A family experiment · rung 1 of the evidence ladder家庭小实验 · 证据阶梯第 1 级
Measure a chicken with a laser用激光量一量鸡肉
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 cheap laser pointer 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. The safety rules below stay exactly as they are. 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).
测鸡肉的肌节，用不着 SWELL。肌肉每隔一两微米就有一道横纹，这些横纹就像一块衍射光栅。用一支便宜的激光笔照穿一缕很细的生鸡胸肉，光束会分成几个光点；光点隔多远，就能算出肌节长度（SL）。这是肉品科学的经典测法，而 SWELL 本来就是设计来在凝胶里读这些横纹的。
这是一个家庭小实验。适合年龄：8 岁及以上，必须有大人一起。凡是碰到激光笔、生鸡肉、刀或玻璃的步骤都由大人来做，最后也由大人把肉扔掉；孩子负责标出光点、测量和计算。透明塑料片比玻璃载玻片更安全。下面的安全规则一条不改。一支标明 2 类的红色激光笔售价 £17.83，含增值税（英国一家分销商的标价，2026 年 10 月 10 日读取；只是价格参考，不是推荐去哪里买）。
cited引用
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 by an AI). doi:10.1080/1828051X.2020.1761271
参考值：肉鸡胸肌用激光衍射测得，正常肉的 SL 是 1.69 µm，"木质化胸肉"（一种常见的肉质缺陷）是 1.91 µm，长了 13%。出处：Soglia、Petracci 与 Puolanne，2020，Italian Journal of Animal Science（AI 读过摘要）。doi:10.1080/1828051X.2020.1761271
You need你需要
- 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.
- 一支红色（650 nm）2 类激光笔（≤ 1 mW），在正规商家买。确认标签上写着 Class 2。家庭实验只用红光：便宜的绿色激光笔可能漏出看不见的红外光，也更晃眼。
- 无论多便宜，都不要用无牌的“大功率”激光笔：它标签上的等级可能不真实（见安全说明）。
- 超市买的生鸡胸肉，指尖大小一块就够。只由大人拿。
- 两小片透明硬塑料（最安全），或两片玻璃显微镜载玻片，只由大人拿。
- 一把锋利的小刀或手术刀，只给大人用（有根牙签帮着撕会更顺手）。
- 一把有毫米刻度的尺子，一支铅笔。
- 一张至少 A4 大小的白卡纸，横着放当屏幕（在 20 厘米处，两侧光点各离中心大约 8 厘米），一个夹子或长尾夹用来夹住玻片，再加一点胶带。
- 一个能关灯变暗的房间，一摞书用来架住激光笔。
Laser safety first.
激光安全第一。
- 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.
- 只用 2 类：可见光，最多 1 mW。英国卫生安全局（UKHSA）的建议：2 类激光短时间照到是安全的，因为人看到强光会本能地避开；但如果故意盯着光束看，就可能有危险。
- 永远不要看光束，也绝不要对着眼睛、人、动物、车辆或飞机照。在英国，用激光照向正在行驶或即将启动的交通工具（汽车、火车、船或飞机），使驾驶者目眩或分心，属于刑事犯罪（《2018 年激光滥用（交通工具）法》），最高可判五年监禁。
- 从正规商家购买。UKHSA 提醒：标着 2 类或"小于 1 mW"的激光笔，实际常常是 3B 类（强 5 到 500 倍），甚至 4 类。别买无品牌的"大功率"激光笔。
- 光路上不要有反光物：镜子、金属、手表、戒指。光束保持水平、低于眼睛高度，把玻片稍微倾斜，让反光朝下。
- 孩子：必须有大人在旁看护；激光笔、刀、玻片和生鸡肉都由大人来拿。UKHSA 记录过因标签不实的激光笔造成的眼睛损伤，受害者尤其多是孩子。
Sources: UKHSA, Laser radiation: safety advice (gov.uk, updated 18 March 2025); Laser Misuse (Vehicles) Act 2018, section 1.
出处：UKHSA《Laser radiation: safety advice》（gov.uk，2025 年 3 月 18 日更新）；《2018 年激光滥用（交通工具）法》第 1 条。
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.
生鸡肉卫生。生鸡肉可能带有弯曲杆菌和沙门氏菌，它们是英国食物中毒的主要原因中的两种。别吃样品，也别吃碰过它的东西。不要在水龙头下冲洗生鸡肉：水花会把细菌溅得到处都是。用热肥皂水马上清洗刀、玻片和台面；碰过生肉后，用肥皂和水洗手。
Sources: NHS, Food poisoning and Meat in your diet; Food Standards Agency, Campylobacter.
出处：NHS《Food poisoning》与《Meat in your diet》；英国食品标准局《Campylobacter》。
Steps步骤
- 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.
- 先关着激光布置光路。把激光笔架在书上，让光束水平、与桌面同高，远低于任何人的眼睛。把白卡纸竖着贴在光点落下的地方，离样品位置大约 20 厘米。把光路上所有反光的东西拿开。
- 切（大人）：顺着纹理切一条火柴棍大小的肉条。肉里能看到的细纹，就是肌纤维的方向。
- 撕（大人）：用刀尖或牙签顺着长边把它撕开，直到只剩几根纤维粗的一缕，薄到能透光。越细，光点越清楚。
- 夹（大人）：把这缕肉丝拉直，夹在两片玻片之间。记下纤维的走向。用夹子夹住玻片，不要用手拿。转动玻片，让纤维水平（左右方向）：这样两侧的亮点落在桌面高度，不会朝着脸。
- 照（大人）：关灯，打开激光。把"玻片三明治"放进光路，玻片正对激光笔，让光束垂直穿过纤维。把玻片稍微倾斜，让玻璃的反光朝下打到桌上，而不是反射回你这边。
- 看：卡纸上中间一个亮点，沿纤维方向两侧各有一个较暗的光点或一小段弧（见下图）。什么都没有？换更细的一缕或换个位置，再把房间弄暗一些。
- 量（孩子）：站在卡纸旁边，绝不站到激光笔和卡纸之间，也不要回头看激光笔，用铅笔标出两个一级光点的中心，然后由大人关掉激光。量两个标记之间的距离，除以 2，就是 x。再量 L：肉到卡纸的距离。量两点之间再除以 2，比从中心亮点量起更准。
- 算（孩子）：用下面的计算器。然后由大人把肉扔掉，用热肥皂水洗玻片和刀，擦干净台面；每个人都要洗手。
What you should see你应该看到什么 predicted预测
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.
中间是直穿过去的亮点（0 级），两侧各有一个较暗的光点（−1 级和 +1 级），沿着纤维的走向排成一线。一缕肉丝里有很多根纤维，角度和长度都略有不同，所以两侧的光点常被拉成横跨纤维方向的短条或小弧。用红光、卡纸放在 20 厘米处，正常鸡胸肉的光点应该在中心两侧约 83.3 mm。卡纸放在 10 厘米时，更外侧约 120 mm 处可能还会出现一对更暗的二级光点。
The maths算法
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, …):
肌节沿纤维每隔一个 SL 重复一次，所以光只会在某些角度 θ 射出：在那些角度上，相邻肌节发出的光步调一致（级次 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):
θ 没法直接量，但能量 x（卡纸上中心到光点）和 L（肉到卡纸）：
sin θ = x / √(x² + L²) ⇒ SL = λ · √(x² + L²) / x (first order, m = 1)（一级，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.
算个例子：λ = 650 nm，L = 20 cm，你量到 x = 83 mm。那么 SL = λ√(x² + L²)/x = 1.70 µm。x 和 L 要用同一个单位（都用毫米：L = 200 mm），λ 用微米（0.650），算出来的就是微米。
| pointer, card distance L激光、卡纸距离 L | normal (1.69 µm)正常（1.69 µm） | wooden breast (1.91 µm)木质化（1.91 µm） | difference相差 |
|---|---|---|---|
| red 650 nm, 10 cm红光 650 nm，10 厘米 | 41.7 mm | 36.2 mm | 5.5 mm |
| red 650 nm, 20 cm红光 650 nm，20 厘米 | 83.3 mm | 72.4 mm | 11.0 mm |
| green 532 nm, 10 cm (labs only, not for families)绿光 532 nm，10 厘米（仅实验室，家庭别用） | 33.2 mm | 29.0 mm | 4.2 mm |
| green 532 nm, 20 cm (labs only, not for families)绿光 532 nm，20 厘米（仅实验室，家庭别用） | 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.
x 读错 1 毫米，卡纸在 10 厘米时 SL 会偏约 0.035 µm，在 20 厘米时只偏 0.017 µm（红光、正常肉）。正常肉和木质化胸肉相差 0.22 µm，所以仔细读到毫米就够了，L 放远一点更好量。
Calculator计算器
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.
作为对照：已发表的鸡胸肉数值是 1.69 µm（正常）和 1.91 µm（木质化）。SL 还会随僵直和冷收缩而变，所以这些数只能当参考，不能当检测标准。