Diffraction Grating
Light, Sound and Waves

Diffraction of light

for 14-16

These experiments enable students to understand and use the diffraction grating, an instrument of enormous importance to physicists. 

It has enabled us to show the wave properties of particles such as electrons and neutrons, to measure wavelengths and to estimate the speed with which stars are approaching or receding. From this, we can infer an expanding universe.

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Coarse diffraction grating

Diffraction Grating
Light, Sound and Waves

Coarse diffraction grating

Practical Activity for 14-16

Class Practical

A diffraction pattern showing bands of colour produced from white light.

Apparatus and Materials

  • Coarse diffraction grating (about 100lines/mm)
  • Retort stand, boss, and clamp
  • Light source, compact
  • Power supply, low voltage, variable, to supply 8A at 12V

Health & Safety and Technical Notes

Read our standard health & safety guidance


The lamp should have a good straight filament.

Each student pair will need a coarse diffraction grating.

The diffraction grating should not be blazed. The laboratory should be darkened. If you don’t have a compact light source (quartz iodine lamp) use a 48W 12V lamp.

Procedure

  1. Mount the compact light source at the end of the laboratory and connect it to the power supply, set at 12 V.
  2. Ask students to hold the grating near to the eye and to look through it at the distant light source.

Teaching Notes

  • Talk students through the observation. They look at the white-hot filament of a lamp with a grating held close to the eye. They should see a central white line where waves of all colours go straight through the grating. Out to each side, they should see a bright band. This is where light arrives from adjacent slits with one wavelength path difference. Since the light is white, each bright fringe is spread into a wide spectrum.
  • Looking further out to each side, students may see a still wider, but fainter spectrum. This corresponds to the next bright fringe out from the centre (two wavelengths’ path difference).
  • For a diffraction grating d sin A = n (wavelength), where A = angle at which the light appears, n is the diffraction order (1,2, ...), d = spacing between slits.
  • Many teachers progress from single slit diffraction to double slit diffraction (Young’s fringes), treating the double slit case as two overlapping single slit diffraction patterns.
  • You can progress through three, four, five, six, etc. slits to the diffraction grating. The use of two distinct equations can mislead students, so they think of double slit interference and gratings patterns as quite different phenomena. Show them the similarities.
  • Young’s fringes:
    • n (wavelength) = distance between slits x fringe separation / distance from slits to screen
    • n is the number of the fringe
  • Diffraction grating:
    • n (wavelength) = distance between slits x sin (angle at which the light appears)
    • n is the diffraction order (1, 2,3,..)

This experiment was safety-tested in February 2006

  • This video shows how to produce a diffraction pattern using a laser source and a thin, straight wire: 

  • And this video can be used with your students in the classroom:

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Projection of spectrum with diffraction gratings

Diffraction Grating
Light, Sound and Waves

Projection of spectrum with diffraction gratings

Practical Activity for 14-16

Demonstration

Exploring the diffraction pattern produced from white light.

Apparatus and Materials

  • Coarse diffraction grating (about 100 lines/mm)
  • Fine diffraction grating (about 300 lines/mm)
  • Cylindrical convex lens
  • Green Filter
  • Red Filter
  • Screen, white
  • Lens holder, 2
  • Light source, compact
  • Power supply, low voltage, variable, to supply 8A at 12V
  • Retort stands, bosses and clamps, 3

Health & Safety and Technical Notes

Read our standard health & safety guidance


The diffraction gratings should not be blazed.

If you don’t have a compact light source (quartz iodine lamp), a 48W 12V lamp will probably be bright enough to project the spectrum across the laboratory. The lens will produce plane waves.

The screen should be a long one, perhaps a 3 to 5 metre roll of white kitchen paper. A white wall is good.

Alternative arrangement

A slide projector can conveniently be used, instead of the compact light source, as illustrated opposite. In this arrangement a single slit must be inserted in the projector as shown.

Procedure

  1. Set up the lamp and lens at one end of the darkened laboratory. Obtain a sharp image of the filament on a screen at the other end.
  2. Place the coarse grating in the beam just beyond the lens.
  3. Place red and then green filters in the beam and show the effect of colour absorption by the filters.
  4. Finally, replace the coarse grating by a finer one and the spectrum will spread out more.

Teaching Notes

  • You many want to let students use a coarse grating for themselves, before they are shown this demonstration.
  • Talk students through the observation. Students look at the white-hot filament of a lamp with a grating held close to the eye. They should see a central white line where waves of all colours go straight through the grating. Out to each side, they should see a bright band, where light arrives from adjacent slits with one wavelength path difference. Since the light is white, each bright fringe is spread into a wide spectrum.
  • Looking further out to each side, they may see a still wider, but fainter spectrum which corresponds to the next bright fringe out from the centre (two wavelengths’ path difference).
  • Help students to make the link between colour and wavelength. Finer gratings will spread the light more.
  • For a diffraction grating d sin A = n (wavelength), where A = angle at which the light appears, n is the diffraction order (1,2, ...), = wavelength, d = spacing between slits.
  • For an image of the fringes produced, see the web link below. Thanks to Jay Jamaican for suggesting that we add this link.
  • ALTERNATIVE: This video, from the National STEM Centre eLibrary, shows how to produce a diffraction pattern using a laser source and a thin, straight wire.

This experiment was safety-checked in February 2006

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Fine cloth as a grating

Diffraction Grating
Light, Sound and Waves

Fine cloth as a grating

Practical Activity for 14-16

Class Practical

Observing a two-dimensional diffraction pattern.

Apparatus and Materials

  • Light source, compact
  • Retort stand, boss, and clamp
  • Power supply, low voltage, variable, to supply 8A at 12V
  • Woven cloth, many pieces

Health & Safety and Technical Notes

Read our standard health & safety guidance


Use a compact light source, rather than a line filament lamp, since this is a two-dimensional grating.

The cloth should be of a simple square weave with holes between the threads to let the light through. The samples should be large enough for students to hold with two hands and try stretching and shearing. Old umbrella cloth works well.

Procedure

Each student looks towards the lamp and holds a piece of cloth in front of one eye. Ask them to observe the pattern as they try stretching and shearing the fabric.

Teaching Notes

  • The cloth acts like crossed gratings. Indeed, as preparation, it might help to show two gratings, initially in line, and then rotate one.
  • Encourage students to be observant outside the lab. Looking at sodium street lamps through domestic net curtains is very effective, especially if there are lamps at various distances.

This experiment was safety-tested in February 2006

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Spectra

The Electromagnetic Spectrum
Light, Sound and Waves

Spectra

Practical Activity for 14-16

Class practical

Students use a diffraction grating as a tool for observing the spectra from a variety of light sources.

Apparatus and Materials

  • Fine diffraction grating (about 300 lines/mm)
  • Green filter
  • Red filter
  • L.T. variable voltage supply (capable of 8 A at 12 V)
  • Lamp, 12 V 24 W
  • Lamp holder on base
  • Spectrum tube, hydrogen
  • Spectrum tube, neon
  • Spectrum tube holder, with integral power supply or 5 kV EHT power supply
  • Sodium flame

Health & Safety and Technical Notes

Where the EHT supply is used, all connections between the tube holder and the supply must be made before the supply is switched on. The tube holder should not have any exposed metal which could become live.

Read our standard health & safety guidance


Each student pair will need a fine diffraction grating, red filter and green filter.

The light sources should be mounted as high up in the laboratory as possible. The spectrum tubes will require an appropriate holder and voltage supply.

Procedure

  1. Set up the bright, white-hot filament high at the end of the laboratory.
  2. Ask students to observe the light source with the fine grating held close to the eye.
  3. Students should then look at the neon spectrum tube; at the hydrogen tube; at a slit held in front of a sodium flame; at the bright, white-hot filament, through red and green filters.

Teaching Notes

  • The capillary-tube gas-filled lamps which operate on (3-5) kV are in fact a fine-line source of light; neon tubes produce spectra consisting of many bright lines of different colours, whereas hydrogen is much fainter and produces fewer spectral lines.
  • Sodium lamps normally need to have a slit placed in front of them in order to produce a line source. Sodium light can also be produced by holding a sodium chloride stick in a Bunsen burner flame, or even by sprinkling sodium chloride into the flame, though that can be quite messy.
  • Alternatively, use the technique in this experiment:

    Interference with air wedge


  • When a tube containing neon gas, for example, 'is connected to a high voltage supply, it produces a 'spectrum because electrons bombard atoms and excite them.' ''The tube gives out red light 'that comes from neon atoms as they recover from that excitation.'' Because many electron transitions are possible, neon produces many other colours too.'' The hydrogen atom is much simpler than neon so there are fewer spectral lines.

This experiment was safety-tested in February 2006

Up next

A CD or gramophone record as a grating

Diffraction Grating
Light, Sound and Waves

A CD or gramophone record as a grating

Practical Activity for 14-16

Demonstration

Students observe the spectrum produced when light falls obliquely on a CD.

Apparatus and Materials

CD or gramophone record

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Students might like to look at the grating spectra formed by reflection when light falls obliquely on a CD.
  2. The rulings are too coarse to be useful at direct incidence. The observer must take an oblique view.
  3. If it is a gramophone record, you can calculate the grating spacing. Use a ruler to measure the part of the radius of the grating used. Play it and count the corresponding number of turns.
  4. Measuring the grating spacing of a CD will be more challenging.
  5. Alternatively, use a known wavelength of light to estimate the grating spacing of a CD.

Teaching Notes

This experiment was safety-tested in February 2006

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Viewing sharp shadows

Diffraction Grating
Light, Sound and Waves

Viewing sharp shadows

Practical Activity for 14-16

Demonstration

Interesting diffraction effects seen in shadows.

Apparatus and Materials

  • Greaseproof paper to make a translucent screen
  • Dressmaking pins (with ball heads), one or more
  • Wax, hard, to attach objects to plate
  • Wax
  • Metal plate with 1.5 mm hole
  • Light source, compact
  • Power supply, low voltage, variable, to supply 8A at 12V
  • Retort stands, bosses and clamps, 3
  • Plate glass, one piece to hold objects for casting shadows

Health & Safety and Technical Notes

Take care when using a razor blade to rule a narrow slit. Single-edged blades are safer. If double-edged ones are used, cover one edge with several layers of PVC tape.

Read our standard health & safety guidance


Avoid inserting lenses, or the magic will be lost.

Place the compact light source at one end of the lab, with the 1.5-mm pinhole just in front. Place the translucent screen near the other end of the room. In the middle, between them, place the objects to cast shadows. The objects should be far from the lamp, at least 3 metres, preferably 5 metres. The screen should be at least 1.5 metres beyond the objects. It is essential to avoid glare from the bench tops (cover with black cloth if necessary}.

A sewing needle, a pin, a sheet of metal or card with small holes drilled in it, and a small disk or steel ball (maximum diameter 5 mm) all make good shadow-casting objects. Stick them with wax on a piece of plate glass. Perhaps because it is natural, a human hair seems to do best of all.

A dressmaking pin, with spherical glass head (diameter about 4 mm) is best of all for showing the white spot. It can be held separately in mid-air. Or several can be used with one source, to cast shadows on several screens.

The room should be fairly dark. However, if you try to show diffraction in a completely blacked-out room then students cannot see what is happening, and discipline problems could ensue. Students will need time to let their eyes become dark-accommodated.

Procedure

  1. Let students stand near the collection of objects and look at them. If they hold a piece of paper just beyond the objects, they may catch sharp shadows.
  2. The diffraction effects...
  3. Ask students to move to the translucent screen, going round behind it to look at the shadows there. Remind students to hold their heads 1/4 metre or more behind the screen (as in reading a book.)
  4. The shadow of a disk...
  5. To a physicist, the strangest shadow of all is that of a small ball or disk: there is a white spot in the centre of the shadow. One can just see this in a long, very dark room, if one expects it. Our source is too large, and students will probably miss it, unless the source is made smaller. Place a metal plate with a hole 1 mm diameter just in front of the lamp. Then ask students what they see.
  6. Diffraction by a slit...
  7. If time permits, change to a set of three prepared slits: wide, medium, and very narrow. Suitable slit widths are 1 or 2 mm, 1/4 to 1/2 mm, 1 to 3 hundredths of a mm. The narrowest slit needs no microscope to check its width; judge it by its diffraction spread, which should be 2 or 3 cm on a screen at 1 metre. These could be ruled on a coated glass slide, the narrowest with a razor blade.

Teaching Notes

  • As part of a higher level course, you may want to show what happens with a V-shaped slit or a variable slit. If students are familiar with pinhole cameras, you might ask able students what this suggests about the diameter of a suitable pinhole.
  • Whether or not you have done this earlier, you may want to bring out a ripple tank and show diffraction through single and multiple slits, and round obstacles.

This experiment was safety-tested in February 2006

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Measuring the wavelength of light

Diffraction Grating
Light, Sound and Waves

Measuring the wavelength of light

Practical Activity for 14-16

Class Practical

A simple and elegant experiment to measure the wavelength of light using a fine diffraction grating.

Apparatus and Materials

  • Fine diffraction grating (about 300 lines mm-1)
  • Metre rules, 2
  • Lamp in holder, 12 V 36 W
  • Green filter
  • Power supply, low voltage, variable, able to supply 6 A

Health & Safety and Technical Notes

Read our standard health & safety guidance


Each student pair will need fine diffraction grating, and 2 metre rules.

Procedure

  1. Set up the 12 V 36 W line-filament lamp high at one end of the laboratory, so that students can see it clearly. Place a green filter in front of the lamp. If necessary, increase the applied voltage to 14–15 V so there is enough light.
  2. Students follow these instructions.
  3. Hold a metre rule straight out in front of you towards the lamp, with the near end of the rule at your face. Hold the diffraction grating against the near end of the metre rule and look at the lamp through it.
  4. Ask your partner to place another metre rule, at 90° to your metre rule at its far end (see the sketch).
  5. Your partner should hold a pencil vertically above their metre rule and move it along until you see it in the green region of your bright spectrum. Note the distance, x, along your partner's ruler from the pencil to the far end of your ruler.
  6. When you have made your observation, record it and change places with your partner so that he or she can take their turn.
  7. Divide your measurement x by the length of your ruler, 100 cm. This gives you tan A, where A is the angle between the line of direct white light and the light to the green in the spectrum marked by the pencil. From tan A, use your calculator to find the angle A, and from this find sin A.
  8. Use the formula d sin A = wavelength to calculate the wavelength of green light. You will need the value of d, spacing, i.e. distance from one ruling to the next. If the grating has 300 lines / mm then the spacing is 1 / 300,000m.

Teaching Notes

  • You must judge whether this experiment is appropriate for your students. If it is likely to represent a burden of strange geometry and unsure measurements, omit it. If they are able to cope, this experiment has the potential to give them a sense of delight and insight. It is a real achievement to make such a small measurement using fairly crude equipment.
  • For a large class, set up a lamp at each end of the room, so that half the class can work facing one way, with half facing the other way, each pair as far as possible from their lamp.
  • Students may need help with the trigonometry and/or with the calculation of wavelength. To avoid the use of tan A , students could do a scale drawing and find sin A from xt , as in this diagram.
  • Then wavelength = sin A × d , where d is the grating spacing.
  • If you supply the value of d , explain where it came from, and make it clear that a mechanical counting during manufacture can supply it.
  • If suitable microscopes are available, students could use them to look at their piece of grating and at the graduations on a finely divided ruler. Although students may not be able to measure the grating space, they will certainly see that a direct measurement could be made.

This experiment was safety-tested in February 2006

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Diffraction patterns using sieves

Diffraction Grating
Light, Sound and Waves

Diffraction patterns using sieves

Practical Activity for 14-16

Demonstration

A laser is used to illustrate how crystal geometry can be inferred from X-ray diffraction patterns.

Apparatus and Materials

  • Laser, preferably mounted and mains powered
  • Sieve or fine mesh (nylon stocking)
  • Screen

Health & Safety and Technical Notes

Check that the laser is labelled 'class 2'.

Take care with reflections from the laser beam so that there is no danger of them striking a student's eye.

Read our standard health & safety guidance


Use sieves with the collar facing the laser.

Sieves are commonly used in geology and biology. You should use the 3 finest or mesh such as from a nylon stocking, stretched over a small embroidery frame.

There should be only a low level of light in the room.

Procedure

  1. Start with laser beam showing a single spot on the screen.
  2. Introduce the coarsest sieve and ask students to observe the pattern.
  3. Remove and introduce finer mesh sieves showing that the pattern will broaden.
  4. Using the finest sieve, move the sieve around but keep the same orientation. There will be no effect on pattern.
  5. Twist the sieve to about 20 degrees from the normal to the beam. The beam expands in one direction.
  6. Return to normal incidence.
  7. Rotate the sieve. The pattern rotates.

Teaching Notes

Hopefully, the students will see that the diffraction pattern is a result of the angle of the mesh to the beam and the geometry of the mesh. If it doesn't work, the meshes available are too coarse. The same principles can be applied to X-ray crystallography or electron beam diffraction in metal films.

This experiment was submitted by David Ferguson, the physics technician at Uppingham School

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Diffraction of light at a narrow opening

Diffraction Grating
Light, Sound and Waves

Diffraction of light at a narrow opening

Practical Activity for 14-16

Demonstration

This is a simple and economic demonstration of the diffraction of light by a narrow opening, from which the wavelength of light can be determined.

Apparatus and Materials

  • Diode laser (designed for educational use NOT a key chain laser) Class 2 either 635 mm or 670 mm
  • Thin aluminium foil (0.1mm thickness available as a sealed wrapper of a milk powder tin)
  • Stand with clamp
  • Meter scale
  • Sharp needle
  • Travelling microscope
  • Screen-white graph sheet pasted on cardboard

Health & Safety and Technical Notes

The power of a class 2 laser is less than 1 mW. This is not harmful even if it is seen directly: the blink response gives adequate protection. Warning: cheap laser pointers have not been tested and cannot be relied upon to be Class 2.

Read our standard health & safety guidance


Keeping the foil over a smooth plane surface, make a small circular hole in it by pressing the needle tip gently into the foil. The rectangular slit can be prepared by using two straight aluminium strips over transparent sticky tape.

Procedure

  1. Arrange the source (laser), aluminium foil with hole, and graph sheet pasted on cardboard as a screen, so that they lie in a straight line. Support the stands with the clamp as shown in the below diagram.
  2. Switch on the source and make fine adjustments so that the aperture in the foil is illuminated evenly.
  3. Move the screen about 1 m from the aluminium foil, so that the diffraction pattern is within the graph sheet pasted on the screen.
  4. The diffraction fringe pattern will be as shown above.
  5. For the calculation of the wavelength of the source used:
    • Note down the diameter of the first minimum, Y , with the help of graph sheet markings.
    • Note the distance between the foil and the screen, D.
    • Using a travelling microscope, measure the diameter of the hole or slit width used, X.
    • Calculate the wavelength of the light source, λ, using the formula λ = XY/ 2D.

Teaching Notes

  • Derivation
  • Let S1 S 2 be the diameter of the hole, acting like secondary coherent sources.
  • Secondary wave fronts coming from these undergo superposition (interference), leading to bright and dark fringes on the screen.
  • The ray diagram is shown above.
  • MN is the path difference between S1 P’ & MP, where P is the centre of the first minimum and O is the centre of central maximum.
  • Then MN = λ /2 ,…………………….(1)
  • From triangle S1 NM, sinӨ = MN/S 1 M = MN / (X/2), or MN = (X/2) sinӨ……………………(2)
  • From triangle OMP, sinӨ = PO /MP ≈Y1 / OM =Y1 / D………………………..(3),
  • From (1),(2) &(3) Y 1 / D = λ / X, or λ = X (2Y 1 ) / 2D =XY/ 2D where the diameter of the first minimum, Y = 2Y 1 Hence λ =XY/ 2D
  • Experimental results conducted at physics lab CMRIT BANGALORE:
  • For circular hole, X = 0\. 018 cm, For rectangular slit, X = 0\. 021 cm
  • D = 177 cm, D = 215 cm
  • Measured Y =1\. 3 cm, Y =1\. 35 cm will give 659 nm< λ <661 nm
  • This is similar to the λ calculated by a diffraction grating experiment, at 653 nm.
  • Actual diffraction pattern recorded by 3.1 Mega pixel digital camera, in the dark room is shown above.

This experiment was submitted by Tukaram Shet, Senior Lecturer in Physics at CMRIT Bangalore

Up next

Diffraction of laser light

Diffraction Grating
Light, Sound and Waves

Diffraction of laser light

Practical Activity for 14-16

Class demonstration

This demonstration shows that a beam of light is diffracted as it passes around a wire, highlighting the wave nature of light.

Apparatus and Materials

  • Laser source
  • Thin, straight wire, approx 25 cm
  • Stand with 2 clamps
  • Screen

Health & Safety and Technical Notes

Read our standard health & safety guidance


You will probably need to work in a darkened room.

Care should be taken to ensure that the laser beam does not shine directly into students’ eyes. This can be avoided by fixing it firmly in a clamp directed away from the students and towards the screen. Ensure that there are no shiny, reflective objects close to the path of the beam.

Procedure

  1. Mount the laser pointer horizontally in a clamp
  2. Mount the wire vertically between two clamps.
  3. Direct the laser light onto the screen. You will see a bright dot.
  4. As suggested in the film, ask your students to predict what they will se when the wire partially blocks the laser beam.
  5. Move the wire into the beam. You should see a diffraction pattern of light and dark ‘fringes’ on the screen.

Teaching Notes

  • We may talk casually about ‘light waves’, but students need to be convinced that light travels as a wave. This demonstration shows it.
  • Students will need to be familiar with two ideas: that waves diffract as they pass around an obstacle, and that waves interfere constructively and destructively when they overlap. These ideas can be shown using a ripple tank.
  • You can show diffraction and interference of light using single, double or multiple slits. However, students may find these difficult to appreciate. Diffraction by a simple wire is a more straightforward situation to explain. Students can also be asked to predict what will be seen on the screen when the wire is placed in the path of the light beam. They will probably expect to see a vertical shadow. The appearance of a diffraction pattern spread across the screen is a surprise worth exploring.
  • A laser is used because it is a convenient source of a narrow beam of light. It has the added advantage that it produces light of a single wavelength; white light would produce a similar effect but the diffraction pattern would not be as wide as different wavelengths (colours) would interfere at different points.
  • It is worth emphasising the extent to which light is diffracted as it passes around the wire. The diffraction pattern may be 50 cm wide when the diffracting wire is one metre from the screen. So light is being diffracted (bent) through an appreciable angle – perhaps 20 degrees.
  • You could investigate the effect of rotating the wire; can students predict what will happen? (A vertical wire produces a horizontal diffraction pattern; a horizontal wire will produce a vertical pattern.)
  • A sequence of experiments to show the diffraction of light and how this can be used to determine the wavelength of light:

    diffraction of light collection


  • The video shows how to demonstrate the diffraction of light using a laser pointer and a wire:

  • This video can be used with your students in the classroom in place of the actual demonstration:

Up next

Classroom management in semi-darkness

Interference
Light Sound and Waves

Classroom management in semi-darkness

Teaching Guidance for 14-16

There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.

Shadows on the ceiling will reveal movements that are not in your direct line of sight.

Up next

Using wave simulations

Interference
Light Sound and Waves

Using wave simulations

Teaching Guidance for 14-16

There are many excellent applets available online that show wave behaviour as if observing a ripple tank or oscilloscope screen.

These cannot substitute for experience of the phenomena themselves but provide a powerful way of helping students to visualize. They provide a valuable complement to experiments by removing extraneous effects.

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