Day 1.5
Diffraction Gratings
1) The interference pattern below was formed by laser light passing through two slits. Sketch the pattern
that would be formed by six slits with the same width and separation, d, between each slit. Use words to
describe what has changed and what has stayed the same. http://hyperphysics.phyastr.gsu.edu/hbase/phyopt/mulslid.html#c4
Instructions: You need special equipment to show this. Cornell plates contain this and many other
demonstrations. Boreal has this and a couple of other items for $100. http://boreal.com/diffractioninterference-resolution-kit/p/IG0023801/ This is pricy, but worth it for key demonstrations like this and the
one below.
The separation between the spots has not changed. However, the spots are more concentrated and more of
the space between the spots is dark.
2) Predict how the pattern will change if you have six slits in the space where you previously had two.
The antinodes will be three times farther apart because the separation is one-third of what it was.
3) A diffraction grating consists of many slits, very close together. It produces spots as shown below.
a) What colours will you see and where will they be if you look at white light through the grating?
Instructions: Plastic replica diffraction gratings just keep getting better and cheaper. Buy a class set.
You will see a full spectrum for the off-centre spots. The violet will be closer to the centre.
The center spot may have no colours or just edges of red because of overlap of the colours.
b) Measure the ratio of the wavelength of red light to violet light.
Instructions: Each student should be looking at the white light source through a grating in their hands. Ask
them to estimate the ratio. For that, they just need to estimate how the distance from the center to the red
compares to the distance from the center to the violet. Then challenge them to measure these distances. This is
challenging because the pattern is projected onto their eyes – not a screen. The students see the pattern
superimposed on their view of the classroom. Have one student tell you where to place marks on the blackboard
for the centre, red and violet. Then make measurements on the blackboard. If this is done as a full-class
demonstration, they will find it easier to do the LED measurement in questions 7.
4) Measure the separation of slits in the diffraction grating.
Instructions: Give each group a laser pointer and diffraction grating. Have them project the light down onto the
table. The students should have the beam perpendicular to the table and grating. The angle will be large, so they
should use the formula with sine.
5) What will you see if a laser beam passes through two diffraction gratings at right angles to each other?
a) Why?
You will not only get the interference pattern of each separate grating at right angles to each other but you
will also get patterns on the diagonal. These are from the diagonal spacings formed by the two gratings
together. (p. 509)
b) What if there are many gratings at different angles?
You will form a series of concentric circles. You can demonstrate this if you have a set of diffraction
gratings with the same spacing.
c) Metal crystals form regular arrays in random orientations like this. What wavelength of light would
show interference like this? (p. 615)
The wavelengths should be similar in size as the separations, 10-9 m and these are x-rays.
6) Measure how far apart the pits on a CD are.
Instructions: This involves interference from reflected sources and all the previous have been transmission
gratings. If you buy large stacks of blank CD’s, you often get one without the metallic coating. Don’t lose it!
Use it to show the transmitted interference pattern. Give each group a laser pointer and CD. The students
should have the beam perpendicular to the table and screen. The angle will be large, so they should use the
formula with sine.
7) Measure the range of wavelengths the LED produces using a diffraction grating, a laser pointer, an LED
and a power supply.
Instruction: Each group needs an LED that produces a different colour. These can be LEDs that will be
used to measure Planck’s constant in the next unit. The LED’s need to be in a circuit of 3.0 V with a resistor
so they don’t burn out. The students will have a similar measuring problem as question #3.
Textbook: 10.3
p. 511 # 1 – 5
8) What is a spectroscope? What sorts of scientists use spectroscopes? Why?