The duck meat I bought was shiny? Was it because I was starving, or was it because the duck had become a spirit?

"A beautiful day is coming to an end. Tomorrow is the weekend, so I have to eat something good tonight." I was tidying up the table while thinking about what to buy later. I decided to eat the famous delicacy from Leshan, Sichuan tonight - sweet-skinned duck!

After get off work, I went to the market, bought the sweet-skinned duck, and went home. Just as I happily opened the box, something strange happened. Why was the duck shining? Was it because I was too hungry to see clearly?

Duck meat shows colorful metallic luster at different angles. Image source: Photographed by the author

After careful observation, I found that the metallic luster only occurs in the areas with solid muscles, and this phenomenon is not observed in bones, white fat layers, and epidermis. Therefore, after reasonable speculation, this should be related to the interaction between light and muscles. For more details, please listen to me slowly.

The structure of muscles

Take skeletal muscle as an example. Skeletal muscle fibers are muscle cells, which are elongated and multinucleated. Skeletal muscle is composed of muscle fiber bundles, which are composed of muscle cells. Myocytes contain multiple myofibrils, which can be divided into several sarcomeres. In the fine structure of muscle, there are thick filaments and thin filaments.

Schematic diagram of muscle microstructure, translated by the author, source: see figure

Looking at the screen full of "muscle", are you almost unable to recognize this word? It doesn't matter. You just need to know that the muscle structure is complicated, but it actually looks very simple. It is just a regular bundle of strands, like wires and cables. When the meat is cut horizontally, the muscle section is composed of small sections. When light hits the cooked, shiny muscle section, the reflection of light is actually completed by the small sections.

So what does this have to do with colored reflections? Read on.

Interference of Light and Grating Dispersion

When raindrops dripped onto the calm lake, the lake surface became active. The water waves on it were drawing their own circles alone, but they seemed to be disturbing and playing with each other.

Light is an electromagnetic wave. Its propagation speed in a vacuum is a constant, usually represented by the letter c, and its approximate value is 300,000 kilometers per second. What is even more amazing is that the speed of light in a vacuum is not related to the motion state of the inertial reference frame, nor is it related to the color of light, that is, the frequency of light. Since the refractive index in the air around us does not change much, we can know from the relationship that "frequency multiplied by wavelength equals the speed of light" that in life we ​​can generally believe that the color of light in the air is related to the wavelength.

Propagation of a single-point light wave source and a double-point light wave source. Image source: provided by the author

As shown in the figure above, when there is only one point light wave source in space, the propagation of light is like a drop of water falling on the water surface, spreading out in circles and gradually weakening as the wave moves away from the light source. This is the result of the conservation of energy, because as the circle expands, the energy is constant, which will inevitably cause the wave to gradually weaken as the propagation distance increases. Imagine that we light a candle, and the flame will become dimmer as it moves away.

When there are two point light sources several wavelengths apart in space, the propagation of waves is no longer uniform in all directions but becomes directional, with strong propagation in some directions and weaker or even zero propagation in others. This is also a manifestation of the conservation of energy - if there are strong propagation in some directions, there must be weaker propagation directions, which concentrates the energy.

Surely the cross section of the muscle is even more amazing? Yes, that's exactly what it is!

Grating and dispersion effects

The structure of the muscle is mathematically and physically modeled. Each muscle fiber bundle is considered as a wave source connected together, and the distance between point light sources is much smaller than the wavelength. The gap separated by the sarcolemma can be regarded as a break in the continuous wave source. According to the microstructure of the muscle, the cross-sectional diameter of the muscle fiber is about 50μm, and the interval is about 10μm. Based on this data, we simulated the propagation of light waves after reflection on the muscle and found that a magical phenomenon appeared!

Scanning electron microscope image of cooked muscle at normal pressure. Image source: Reference [2]

First, no matter what the color of light is, their propagation has obvious directionality. Secondly, in the order of "red, orange, yellow, green, cyan, blue, and violet", red light has a long wavelength and blue light has a short wavelength. In this process, except for the central symmetry line perpendicular to the muscle surface, their propagation directions are consistent, and the deflection angle of red light is smaller than that of blue and violet light. This is exactly the opposite of the refraction behavior of glass.

This structure is like a fence, so it is called "light grating". Why it is pronounced as "light grating" is another story.

Grating with 1200 reflective lines per millimeter. Image source: Photographed by the author

Since different colors of light have different propagation angles except for the center, the grating structure has the function of separating light by color (more precisely, wavelength and frequency), so we say that the grating has dispersion ability. One of its most important applications is the spectrometer.

Fiber optic spectrometer structure, Image source: Photographed by the author

Grating dispersion effect diagram, Image source: provided by the author

Is there a grating in life that allows us to see rainbow-colored light? Of course there is, that is the optical disc.

Optical disc and hexagonal wrench (note the shadow and color order) and electron microscopic structure of the optical disc. Image source: Photographed by the author

The microstructure of the optical disc is also a series of fences, and the dots on them are the recorded information. In the picture, we can see that the shadow is on the left, indicating that the sun is on the upper right. The red light is on the left and the blue light is on the right, indicating that the red light has a larger deflection angle than the blue light, which is consistent with our simulation results.

Therefore, if you also want to see the glittering meat, you can try to observe it at an angle, because the cross section of the muscle is basically flat, and only when you look at it at an angle can you see the dispersion effect of the grating. Through the above simulation experiment, I believe that we have solved the doubts at the beginning. Facing this piece of glittering meat, there is no need to worry about whether you can eat it, just enjoy it!

References:

[1] Zhong Xihua. (2012). Foundations of Modern Optics. Peking University Press.

[2] Wang Zhijiang, & Jiang Aimin. (2015). Effects of ultra-high pressure treatment on the microstructure of cooked chicken. Food and Machinery (1), 4.

Source: This article is produced by Science Popularization China, produced by Tang Hongyang (Institute of Optoelectronics Technology, Chinese Academy of Sciences), and supervised by China Science Popularization Expo

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