mm and 1.00 mm.
Investigate the effect of changing the slit separation g on the diffraction pattern.
4. Measuring the wavelength of the laser
4. Measuring the wavelength of the laser
The wavelength of the light used is related to the geometry of the experiment by the following equation:
where:
s is the separation of the fringes on the screen [m],
λ = wavelength of light [m]
D is the distance between the slits and the screen [m] and
d is the distance between the slits [m].
Set up the optics bench so that the red or green laser light passes through the double slit with slit separation 0.25 mm and slit width 0.15 mm. The slit separation d in the equation is therefore 0.25 mm.
Adjust the positions of the laser, slide and screen so that the diffraction pattern is as wide as possible.
Measure the distance from the slide to the screen D.
Measure the separation of the fringes s on the screen by measuring a larger number of separations e.g. 10 and then calculating the average.
Use the equation above to calculate the wavelength of the laser light in nm and compare it with the stated value of the laser.
5. Measuring the width of a human hair
5. Measuring the width of a human hair
Attach a human hair to a slide holder with tape.
The equation,
s = fringe separation [m]
λ = wavelength of light [m]
D = distance from hair to screen [m]
d = width of hair [m].
can be used as before but in the situation where light diffracts around an object d in the equation represents the width of the object.
Set up the optical bench so that the laser passes across the hair and a diffraction pattern is visible on the screen.
Adjust the positions of the laser, slide and screen so that the diffraction pattern is as wide as possible.
Measure the distance from the hair to the screen D.
Measure the separation of the fringes s on the screen by measuring a larger number of separations e.g. 10 and then calculating the average.
Write down the wavelength λ of the laser used.
Calculate the width of the hair d using the equation above and compare it with values found using the internet.
The equation for the double slit and the width of a human hair is the same, except for the measurement of d. Compare the variable d in both equations.
6. Ripple tank simulation by Paul Falstad
6. Ripple tank simulation by Paul Falstad
Launch the Ripple Tank Simulation by Paul Falstad.
The simulation can be used to observe the wave patterns formed by diffraction and interference. There are pre-set simulations that can be selected. e.g. click on the Example tab and choose Example: Obstacle as shown below.
As the simulations runs you can observe points of constructive and destructive interference, as shown below.
Try some of the other pre-set examples e.g. single slit and double slit or set up your own simulation using the Add tab.
7. Two Source Interference - Desmos
7. Two Source Interference - Desmos
Using the time slider (t), expand the waves until they hit the barrier.
Describe the pattern of waves that represents constructive and destructive interference.
You can move top blue point (d).
Sketch a graph of the relationship between the distance between sources (d) and the fringe spacing (Y).
Changes to the coding:
Some of the variables are labeled using different letters. Using the code on the left, change the labels of 'L', 'w', and 'Y' to fit the letters used in the IB data booklet.
Diffraction Theory
Diffraction Theory
General Resources:
General Resources:
Kognity Textbook Chap 4 - Use you ACS Login
IB Physics Site: Topic 4 - Comprehensive notes
IB Physics Site: Topic 4 - More notes
Topic 4 Flashcards - Vocab Devo.
The Physics Classroom - Two-point Interference
openstax - Diffraction
Sample Problem:
Sample Problem:
Coherent light of wavelength 633 nm from a He-Ne laser falls on a double slit with a slit separation of 0.103 mm. An interference pattern is produced on a screen 2.56 m from the slits.
Calculate the distance to the first bright fringe. (be sure to check the units of your measurements.)
Two-Slit Diffraction Problem Set -
Not part of the 2020-21 School Year.
Not part of the 2020-21 School Year.
This activity is about diffraction and two-slit interference. You will investigate qualitatively the effect of
slit width,
slit separation and
wavelength
on diffraction through a double slit and around an object. You will investigate qualitatively the interference of light through a double slit by applying an equation to calculate the wavelength of the light.
Visible light has wavelengths of less than a thousandth of a millimeter therefore the diffraction of light goes unnoticed unless the light is passing through very small gaps. A typical diffraction pattern for green light passing through a single slit is shown above.
Set up the 2 m optical bench kit, as shown below, with either the red or green laser.
Warning! Do not look into the laser beam
Effect of slit width - Not evaluated in IB Curric.
Use the double slit slide with slit separation g = 0.30 mm and slit width b = 0.10 mm, 0.15 mm and 0.20 mm.
Investigate the effect of changing the slit width b on the diffraction pattern.
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