Laser
Light, Sound and Waves

Using lasers in the classroom (16-19 physics)

for 16-19

A collection of practicals and classroom activities for the use of a lasers in the teaching of 16-19 physics. 

When using lasers it is important to check up-to-date health and safety guidance. Please refer to guidance from CLEAPSS or SSERC when attempting demonstrations or practical activities in the classroom and incorporate this into your risk assessments. 

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. Additionally, ensure that there are no shiny, reflective objects close to the path of the beam.

Laser lines and laser pointers can be used in some experiments, but check that they are Class 2 permitted for use in schools and colleges.

Interference
Light, Sound and Waves

Episode 319: Preparation for superposition of waves topic

Teaching Guidance for 16-19

Many demos are suggested using many different types of wave motion. The first time you set these up keep diagrams or exact notes of what works best with the actual equipment you have to hand. This will save you time on the next occasion you teach this topic!

A stick wave machine is useful. It consists of two parallel strings with many sticks threaded on in the plane defined by the two strings. The strings are held at each end to provide sufficient tension. Giving the stick at one end a twist produces a slow ripple as the torsional wave travels down the strings.

Lasers

You will find it useful to work with lasers in some of the activities described here, so it is a good idea to familiarize yourself with any available lasers, and to ensure that you know how to use them safely.

Laser light is particularly suitable for optical work. He-Ne gas lasers used in schools and colleges have a wavelength of 633 nm. Laser lines and laser pointers can also be used, but check that they are Class 2 permitted for use in schools and colleges. Laser pointers tend to need new batteries quite often when used for experiments rather than pointing.

Laser diodes can now be bought from various suppliers. For example, a Class 2 laser diode from a reputable supplier can be run off a 3 V battery. It can be mounted it in a bracket 19 mm diameter. With a set of taps and dies put three bolt holes in it at 120 ° intervals and then put in three bolts. This gives an excellent mounting (and adjustment) system for the laser diode and also acts as a good heat sink, which is needed for fairly continuous operation. Make sure that a laser safety label is prominent alongside the device.

Safety with lasers

Never shine laser light directly into the eye, or allow it to reflect from a shiny surface. Direct the light onto the ceiling: all students in the room can see a spot. The beam has been diffusely reflected into all angles, so the intensity in any one direction is low and safe to view.

Ripple tanks

It’s worth practicing with a ripple tank before going public in front of the class. Here are some tips for setting up a ripple tank with an OHP.

Add a single small drop of detergent to the water – this helps to wet the wave bar.

Adjust the position of the wave bar and/or its frequency so you get standing waves between it and the adjacent edge – the resulting increased amplitude of the wave bar helps to get better amplitude waves where you want them.

Use a variable power supply to control the illumination of the OHP for optimum contrast.

It’s worth taking some care to level the tank – varying water depth leads to refraction if the wave bar is not perpendicular to the long axis of the tank. Even with good alignment, if the depth changes this leads to a change of wave speed and hence wavelength. (You will notice optimum standing waves when the wave bar is parallel to the short side.)

Main aims of this topic

Superposition

Students will:

  • use the principle of superposition of waves to determine the resultant displacement of two or more waves
  • understand how diffraction and interference effects result from superposition
  • use the equations for single, double and multiple slit diffraction
  • use double and multiple slit diffraction to determine wavelength
  • explain how standing waves arise from travelling waves
  • use standing waves to determine wavelength and wave speed

Prior knowledge

Students should have studied the basic representation of sinusoidal waves. They should be familiar with the wave speed equation c = f ×  λ .

It will be helpful if they are familiar with some aspects of trigonometry (small angle approximations sin( θ ) ~ θ ~ tan θ , maximum value of sin( θ ) = 1 etc; simple triangle trigonometry).

Where this leads

The ideas of diffraction and interference can lead to an understanding of many different scientific instruments and measurement techniques – interferometers, spectrometers etc – used in many different branches of science.

Ideas of waves, including standing waves, proved vital in the development of quantum mechanics.

Interference
Light, Sound and Waves

Episode 321: Interference patterns

Lesson for 16-19

When two or more waves meet, we may observe interference effects. It is likely that your students will have already met the basic ideas of constructive and destructive interference.

Lesson Summary

  • Demonstrations: Simple interference phenomena (20 minutes)
  • Demonstration: Two sound sources (15 minutes)
  • Demonstration: Young’s two-slit experiment (15 minutes)
  • Discussion: Deriving and using the formula (20 minutes)
  • Student experiments: Double slit analogues (30 minutes)
  • Student questions: Using the Young’s slits formula (40 minutes)
  • Demonstrations: For students to explain (20 minutes)

The following simple demonstrations could be used to introduce this section; don’t feel that you have to give detailed explanations at this stage.

  • Laser speckle pattern. Shine a laser onto a screen. Move your head side to side and observe the dark and light speckles, due to the different path lengths to the eye from different positions on the spot of laser light (If the beam is too small to show the speckles, try expanding it by passing it through a low-power lens, either converging or diverging.)
  • Observe the colours in soap bubbles or oil films. Light is partly reflected by the upper surface of the film, partly by the lower surface. Depending on the thickness of the film, these two light rays will superpose constructively or destructively, depending on the wavelength. Thus two paths giving constructive superposition at one wavelength will not give constructive superposition for other wavelengths – hence only the colour with the correct wavelength is seen.
  • Tuning forks. Hold the vibrating fork with its prongs vertical and close to the ear. Twist the fingers so the fork slowly rotates about a vertical axis. The loudness of the sound will rise and fall, four times per complete rotation. (Each prong acts as a source of sound waves; twisting the fork alters the distance between each prong and the eardrum.)

Emphasise that, in each case, there are two or more sources of light or sound reaching the eye or ear. You are going to look at an experiment designed to have two sets of light waves meeting in a very controlled way, i.e. Young’s two-slit experiment.

Safety

In any work with lasers, it is worth pointing out to the class the label in the laser. It should say Class 2: do not stare down the beam. With such a laser, a momentary reflection of the beam into someone’s eye will not cause an injury.

Demonstration: Two sound sources

Because the wavelength of light is very small, it is worth setting up an equivalent experiment with sound waves. Use two loudspeakers connected to a single signal generator. At this stage, it is not necessary to make detailed measurements.

Episode 321-1: Hearing superposition (Word, 84 KB)

Demonstration: Young’s two-slit experiment

Young’s two-slit experiment is perhaps one of the most famous experimental arrangements in physics. It was inspired by Young’s discovery of interference that he related in May 1801: Given a pond with a canal connected to it. At two places in the pond waves are excited. In the canal two waves superpose forming a resultant wave. The amplitude of the resultant wave is determined by the phase difference with which the two waves arrive at the canal.

Shine laser light through a double slit on to a screen. You should see a series of evenly-spaced bright spots (fringes or maxima). Ask students to relate this to the sound experiment. (The bright fringes are the equivalent of the loud points in the sound field.)

Point to the central bright fringe. Emphasise that two light rays reach this point, one from each slit. They have travelled the same distance, so there is no path difference between them. They started off in step (in phase) with each other, and now they arrive at the screen in phase with each other. Hence their displacements add up to give a brighter ray.

The next bright fringes (on either side of the central one) represent points where one ray has travelled λ further than the other, so they are back in phase. Why is there a dark fringe in between? (One ray has travelled λ/2 further than the other, so they are out of phase and interfere destructively.)

If we could measure these distances approximately, we could determine λ .

Show the effects of:

  • Using two slits with a smaller separation (the fringes are further apart).
  • Moving the screen closer to the slits (the fringes are closer together).

It is clear that we might use this experiment to determine the wavelength of light, but how?

A modern version of the Young’s Two Slit experiment was voted the most beautiful experiment in physics in a Physics World readers’ poll in 2002. It still forms the basis of ongoing research into the fundamental quantum nature of matter.

If you have already covered the photon model for light, you may want to refer back to this. As early as 1909, it was established that fringes were found even if the source was so faint that only one photon at a time was in the apparatus. Fringes can also be seen using de Broglie (or matter) waves. The most massive particles used to generate fringes to date (March 2005) are fluorinated buckyballs C60F48 (i.e. 1632 mass units).

Discussion: Deriving and using the formula

Now derive or quote the formula (depending upon your specification). A good way to start is to ask your students to identify the important variables (they are all lengths), and to give their approximate sizes:

λ , wavelength of the light (~500 nm)

d, separation of the two slits (~1 mm)

s, separation of the fringes (bright to bright or dark to dark) (~1 mm)

L, distance between slits and screen (~1 m)

How can we make a balanced equation from four quantities that are so different in magnitude? The simplest solution is that the product of the biggest and smallest is equal to the product of the two in-between quantities. Hence:

λ L = sd

or

λ d = sL

Episode 321-2: Calculating wavelength in two-slit interference (Word, 79 KB)

Episode 321-3: Two-slit interference (Word, 51 KB)

Student experiments: Double slit analogues

Set up a circus of Young’s two-slit arrangements (depending upon the available equipment to hand) using light, microwaves, 3GHz radio waves, ultra-sound (less noisy than audible sound!) and a ripple tank, and get students to determine the wavelength of the waves being used in each case.

You may prefer to set some up as demonstrations.

Episode 321-4: Interference patterns in a ripple tank (Word, 48 KB)

Episode 321-5: Measuring the wavelength of laser light (Word, 42 KB)

Student questions: sing the Young’s slits formula

The first set of questions covers the principles of Young’s experiment.

The second set is questions for practice in using the equation.

Episode 321-6: Questions on the two-slit experiment (Word, 28 KB)

Episode 321-7: Two-source interference: some calculations (Word, 23 KB)

Demonstration: For students to explain

Here are two fun demonstrations to round off this episode. Demonstrate them, and ask your students to provide explanations.

  1. A nice demo using audible sound is to fix two loudspeakers at each end of a longish piece of wood. Mount the wood on a suitable pivot (large nail) mid-way between the two speakers. Drive both in parallel from signal generator. Slowly scan the class. As the interference fringes sweep across the audience, they hear the regular change in volume.
  2. Make a sound trombone. Mount a small loudspeaker in the wide end of a small plastic funnel. Tubing from the other end divides into two tubes: one takes a direct route, the other a route whose length can be varied by a U-shaped glass tube sliding trombone section. The two routes combine and are fed into another funnel that acts as an earpiece.
  3. The loudness of the sound depends upon the position of the trombone slider. It is obvious that there are two paths by which the sounds reach the ear. There may be a path difference between them. If the path lengths differ by an exact number of wavelengths, constructive interference increases the volume; integral half wavelength path differences mute the sound due to destructive interference. Do not confuse this with beats; here, only one frequency is involved, whereas beating is an effect due to two close frequencies (see below).
Multi-Slit Interference
Light, Sound and Waves

Episode 322: Diffraction gratings

Lesson for 16-19

The diffraction grating was named by Fraunhofer in 1821, but was in use before 1800. There is a good case for describing it is the most important invention in the sciences.

Lesson Summary

  • Demonstration: Looking through gratings (10 minutes)
  • Discussion: Deriving the grating formula (20 minutes)
  • Student experiment: Measuring wavelength (30 minutes)
  • Student questions: Using the grating formula (30 minutes)
  • Student experiment: A CD as a grating (home experiment) (30 minutes)
  • Discussion: The meaning of coherence (10 minutes)

Demonstration: Looking through gratings

Pass around some diffraction gratings or hand-held spectroscopes. Invite students to look through them at various light sources: a torch bulb, LEDs of different colours, a sodium or mercury lamp. They should see a continuous spectrum (red to violet) for a white light source, but multiple images of different colours for coloured sources.

Shine a laser through a grating (as in the Young’s slits experiment above). Explain that light rays are emerging from each slit. What condition must be met for a maximum on the screen? (All rays must be in phase with each other.)

Observe the effect of moving the screen further away (the maxima are further apart). Try a grating with a different slit spacing. (Closer slits give maxima that are further apart.)

Shine a bright white light through the grating and observe the resulting spectra.

Describe the central maximum as zero order. The next maximum on either side is then first order ( n = 1), and so on. You will probably observe two or three on each side, far fewer than with a double slit. (It’s a much more demanding condition to have many rays in phase than just two.)

It is hard to overstate the importance of the diffraction grating to the progress of the sciences. Some would claim it is the most important invention, used in investigations from the structure of matter including DNA (being used as its own grating) to the structure of the Universe (from Doppler shifted spectral lines), and the determination of the composition of everything from stars to chemical compounds, by analysis of their spectra.

Discussion: Deriving the formula

You may need to derive (or simply present) the grating formulafirst derived by Young 1801:

d sin( θ ) = n λ

where d is the grating separation (the distance between slits). NB gratings are often specified as so many lines per metre (often on old gratings as per inch, so beware if doing quantitative work!). Separations as small as 100 nm possible, so 10 million lines per metre (equivalent to about 40 000 lines per inch).

Note that this formula gives the directions for the maximum intensities ( cf the single slit formula which gives directions for the minimum intensity).

Point out that smaller wavelengths give smaller angular separations. (You should have observed that violet light is diffracted less than red light.)

Episode 322-1: A transmission grating (Word, 40 KB)

Student experiment: Measuring wavelength

Students can determine the wavelength of laser light using a diffraction grating. They could discuss the relative precision of this method, compared with the two-slit arrangement.

Episode 322-2: Measuring the wavelength of laser light (Word, 44 KB)

Student questions: Using the formula

Students can perform calculations relating to diffraction gratings.

Episode 322-3: Grating calculations (Word, 37 KB)

They can interpret some images made using gratings.

Episode 322-4: Using diffraction gratings (Word, 157 KB)

Student experiment: A CD as a grating (home experiment)

Use a CD as a reflection diffraction grating. This experiment can be done at home.

Episode 322-5: Using a CD as a reflection grating (Word, 27 KB)

Discussion: The meaning of coherence

Now that your students have had practical experience of observing maxima and minima, further discussion of the necessary conditions should make sense. For superposition effects to be observable, the conditions must persist for a time long enough for them to be observed and extend far enough in space for a reasonably-sized pattern. It is easier for this to be the case if the waves have a well-defined frequency or wavelength (they must be monochromatic).

Ordinary light sources emit light in overlapping bursts, are not usually monochromatic (so the number of wavelengths in a given path can vary) and no source is a true point (so a range of path lengths is inevitably involved).

Laser light is particularly useful for showing interference effects because it is intense, highly monochromatic, and emitted in long wave trains and so maintains a constant phase relationship over large distances. We say that the waves arriving along two or more paths are said to be coherent .

Episode 322-6: Coherence (Word, 30 KB)

Laser
Light, Sound and Waves

Diffraction of laser light

Practical Activity for 16-19

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

Notes

You will probably need to work in a darkened room

Health & Safety and Technical Notes

Read our standard health & safety guidance

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

This film shows how to demonstrate the diffraction of light using a laser source and a wire

  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 see 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

  1. We may talk casually about ‘light waves’, but students need to be convinced that light travels as a wave. This demonstration shows it.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.)

Related Guidance

Classroom management in semi-darkness
 

 

Related Experiments

A sequence of experiments to show the diffraction of light and how this can be used to determine the wavelength of light

Coherence
Quantum and Nuclear | Light, Sound and Waves

Episode 503: Preparation for lasers topic

Teaching Guidance for 16-19

As regards post-16 examinations, the formal requirements laid down are modest, or non-existent, so that the level of treatment here is not detailed.

Main aims of this topic

Lasers

Students will:

  • outline the principles of operation of a laser
  • state some uses of high and low energy lasers

Prior knowledge

Students should know about the wave nature of light, including interference. They should also know how light is emitted by electron transitions within atoms.

Coherence
Quantum and Nuclear | Light, Sound and Waves

Episode 504: How lasers work

Lesson for 16-19

This episode considers uses of lasers, and the underlying theory of how they work.

Lesson Summary

  • Demonstration: Seeing a laser beam (10 minutes)
  • Discussion: Uses of lasers (15 minutes)
  • Discussion: Safety with lasers (10 minutes)
  • Discussion: How lasers work (20 minutes)
  • Worked examples: Power density (10 minutes)
  • Student calculations (10 minutes)

Safety

Ensure that you are familiar with safety regulations and advice before embarking on any demonstrations (see

episode 504-2 (Word, 26 KB)

Demonstration: Seeing a laser beam

A laser beam can be made visible by blowing smoke or making dust in its path. Its path through a tank of water can be shown by adding a little milk.

Show laser light passing through a smoke filled box or across the lab and compare this with a projector beam or a focussed beam of light from a tungsten filament light bulb.

Show the principle of optical fibre communication by directing a laser beam down a flexible plastic tube containing water to which a little milk has been added.

Show a comparison between the interference pattern produced by a tungsten filament lamp (with a monochromatic filter) and that produced by a laser.

Discussion: Uses of lasers

Talk about where lasers are used – ask for suggestions from the class. As far as possible this should be an illustrated discussion with a CD player, a laser pointer, a set of bar codes, a bar code reader and the school’s laser with a hologram available for demonstration.

Episode 504-1: Uses of lasers (Word, 25 KB)

Show the list of uses. Invite students to consider the uses shown in the list. Can they say why lasers are good for these? The reasons might be:

  • a laser beam can be intense
  • a laser beam is almost monochromatic
  • a laser beam diverges very little
  • laser light is coherent

Discussion: Safety with lasers

Lasers must be used with care. Use the text as the basis of a discussion of the precautions which must be taken.

Episode 504-2: Lasers and safety (Word, 26 KB)

Discussion: How lasers work

If students are familiar with energy level diagrams for atoms, and of the mechanisms of absorption and emission of photons, you can present the science behind laser action. Point out the difference between:

(a) excitation – an input of energy raises an electron to a higher energy level

(b) emission – the electron falls back to a lower energy level emitting radiation and

(c) stimulated emission – the electron is stimulated to fall back to a lower energy level by the interaction of a photon of the same energy.

Define population inversion: Usually the lower energy levels contain more electrons than the higher ones (a).

In order for lasing action to take place there must be a population inversion. This means that more electrons exist in higher energy levels than is normal (b).

For the lasing action to work the electrons must stay in the excited (metastable) state for a reasonable length of time. If they fell to lower levels too soon there would not be time for the stimulating photon to cause stimulated emission to take place.

Laser stands for Light Amplification by Stimulated Emission of Radiation. The diagrams in

episode 503

up to the higher energy level using photons. hey then drop down and accumulate in a relatively stable energy level, where they are stimulated to all drop back together to the ground state by a photon whose energy is exactly the energy difference to the ground state.

Discuss coherent and non-coherent light. Coherent light is light in which the photons are all in step – in other words the change of phase within the beam occurs for all the photons at the same time. There are no abrupt phase changes within the beam. Light produced by lasers is both coherent and monochromatic (of one colour).

Incoherent sources emit light with frequent and random changes of phase between the photons. (Tungsten filament lamps and ordinary fluorescent tubes emit incoherent light).

Worked examples: Power density

The laser beam also shows very little divergence and so the power density (power per unit area) diminishes only slowly with distance. It can be very high.

For example consider a light bulb capable of emitting a 100 W of actual luminous radiation.

At a distance of 2 m the power density is

100 W4 π 22 = 2 W m-2.

The beam from a helium-neon gas laser diverges very little. The beam is about 2 mm in diameter close to the laser spreading out to a diameter of about 1.6 km when shone from the Earth onto the Moon!

At a distance of 2 m from a 1 mW laser the power density in the beam would be

0.001 W4 π 0.0012 = 320 W m-2.

This is why you must never look directly at a laser beam or its specular reflection.

Student calculations

Ask the class to calculate the power densities for a 100 W lamp and a 1 mW laser at the Moon.

(Distance to Moon is 400 000 km; diameter of laser beam at Moon is 1.6 km)

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