The Science of 700 nm: Red Light in Physics and Biology | 700 纳米的科学：物理与生物学中的红光

📚 The Science of 700 nm: Red Light in Physics and Biology | 700 纳米的科学：物理与生物学中的红光

At first glance, the number 700 may look like just a value on a page. In physics and biology, however, 700 represents something precise and powerful: a wavelength of 700 nanometres. This light lies at the very edge of the human visible spectrum, deep in the red region. Understanding what 700 nm means helps IGCSE students connect the electromagnetic spectrum, energy calculations, and plant biology in one elegant story.

乍看之下，数字 700 可能只是页面上的一个数值。但在物理学和生物学中，700 代表着一个精确而强大的含义：700 纳米的波长。这种光位于人眼可见光谱的最边缘，属于深红色区域。理解 700 nm 的含义，能帮助 IGCSE 学生将电磁波谱、能量计算和植物生物学串联成一个优雅的故事。

1. The Electromagnetic Spectrum | 电磁波谱

The electromagnetic spectrum is the full range of electromagnetic waves, ordered by frequency and wavelength. It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Visible light occupies only a tiny central band, roughly from 380 nm (violet) to 700 nm (red).

电磁波谱是电磁波的完整范围，按频率和波长排列。它包括无线电波、微波、红外线、可见光、紫外线、X 射线和伽马射线。可见光只占据一个狭小的中心区域，大约从 380 nm（紫色）到 700 nm（红色）。

Wavelength is the distance between two consecutive peaks of a wave. It is usually measured in metres, but for light it is convenient to use nanometres (nm). One nanometre is one-thousand-millionth of a metre:

波长是波的两个相邻波峰之间的距离。通常以米为单位测量，但对于光来说，使用纳米（nm）更方便。1 纳米等于 10 亿分之 1 米：

1 nm = 1 × 10⁻⁹ m

Therefore, 700 nm can be written as 7.00 × 10⁻⁷ m. In IGCSE physics questions, you may need to convert nm to m before using the wave equation.

因此，700 nm 可以写成 7.00 × 10⁻⁷ m。在 IGCSE 物理题中，你可能需要先将 nm 转换为 m，再使用波方程。

2. What is 700 nm? | 什么是 700 nm？

Light with a wavelength of 700 nm is perceived by human eyes as red light. It is the longest wavelength our eyes can detect. Beyond 700 nm lies infrared radiation, which we cannot see but can feel as heat.

波长 700 nm 的光被人眼感知为红光。它是我们眼睛能检测到的最长波长。超过 700 nm 就是红外线，我们看不见，但能感觉到热。

The frequency of 700 nm light can be found using the wave equation:

700 nm 光的频率可以用波方程求出：

v = f × λ

where v is the speed of light (3.00 × 10⁸ m/s), f is frequency in hertz (Hz), and λ is wavelength in metres. For 700 nm:

其中 v 是光速（3.00 × 10⁸ m/s），f 是频率，单位赫兹（Hz），λ 是波长，单位米。对于 700 nm：

f = v ÷ λ = (3.00 × 10⁸) ÷ (7.00 × 10⁻⁷) ≈ 4.29 × 10¹⁴ Hz

This is a very high frequency, wiggling about 429 trillion times every second.

这是非常高的频率，每秒振动约 429 万亿次。

3. Photon Energy and the Red Edge | 光子能量与红边

Light travels as packets of energy called photons. The energy of one photon depends on its frequency. The higher the frequency, the greater the energy. Red light has a lower frequency than blue light, so red photons are less energetic than blue photons.

光以称为光子的能量包形式传播。一个光子的能量取决于其频率。频率越高，能量越大。红光的频率低于蓝光，所以红色光子比蓝色光子能量低。

The energy of a photon can be calculated using:

光子的能量可以用以下公式计算：

E = h × f

where E is energy in joules, h is Planck’s constant (6.63 × 10⁻³⁴ J·s), and f is frequency. For 700 nm light, E ≈ 6.63 × 10⁻³⁴ × 4.29 × 10¹⁴ ≈ 2.84 × 10⁻¹⁹ J.

其中 E 是能量，单位焦耳（J），h 是普朗克常数（6.63 × 10⁻³⁴ J·s），f 是频率。对于 700 nm 光，E ≈ 6.63 × 10⁻³⁴ × 4.29 × 10¹⁴ ≈ 2.84 × 10⁻¹⁹ J。

This energy is enough to trigger certain chemical reactions, such as the light-dependent reactions of photosynthesis, but not enough to break strong covalent bonds. This is why red light is useful in biology rather than harmful like ultraviolet light.

这个能量足以触发某些化学反应，例如光合作用的光反应，但不足以断裂强共价键。这就是为什么红光在生物学中有用，而不像紫外线那样有害。

4. Why Does 700 nm Look Red? | 为什么 700 nm 看起来是红色的？

Our eyes contain cone cells that are sensitive to different ranges of wavelengths. Long-wavelength-sensitive cones respond most strongly to light around 560 nm, but they still respond to 700 nm. When 700 nm light enters the eye, the brain interprets the combined signal as red.

我们的眼睛包含对不同波长范围敏感的视锥细胞。长波敏感视锥细胞对 560 nm 附近的光反应最强，但它们对 700 nm 也有反应。当 700 nm 光进入眼睛时，大脑将综合信号解读为红色。

Red light also scatters less than blue light because scattering is stronger for shorter wavelengths. This is why the Sun appears red at sunrise and sunset: its light travels through more atmosphere, blue light is scattered away, and red light reaches your eye directly.

红光也比蓝光散射得少，因为散射对较短波长更强烈。这就是为什么太阳在日出和日落时看起来是红色的：阳光穿过更多大气层，蓝光被散射掉，红光则直接到达你的眼睛。

Visible spectrum: 380 nm – 700 nm

可见光谱：380 nm – 700 nm
Red light: ~620 nm – 700 nm

红光：约 620 nm – 700 nm
Invisible beyond 700 nm: infrared

700 nm 以外不可见：红外线

5. 700 nm and Photosynthesis | 700 nm 与光合作用

In biology, plants absorb light for photosynthesis using pigments such as chlorophyll a and chlorophyll b. Chlorophyll a absorbs strongly in the blue-violet region (~430 nm) and the red region (~660-680 nm). It absorbs less light in the green region, which is why plants look green.

在生物学中，植物利用叶绿素 a 和叶绿素 b 等色素吸收光进行光合作用。叶绿素 a 在蓝紫区（约 430 nm）和红区（约 660-680 nm）强烈吸收。它在绿光区域吸收较少，这就是为什么植物看起来是绿色的。

However, 700 nm is slightly beyond the main absorption peak of chlorophyll a. This wavelength is important because it is the approximate “red edge” of plant reflectance. Live vegetation strongly reflects infrared light just beyond 700 nm, while absorbing red light just before 700 nm. This sudden change is called the red edge.

然而，700 nm 稍稍超出了叶绿素 a 的主要吸收峰。这个波长很重要，因为它大约是植物反射的“红边”。活植被强烈反射 700 nm 之后的红外光，同时吸收 700 nm 之前的红光。这种突然的变化被称为“红边”。

In an IGCSE experiment, you might measure the absorption spectrum of a leaf. You would notice that absorption falls rapidly around 700 nm. Past 700 nm, in the infrared, absorption is low and reflectance is high. This property is used by satellites to monitor the health of plants.

在 IGCSE 实验中，你可能会测量叶片的吸收光谱。你会注意到，吸收率在 700 nm 附近急剧下降。超过 700 nm，进入红外区后，吸收率低而反射率高。这种特性被卫星用来监测植物的健康状况。

6. Red Light in Plant Growth | 红光在植物生长中的作用

Red light is not only absorbed by chlorophyll; it also controls plant development through a pigment called phytochrome. Phytochrome exists in two forms: Pr (red-absorbing form) and Pfr (far-red-absorbing form).

红光不仅被叶绿素吸收，还通过一种叫做光敏色素的色素控制植物发育。光敏色素有两种形式：Pr（吸收红光的形式）和 Pfr（吸收远红光的形式）。

When a plant is exposed to red light near 660 nm, Pr is converted into Pfr. Pfr promotes responses such as seed germination, stem elongation and flowering. Exposure to far-red light (about 730 nm) converts Pfr back to Pr. Because 700 nm is at the boundary between red and far-red, it can influence the balance between Pr and Pfr.

当植物暴露在 660 nm 附近的红光下，Pr 被转化为 Pfr。Pfr 促进种子萌发、茎伸长和开花等反应。暴露在远红光（约 730 nm）下，Pfr 会转回 Pr。由于 700 nm 位于红光和远红光的边界，它可以影响 Pr 与 Pfr 之间的平衡。

This is why red light is used in plant growth chambers. Light-emitting diodes (LEDs) that emit around 660-700 nm are highly effective at driving photosynthesis and photomorphogenesis.

这就是为什么红光常被用于植物生长箱。发射约 660-700 nm 光的发光二极管（LED）在驱动光合作用和光形态发生方面非常有效。

7. Photosynthesis: Light-Dependent Reactions | 光合作用：光依赖反应

In the light-dependent reactions of photosynthesis, chlorophyll molecules absorb light energy and use it to split water molecules. This produces oxygen, protons and electrons. The electrons enter an electron transport chain, which powers the synthesis of ATP and NADPH.

在光合作用的光依赖反应中，叶绿素分子吸收光能并利用它分解水分子。这个过程产生氧气、质子和电子。电子进入电子传递链，驱动 ATP 和 NADPH 的合成。

Although chlorophyll a absorbs most strongly at 680 nm (P680) and 700 nm (P700), the absorption at 700 nm falls within photosystem I. Photosystem I has a special pair of chlorophyll molecules called P700, named because its absorption peak is at 700 nm.

虽然叶绿素 a 在 680 nm（P680）和 700 nm（P700）处吸收最强，但 700 nm 处的吸收属于光系统 I。光系统 I 有一对特殊的叶绿素分子，称为 P700，因其吸收峰在 700 nm 而得名。

When light of 700 nm hits P700, it boosts electrons to a high energy level. These electrons reduce ferredoxin, which is used to make NADPH. Thus 700 nm is directly involved in the energy conversion of photosynthesis.

当 700 nm 的光照射 P700 时，它会将电子推高到高能级。这些电子还原铁氧还蛋白，用于制造 NADPH。因此，700 nm 直接参与光合作用的能量转换。

For IGCSE, remember the word equation for photosynthesis:

对于 IGCSE，记住光合作用的文字方程：

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

8. Remote Sensing and the Red Edge | 遥感与红边

Scientists use the difference in reflectance between red and near-infrared light to assess vegetation health. Healthy plants absorb red light (around 660-700 nm) and reflect much more near-infrared light (700-900 nm). If a plant is stressed or diseased, its red reflectance may increase and its near-infrared reflectance may decrease.

科学家利用红光和近红外光反射率的差异来评估植被健康状况。健康植物吸收红光（约 660-700 nm）并反射更多近红外光（700-900 nm）。如果植物受到胁迫或患病，其红光反射率可能增加，近红外反射率可能下降。

A common satellite index is the Normalized Difference Vegetation Index (NDVI). It compares the brightness in the near-infrared band (NIR) and the red band:

常用的卫星指数是归一化植被指数（NDVI）。它比较近红外波段（NIR）和红色波段的亮度：

NDVI = (NIR – Red) ÷ (NIR + Red)

Values near +1 indicate dense, healthy vegetation; values near 0 indicate bare soil or water. The red band used by satellites is often centered near 645-670 nm, while 700 nm marks the beginning of the near-infrared region.

接近 +1 的值表示浓密健康的植被；接近 0 的值表示裸土或水体。卫星使用的红色波段通常以 645-670 nm 为中心，而 700 nm 标志着近红外区域的开始。

9. Medical Applications of 700 nm Light | 700 nm 光的医学应用

Red light has been used in medicine for many years. Low-level laser therapy, also called photobiomodulation, uses red or near-infrared light to reduce pain and inflammation. Wavelengths around 660-700 nm can penetrate skin to shallow depths and stimulate mitochondria in cells.

红光在医学中已使用多年。低强度激光疗法，也称为光生物调节，利用红光或近红外光减轻疼痛和炎症。660-700 nm 左右的波长可以穿透皮肤至较浅深度，并刺激细胞中的线粒体。

Mitochondria contain a pigment called cytochrome c oxidase, which absorbs light in the red and near-infrared range. When it absorbs light, the production of ATP can increase, helping cells to repair themselves. This is why red light lamps are used in skin clinics and sports rehabilitation.

线粒体中含有一种叫做细胞色素 c 氧化酶的色素，它能吸收红光和近红外光。当它吸收光时，ATP 的产生会增加，帮助细胞自我修复。这就是为什么红光灯被用于皮肤科诊所和运动康复。

Wound healing: red light encourages collagen production

伤口愈合：红光促进胶原蛋白产生
Pain relief: reduces inflammation in joints and muscles

镇痛：减轻关节和肌肉炎症
Hair growth: stimulates hair follicles

生发：刺激毛囊

10. Optical Fibres and 700 nm Light | 光纤与 700 nm 光

In physics, the behaviour of light at different wavelengths is useful in fibre optics. Most optical communication uses infrared light (around 850 nm, 1300 nm or 1550 nm) because it has less loss in glass. However, red light around 700 nm is still used in short-distance plastic optical fibres and in education to demonstrate total internal reflection.

在物理学中，不同波长的光行为可用于光纤。大多数光通信使用红外光（约 850 nm、1300 nm 或 1550 nm），因为它在玻璃中损耗更小。然而，700 nm 左右的红光仍用于短距离塑料光纤，并用于教学演示全内反射。

Because red light has a longer wavelength than blue light, it refracts less when entering glass. This can be seen when white light passes through a prism: red light bends the least, while violet light bends the most. This separation of white light into colours is called dispersion.

因为红光的波长比蓝光长，进入玻璃时的折射程度较小。这在白光通过棱镜时可以看到：红色光偏折最小，而紫色光偏折最大。这种白光分离成颜色的现象称为色散。

11. Comparing 700 nm to Other Wavelengths | 比较 700 nm 与其他波长

To understand why 700 nm is special, compare it with other regions of the electromagnetic spectrum. Each region interacts with matter differently.

为了理解 700 nm 的特殊性，我们可以将其与电磁波谱的其他区域进行比较。每个区域与物质相互作用的方式都不同。

Type 类型	Approximate wavelength 近似波长	Interaction with matter 与物质的作用
Ultraviolet 紫外线	10 – 380 nm	Can damage DNA; causes sunburn 可损伤 DNA；导致晒伤
Blue light 蓝光	450 – 495 nm	Strongly absorbed by chlorophyll; scatters in atmosphere 被叶绿素强烈吸收；在大气中散射
Red light 红光	620 – 700 nm	Absorbed by chlorophyll and phytochrome; penetrates tissue 被叶绿素和光敏色素吸收；穿透组织
Near-infrared 近红外	700 – 1400 nm	Reflected by vegetation; used in remote sensing 被植被反射；用于遥感

Notice that 700 nm sits exactly at the boundary between red light and near-infrared. This is why it is so important in both physics and biology.

注意，700 nm 恰好位于红光和近红外的边界。这就是为什么它在物理学和生物学中都如此重要。

12. Exam Tips and Key Equations | 考试技巧与关键方程

For Edexcel IGCSE Science, you may be tested on the wave equation, the electromagnetic spectrum and photosynthesis. Here are some revision points:

对于 Edexcel IGCSE 科学，你可能会被考查波方程、电磁波谱和光合作用。以下是一些复习要点：

Convert wavelength to metres before applying v = f × λ.

在应用 v = f × λ 之前，先将波长转换为米。
Remember that red light has lower frequency and lower energy than blue light.

记住红光的频率和能量都低于蓝光。
Know the order of the electromagnetic spectrum: radio, micro, infrared, visible, ultraviolet, X-ray, gamma.

知道电磁波谱的顺序：无线电、微波、红外、可见光、紫外、X 射线、伽马。
Link 700 nm to photosystem I (P700) and photosynthesis.

将 700 nm 与光系统 I（P700）和光合作用联系起来。
Explain why plants appear green: they absorb blue and red light, but reflect green light.

解释植物为什么呈现绿色：它们吸收蓝光和红光，但反射绿光。

v = f × λ and E = h × f

v = f × λ 和 E = h × f

The number 700 nm is not just a value; it is a natural boundary where light becomes nearly infrared, a key player in photosynthesis, and a tool for medical and environmental technology. By understanding 700 nm, you are understanding how science connects the tiny world of atoms to the giant world of satellites.

数字 700 nm 不仅仅是一个数值；它是一个自然的边界，在这个边界上光几乎变为红外光，是光合作用的关键角色，也是医学和环境技术的工具。通过理解 700 nm，你正理解科学如何将原子的微观世界与卫星的宏观世界连接起来。

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The Science of 700 nm: Red Light in Physics and Biology | 700 纳米的科学：物理与生物学中的红光