1Ripple Tank β Reflection, Refraction, Diffraction, Interference
Ripple tank with transparent base, water, light source (stroboscope)
Plane wave dipper (bar vibrator) and point source dipper
Metal barriers and small gaps
White paper below tank to observe shadow wave patterns
Power supply
Fill the ripple tank with water to approximately 1 cm depth. Place white paper below.
Attach the bar vibrator (plane waves) and switch on β observe straight parallel waves.
Reflection: place a metal barrier at an angle. Observe waves reflecting off it β angle of incidence equals angle of reflection.
Refraction: place a flat piece of perspex in the tank (raises the floor, creating shallow water). Observe waves slow down and change direction in shallow water.
Diffraction: use barriers with a small gap. Observe circular spreading of waves beyond the gap.
Interference: use the two-point source dipper. Observe alternating regions of constructive and destructive interference.
Reflection: plane waves reflect off a barrier with equal angles.
Refraction: waves change direction and wavelength at the shallow/deep boundary (speed changes, frequency stays constant).
Diffraction: waves spread out in circular arcs after passing through a narrow gap (most pronounced when gap β wavelength).
Interference: stable pattern of alternating loud (constructive) and quiet (destructive) regions.
The ripple tank is like a physics lab in a tray β every major wave property can be seen with your own eyes in the water surface.
For refraction: wavelength decreases in shallow water (speed decreases); frequency stays constant. v = fΞ».
2Demonstration of the Doppler Effect
Electronic buzzer or small loudspeaker emitting a constant frequency tone
Battery
String (~1 m)
Open space or long corridor
Attach the buzzer (connected to battery) to the end of the string.
Swing the buzzer in a circle above your head, or whirl it in a large horizontal circle.
Listen to the pitch as the buzzer moves towards and then away from the ear.
Alternative: record the sound with a smartphone while walking past a stationary buzzer, then play it back.
When the buzzer moves towards the listener, the pitch is higher than the actual frequency (waves compressed β shorter wavelength).
When the buzzer moves away, the pitch is lower (waves stretched β longer wavelength).
The faster the speed of the source, the greater the frequency shift.
Like a police siren β as the car approaches you it sounds higher, and as it drives away it drops to a lower pitch. The sound frequency hasn't changed; it's the compression of waves in front and stretching behind.
State: 'The Doppler effect is the apparent change in frequency due to relative motion between source and observer.' f_observed > f_source when approaching; f_observed < f_source when receding.
3Demonstration of Interference (Sound)
Two loudspeakers connected to the same signal generator (same frequency, same phase)
Signal generator (set to ~1000 Hz)
Long room or open space
Connect both loudspeakers to the same signal generator output so they produce identical sound waves in phase.
Place the speakers about 1β2 metres apart, facing the same direction.
Walk slowly in a line parallel to the speakers, at a distance of about 3β4 metres.
Listen carefully as you walk β note the alternating loud and quiet regions.
Clear alternating regions of loud sound (constructive interference: path difference = nΞ») and quiet/silence (destructive interference: path difference = (n+Β½)Ξ»).
The spacing of loud/quiet regions increases as frequency decreases (wavelength increases).
Moving closer to the speakers makes the pattern more widely spaced.
Like two people in a pool making identical ripples β where the ripple crests meet you get a big splash (constructive), where a crest meets a trough they cancel out (destructive).
Path difference for constructive interference = nΞ». For destructive = (n + Β½)Ξ». Speakers must be coherent (same frequency and phase).
4Sound Needs a Medium β Bell in Vacuum Jar
Electric bell (battery-operated or wired)
Large glass bell jar with airtight seal
Vacuum pump
Rubber mounting pad to suspend the bell (reduces solid-conducted vibration)
Battery and connecting wires passed through airtight grommet
Mount the electric bell inside the bell jar on a rubber pad β rubber reduces vibration conducted through solid supports.
Connect the bell to a battery via wires sealed through the base.
Switch on the bell β you can hear it ringing clearly.
Switch on the vacuum pump to remove air from the jar.
Observe the sound level as air is progressively removed.
As air is pumped out, the ringing sound becomes progressively quieter.
In a near-vacuum, the bell is almost inaudible (you may still hear faint sound through the rubber mount and jar itself).
When air is let back in, the sound returns.
This demonstrates that sound requires a material medium to travel β it cannot propagate through a vacuum.
Astronauts on the Moon cannot speak to each other through the air (there is none) β they must use radio waves (EM radiation), which unlike sound, require no medium.
State: 'Sound is a mechanical (longitudinal) wave and requires a medium for propagation. EM waves do not require a medium.' This experiment provides evidence for both.
5Demonstration of Resonance
Two identical tuning forks (same frequency, e.g. 440 Hz) mounted on resonance boxes
OR Barton's pendulums: several pendulums of different lengths on a shared support string, one matching the 'driver' length
Stopwatch and metre rule
Strike one tuning fork (Fork A) hard with a rubber mallet.
After a few seconds, stop Fork A with your hand (touch it to stop vibration).
Hold Fork A close to Fork B's opening. Remove Fork A and listen to Fork B.
Alternative β Barton's pendulums: set the driver pendulum swinging; observe which other pendulum swings most.
Repeat with a fork of different frequency β Fork B does not resonate.
Fork B vibrates and emits sound even though it was not struck directly β resonance has occurred.
This only happens when Fork A and B have the same natural frequency.
With Barton's pendulums: only the pendulum with the same natural frequency as the driver swings with large amplitude.
This is resonance: maximum energy transfer when driving frequency equals natural frequency.
A child on a swing: if you push at exactly the right frequency (matching its natural frequency), the amplitude builds up dramatically. Push at the wrong time and nothing happens.
State both conditions: (1) driving frequency = natural frequency, (2) large amplitude oscillations result. Mention resonance can be harmful (Tacoma Narrows Bridge) or useful (tuning a radio).
6No-Parallax β Locating Image in a Plane Mirror
Plane mirror (rectangular, ~20 cm)
Retort stand and clamp to hold mirror vertically
Two optical pins (or sewing pins)
Soft board and paper
Ruler and pencil
Pin the mirror vertically using the retort stand. Mark the mirror position on the paper.
Place object pin Pβ in front of the mirror at a known distance.
Look into the mirror and position the search pin Pβ behind the mirror so that it coincides with the image of Pβ β move your eye laterally to check for parallax.
When there is no parallax (the search pin and image move together as you move your head), Pβ is at the image position.
Measure the image distance and compare with the object distance.
Image distance = object distance (for a plane mirror).
Image is virtual (behind the mirror), upright, and the same size as the object.
When the search pin is correctly placed, it does not appear to separate from the image when viewed from different positions (no parallax).
Image is laterally inverted.
No-parallax is like lining up the back and front sights of a gun β when both are perfectly aligned in your line of sight, there is no offset between them regardless of small head movements.
State: 'No parallax occurs when the image and the search pin are at the same position in space.' The image in a plane mirror is: virtual, same size, same distance behind mirror, upright, laterally inverted.
7Demonstration of the Second Law of Reflection
Plane mirror on a stand
Soft board and white paper
Ray box (with single-slit attachment) or laser
Protractor and ruler
Pencil
Place the mirror on the paper and draw the mirror line. Mark the point of incidence (where ray hits mirror).
Draw the normal (perpendicular to mirror surface at the point of incidence).
Direct the ray box beam at the mirror at a known angle of incidence i.
Trace the incident and reflected rays on the paper.
Measure the angle of incidence and angle of reflection (both from the normal) using a protractor.
Repeat for at least three different angles of incidence.
For every angle of incidence measured, the angle of reflection is equal: i = r.
Both the incident ray, reflected ray, and normal all lie in the same plane (the paper).
This confirms the Second Law of Reflection.
Exactly like a snooker ball bouncing off a cushion β the ball leaves at the same angle it arrived at, measured from the perpendicular to the cushion.
State BOTH laws of reflection for full marks: (1) The incident ray, reflected ray and normal lie in the same plane. (2) The angle of incidence equals the angle of reflection. Angles measured from the normal, not from the surface.
8Total Internal Reflection and Critical Angle
Semi-circular glass or perspex block
Ray box (single slit) or laser
Protractor and white paper
Ruler and pencil
Place the semi-circular block flat-side down on white paper. Trace its outline.
Direct the ray into the curved surface so it hits the flat surface perpendicularly (to avoid refraction at entry).
Gradually increase the angle of incidence at the flat surface from 0Β° upward.
At each angle, trace the incident ray, refracted ray (if present), and reflected ray.
Identify the critical angle C as the angle at which the refracted ray just grazes the surface (refraction angle = 90Β°).
Beyond this angle, observe total internal reflection β no refracted ray, only reflected ray.
For angles less than the critical angle: a refracted ray (in air) and a weaker reflected ray are observed.
At exactly the critical angle: the refracted ray travels along the glassβair boundary (90Β° to normal).
For angles greater than the critical angle: no refracted ray β only a bright reflected ray inside the glass (TIR).
Measured critical angle can be used to calculate refractive index: n = 1/sin C.
Think of trying to escape from a swimming pool by climbing up the side β at a shallow angle you can get out, but beyond a certain angle the wall just bounces you back. The glassβair boundary acts the same way for light rays.
Two conditions for TIR: (1) light travelling from denser to less dense medium, (2) angle of incidence > critical angle. Formula: sin C = 1/n. Use this to calculate n or C.
9Wave Nature of Light (Double Slit / Young's Experiment)
Laser (red, 650 nm) or bright monochromatic light source
Double slit card (or diffraction grating)
White screen (or wall) at least 1β2 m from slits
Metre rule
Optional: ruler for measuring fringe spacing
Set up the laser to shine through the double slit onto a white screen approximately 2 m away.
Observe the pattern produced on the screen.
Measure the fringe spacing (distance between adjacent bright fringes).
Block one slit β observe the diffraction pattern from a single slit changes to a single-slit diffraction envelope.
Replace double slit with a diffraction grating β observe clearly separated bright maxima.
A series of alternating bright and dark vertical fringes is observed on the screen.
This interference pattern is only possible if light is a wave.
Dark fringes: destructive interference (path difference = (n+Β½)Ξ»).
Fringe spacing increases with wavelength and distance to screen; decreases with slit separation.
Just like the sound interference pattern with two speakers β where two waves from coherent sources meet, you get reinforcement (bright) or cancellation (dark).
Formula: Ξ» = ax/d (a = slit separation, x = fringe spacing, d = slit to screen distance). State this demonstrates the wave nature of light by producing an interference pattern β a particle model could not explain this.
10Demonstration of Addition of Colours
Three identical projectors or spotlights with red, green, and blue filters (or coloured torches)
White screen in a darkened room
Optional: Newton's colour wheel (motor-driven disc with coloured sectors)
In a darkened room, project three overlapping circles of red, green, and blue light onto a white screen.
Observe the colour where each pair of colours overlaps, and where all three overlap.
Alternative with Newton's wheel: spin the colour wheel at high speed and observe the colour perceived.
Red + Green = Yellow
Green + Blue = Cyan
Red + Blue = Magenta
Red + Green + Blue = White light
Newton's wheel: spinning rapidly produces an impression of white or grey due to persistence of vision.
This demonstrates additive colour mixing.
Colour monitors (TV, phone screens) use exactly this system β each pixel is made of tiny red, green, and blue dots. Your brain adds them together to perceive millions of colours.
Primary colours of light: Red, Green, Blue (RGB). Different from paint/pigment primaries. State these are 'additive primary colours.' Complementary pairs: R+Cyan=White, G+Magenta=White, B+Yellow=White.
11Demonstration of Dispersion of White Light
Glass prism (equilateral or right-angle prism)
Ray box or narrow beam of sunlight through a slit
White card or screen
Place the prism on a white card in a darkened room. Direct a narrow beam of white light at one face of the prism.
Observe the light emerging from the opposite face on the white screen or card.
Mark the colours and measure the angular spread.
Optional: use a second prism to recombine the colours back to white light.
White light is dispersed into a continuous visible spectrum: red, orange, yellow, green, blue, indigo, violet (ROYGBIV).
Violet light is refracted most (highest refractive index in glass, shortest wavelength).
Red light is refracted least (lowest refractive index in glass, longest wavelength).
A second inverted prism recombines the colours back to white β proving the prism did not add colour.
A prism is like a slow-motion camera for light β white light enters as a single beam but the different wavelengths travel at slightly different speeds in glass, causing them to separate and fan out into the rainbow.
Dispersion occurs because different wavelengths have different refractive indices in glass (violet highest, red lowest). State: 'Dispersion is the separation of white light into its constituent colours due to different wavelengths being refracted by different amounts.'
12Detection of Infrared Radiation
Infrared source: Leslie's cube with hot water, or a radiant heater, or TV remote control
Thermopile connected to a galvanometer (or sensitive IR detector/camera)
Black card to block visible light (proves it is IR, not visible light)
Prism to show IR is beyond red in the spectrum
Direct the thermopile at the hot object (Leslie's cube or heater). Record galvanometer reading.
Interpose a glass plate β glass blocks IR partially. Note change in reading.
Place a black card in the path β the card blocks visible light but passes IR. Note galvanometer still deflects.
Classic spectrum method: produce a spectrum with a prism. Move the thermopile beyond the red end of the visible spectrum β galvanometer still deflects, showing IR radiation beyond visible red.
The thermopile registers a deflection when facing the hot source, even when visible light is blocked.
The detector responds beyond the red end of the visible spectrum β confirming IR is beyond red light.
This demonstrates that infrared radiation exists and carries energy.
Dull black surfaces emit most IR (as shown by Leslie's cube).
Your skin is a natural IR detector β you feel warmth from a fire even when looking away or in a darkened room. This shows IR is distinct from visible light but transfers heat energy the same way.
State: 'Infrared radiation is EM radiation with wavelength greater than visible red (~700 nm to 1 mm). It is detected by a thermopile or thermocouple. It transfers heat energy without a medium.'
13Detection of Ultraviolet Radiation
UV lamp (long-wave UV, e.g. black light)
Fluorescent material (fluorescent paper, tonic water, washing powder residue on white paper, UV-reactive paint)
Piece of glass (blocks UV) and quartz glass (passes UV)
Dark room
In a darkened room, direct the UV lamp at a piece of fluorescent material (e.g. white paper washed with fluorescent detergent, or tonic water in a glass).
Observe the bright blue-white glow β fluorescence shows UV is present.
Interpose a piece of ordinary glass between lamp and material β fluorescence stops (glass absorbs UV).
Replace with quartz glass β fluorescence resumes (quartz passes UV).
Fluorescent materials glow brightly when exposed to UV β they absorb UV and re-emit as visible light.
Ordinary glass blocks UV radiation (no fluorescence through glass).
Quartz glass transmits UV (fluorescence continues through quartz).
This demonstrates UV radiation exists beyond the violet end of the visible spectrum.
UV is to our eyes what dog-whistle frequency is to our ears β our senses can't detect it directly, but we can see its effect through fluorescence, just as we can hear a dog react to sounds we cannot.
State: 'UV radiation has wavelength shorter than visible violet (~10 nm to 400 nm). It cannot be detected by the eye directly but causes fluorescence. Glass absorbs UV; quartz transmits it.'
14Light is a Transverse Wave β Polarisation
Two Polaroid filters (polarising sheets or lenses)
Light source (lamp or projector β NOT a laser unless safety-rated)
Rotating mount for second Polaroid
Place the first Polaroid (Polariser P1) in front of the light source β it only transmits light vibrating in one plane.
Look through the second Polaroid (Analyser P2) at the light through P1.
Slowly rotate P2 through 360Β°.
Observe the intensity of transmitted light as P2 is rotated.
When P1 and P2 are aligned (transmission axes parallel): maximum brightness.
As P2 is rotated, intensity gradually decreases.
When P2 has been rotated 90Β° (crossed polaroids): no light passes β minimum brightness (extinction).
At 180Β° alignment: maximum brightness again.
This variation in intensity with rotation is only possible if light is a TRANSVERSE wave (vibrations perpendicular to direction of travel).
Longitudinal waves (like sound) cannot be polarised.
Like trying to push a fence post through a set of railings β if the post is aligned with the gaps it passes through; rotate it 90Β° and it's blocked. Polaroid filters are like railings for light waves.
State: 'Polarisation can only occur with transverse waves. Since light can be polarised, this is evidence that light is a transverse wave.' Longitudinal waves (sound) cannot be polarised β a key distinction.