A family experiment · rung 1 of the evidence ladder
Measure a chicken with a laser
You don't need SWELL to measure a chicken's sarcomeres. Muscle is striped every couple of micrometres, and the stripes act as a diffraction grating. Shine a labelled Class 2 red laser pointer (the adult holds and aims it) through a thin thread of raw chicken breast and the beam splits into spots; how far apart they land gives the sarcomere length (SL). It is the classic meat-science method, and SWELL was designed to read the same stripes inside its gels.
A family experiment. Age guide: 8 and up, with an adult. The adult does every step that touches the laser, the raw chicken, the knife or the glass, and bins the meat at the end; children mark the spots, measure and do the maths. Clear plastic is safer than glass slides. Follow every safety rule below. A labelled Class 2 red pointer costs £17.83 including VAT (one UK distributor's listed price, read 10 October 2026; a price, not a recommendation where to buy).
cited
For grown-ups: the reference
Reference: in broiler breast muscle, laser diffraction gave SL 1.69 µm in normal meat and 1.91 µm in "wooden breast", a common texture defect (13% longer). Soglia, Petracci & Puolanne 2020, Italian Journal of Animal Science (abstract read). doi:10.1080/1828051X.2020.1761271
You need
- A red (650 nm) class-2 laser pointer (≤ 1 mW) from a reputable seller. Check that the label says Class 2. For families, red only: cheap green pointers can leak invisible infrared and dazzle more.
- Never an unbranded "high-power" pointer, however cheap: the class on its label may not be true (see the safety box).
- Raw chicken breast from the supermarket. A fingertip-sized piece is plenty. Only the adult handles it.
- Two small pieces of clear rigid plastic (safest), or two glass microscope slides, which only the adult handles.
- A sharp knife or scalpel, for the adult only (a cocktail stick helps with teasing).
- A ruler marked in millimetres, and a pencil.
- A white card at least A4, turned landscape, for a screen (the side spots land about 8 cm either side at 20 cm), a peg or bulldog clip to hold the slides, and some tape.
- A dark room, and a pile of books to rest the pointer on.
Laser safety first.
- Class 2 only: visible light, at most 1 mW. UK Health Security Agency (UKHSA) advice: class 2 is safe for short exposures because we naturally look away from bright light, but it can be hazardous if you deliberately stare into the beam.
- Never look into the beam, and never point it at eyes, people, animals, vehicles or aircraft. In the UK, shining a laser at a vehicle that is moving or ready to move (car, train, boat or aircraft) so that it dazzles or distracts the person in control is a crime under the Laser Misuse (Vehicles) Act 2018, with up to five years in prison.
- Buy from a reputable seller. UKHSA warns that pointers marked class 2 or "less than 1 mW" are often really class 3B (5–500 times stronger) or even class 4. Avoid unbranded "high-power" pointers.
- Keep shiny things out of the beam: mirrors, metal, watches, rings. Keep the beam level and below eye height, and tilt the glass so its reflection goes down.
- Children: only with an adult supervising. The adult handles the laser, the knife, the glass and the raw chicken. UKHSA records eye injuries, especially to children, from mislabelled pointers.
For grown-ups: the sources
Sources: UKHSA, Laser radiation: safety advice (gov.uk, updated 18 March 2025); Laser Misuse (Vehicles) Act 2018, section 1.
Raw chicken hygiene. Raw chicken can carry Campylobacter and Salmonella, two of the main causes of food poisoning in the UK. Don't eat the sample or anything it touched. Don't rinse the chicken under the tap: splashes spread the bacteria. Clean the knife, slides and surfaces with hot soapy water straight away, and wash your hands with soap and water after touching raw meat.
For grown-ups: the sources
Sources: NHS, Food poisoning and Meat in your diet; Food Standards Agency, Campylobacter.
Steps
- Set up the beam first, laser off. Rest the pointer on books so the beam runs level at table height, well below anyone's eyes. Tape the white card upright where the beam lands, about 20 cm from where the sample will sit. Clear anything shiny from the path.
- Cut (adult): a strip about the size of a matchstick, along the grain; the fine lines you can see in the meat run along the fibres.
- Tease (adult): pull it apart lengthways with the knife tip or a cocktail stick until the thread is only a few fibres thick, thin enough to see light through. Thinner gives sharper spots.
- Sandwich (adult): lay the thread straight between the two slides, note which way the fibres run, and hold the slides in the peg or clip, not fingers. Turn the sandwich so the fibres run level (left to right): the side spots then land at table height, not up towards faces.
- Shine (adult). Darken the room and switch on. Put the sandwich in the beam with the glass facing the pointer, so the beam crosses the fibres at right angles. Tilt the slides slightly so the glass reflects down onto the table, not back at you.
- Look at the card: a bright central spot, with a fainter spot or short arc on each side along the fibre direction (picture below). Nothing? Try a thinner thread or another part of it, and make the room darker.
- Measure (children). Standing beside the card, never between the pointer and the card, and without looking back towards the pointer, mark the centres of the two first-order spots in pencil; then the adult switches the laser off. Measure the distance between the marks and halve it: that is x. Then measure L, from the meat to the card. Spot-to-spot and halve is more accurate than measuring from the bright centre.
- Calculate (children) with the calculator below. Then the adult bins the meat, washes the slides and knife in hot soapy water and wipes the surfaces, and everyone washes their hands.
What you should see predicted
The bright straight-through beam in the middle (order 0) and one fainter spot on each side (orders −1 and +1), lined up along the direction the fibres run. A thread holds many fibres at slightly different angles and lengths, so the side spots are often smeared into short bars or arcs that cross the fibre direction. With a red pointer and the card at 20 cm, normal breast meat should put them about 83.3 mm either side of the centre. With the card at 10 cm, a second, fainter pair may show up about 120 mm out.
The maths
The sarcomeres repeat every SL along the fibre, so light only leaves at angles θ where neighbouring sarcomeres send it out in step (order m = 1, 2, …):
SL · sin θm = m · λ ⇒ SL = m · λ / sin θm
You can't measure θ directly, but you can measure x (centre to spot, on the card) and L (meat to card):
sin θ = x / √(x² + L²) ⇒ SL = λ · √(x² + L²) / x (first order, m = 1)
Worked example: λ = 650 nm, L = 20 cm, and you read x = 83 mm. Then SL = λ√(x² + L²)/x = 1.70 µm. Put x and L in the same unit (both in millimetres: L = 200 mm) and λ in micrometres (0.650), and the answer comes out in micrometres.
| pointer, card distance L | normal (1.69 µm) | wooden breast (1.91 µm) | difference |
|---|---|---|---|
| red 650 nm, 10 cm | 41.7 mm | 36.2 mm | 5.5 mm |
| red 650 nm, 20 cm | 83.3 mm | 72.4 mm | 11.0 mm |
| green 532 nm, 10 cm (labs only, not for families) | 33.2 mm | 29.0 mm | 4.2 mm |
| green 532 nm, 20 cm (labs only, not for families) | 66.3 mm | 58.0 mm | 8.3 mm |
Misreading x by 1 mm moves SL by about 0.035 µm with the card at 10 cm, but only 0.017 µm at 20 cm (red light, normal meat). Normal and wooden-breast meat differ by 0.22 µm, so careful millimetre readings are enough, and a longer L makes it easier.
Calculator
For scale: the published breast-meat values are 1.69 µm (normal) and 1.91 µm (wooden breast). SL also changes with rigor and cold shortening, so treat them as a guide, not a test.
Predicted for red light with the card at 10 cm: side spots 41.7 mm from the centre. Nobody at X-WOW has tried it yet.