电磁波谱
爱德思国际普通中等教育证书（IGCSE）物理· 3(c) 3.10–3.13· 15 分钟阅读
1. 电磁波谱的核心性质★☆☆☆☆⏱ 3 min
所有电磁波（EM）都属于横波，它们共有一个普适性质：在真空（自由空间）中的传播速度均约为3 × 10⁸ m/s。可见光只是这个范围极广的连续光谱中一个很小的窄波段。
电磁波谱
所有都能以光速在真空中传播的横波连续谱系，根据波长和频率可划分为7个主要波段。
写出所有电磁波在真空中传播时共有的一项性质。
- 1
回忆电磁波在真空中共有的核心性质
- 2
所有电磁波在真空中的传播速度都相同（约3 × 10⁸ m/s），这是电磁波谱所有波段共有的唯一性质。
Exam tip:
这是本知识点最常考的1分小题之一：答题时一定要说明是在真空中传播速度相同，除非题目明确要求写出波的类型，否则不要只回答“它们是横波”。
2. 电磁波谱的排序★★☆☆☆⏱ 4 min
电磁波谱按照两个关联的性质排序：频率升高（能量也随之升高）与波长减小直接对应。从最低频率/最长波长到最高频率/最短波长的7个波段依次为：无线电波、微波、红外线、可见光、紫外线、X射线、伽马射线。
可见光部分也遵循同样的规律：红光的波长最长、频率最低，之后依次是橙、黄、绿、蓝、靛、紫（助记词ROYGBIV），其中紫光在可见光中波长最短、频率最高。
将以下电磁辐射按波长递增的顺序排列：紫外线、无线电波、红外线、伽马射线。
- 1
回忆从低频到高频的排列顺序（该顺序对应波长从高到低）
- 2
将频率排序反转即可得到波长递增的顺序：波长最短的伽马射线排在最前，之后是紫外线、红外线，波长最长的无线电波排在最后
- 3
最终排序：伽马射线、紫外线、红外线、无线电波
Exam tip:
答题时务必确认题目要求的是按波长/频率递增还是递减排序：将二者混淆是非常常见的失分错误。
3. 电磁辐射的用途★★☆☆☆⏱ 4 min
电磁波谱的每个波段都有独特的性质，使其适用于特定场景。在“解释类”考题中，你必须将波的性质与其用途关联起来才能拿到满分。
EM Band | Key Property | Common Uses |
|---|---|---|
Radio waves | Long wavelength, diffracts easily around obstacles | Broadcasting, terrestrial communications |
Microwaves | Passes through atmosphere, absorbed by water molecules | Cooking food, satellite transmission |
Infrared | Emitted by warm objects, transfers thermal energy | Electric heaters, night vision equipment |
Visible light | Detected by eyes, transmitted through glass | Optical fibre communications, photography |
Ultraviolet | Causes fluorescent materials to emit visible light | Fluorescent lamps, security marking |
X-rays | Passes through soft tissue, absorbed by dense material (bone) | Medical imaging, airport security scanners |
Gamma rays | Kills microorganisms and living cells | Sterilisation of medical equipment, food pasteurisation |
解释为什么微波被用于烹饪食物。
- 1
找出微波与加热作用相关的对应特性
- 2
微波会被所有食物中都含有的水分子吸收
- 3
这种吸收过程会将能量传递给水，从内到外快速均匀地加热食物。
4. 电磁辐射的危害与防护措施★★★☆☆⏱ 4 min
过度暴露在电磁辐射下会引发一系列有害影响，通常频率越高（能量越高）的辐射造成的损伤越严重。备考时你需要将每种危害对应到正确的电磁辐射波段，同时掌握基础的防护方法。
EM Band | Hazard of Excessive Exposure | Protective Measure |
|---|---|---|
Microwaves | Internal heating of body tissues, damaging cells | Shielding microwave oven doors with metal mesh to contain radiation |
Infrared | Skin burns from overexposure to thermal energy | Wearing heat-resistant clothing when working with industrial heaters |
Ultraviolet | Damage to surface skin cells (skin cancer) and permanent eye damage/blindness | Wearing high-factor sunscreen, UV-blocking sunglasses, and covering skin in strong sunlight |
Gamma rays | Mutation of DNA, leading to cancer or genetic damage | Using lead shielding, thick concrete barriers, and increasing distance from the radiation source |
医院放射科工作人员每日操作X光机。描述一种他们可以用来减少辐射暴露的防护措施，并解释其工作原理。
- 1
确定适用于X射线的防护措施：铅衣
- 2
解释原理：X射线是高能电磁波，会被铅这类高密度材料吸收
- 3
铅衣可以阻挡X射线接触工作人员的身体，避免细胞受到损伤。
Exam tip:
切勿混淆不同辐射的危害：例如，紫外线会造成表层细胞损伤，而非内部加热；伽马射线会引发DNA突变，而不仅仅是皮肤灼伤。作答时务必将危害与对应的正确波段直接关联。
5. 常见陷阱
错误做法:
Stating that EM waves travel at different speeds in a vacuum
原因:
All EM waves travel at exactly the same speed (~3 × 10⁸ m/s) in free space, regardless of frequency or wavelength
正确做法:
Always state that speed is identical in a vacuum as a core property of EM radiation
错误做法:
Mixing up the order of EM spectrum by frequency and wavelength
原因:
Examiners often ask for order in one direction, and reversing it will lose all marks for the question
正确做法:
Use the Rabbits Mate In Very Unusual eXpensive Gardens mnemonic to recall the low frequency → high frequency order, then reverse it if asked for longest to shortest wavelength
错误做法:
Attributing the wrong use to an EM band, e.g. saying X-rays are used for sterilising equipment
原因:
Uses are strictly tied to each band's unique properties, and incorrect matches get no marks
正确做法:
Learn the property-use pairings from the specification list to avoid mix-ups
错误做法:
Describing ionisation effects as a hazard of UV radiation
原因:
Only X-rays and gamma rays are classed as ionising radiation for this specification; UV causes surface cell damage, not DNA mutation from ionisation
正确做法:
Link UV hazards only to surface skin cell damage and eye damage, not cancer from ionisation (reserved for gamma/X-rays)
错误做法:
Forgetting to link a property to a use/hazard in 'explain' questions
原因:
2/3 mark questions require a reason, not just a statement of use/hazard, to get full marks
正确做法:
Always add the relevant property, e.g. 'microwaves heat food because they are absorbed by water molecules' not just 'microwaves are used to cook food'
6. 速查表
Category | Key Details |
|---|---|
Core Property | All EM waves travel at 3 × 10⁸ m/s in free space, are transverse |
EM Order (low f → high f) | Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma |
Visible Order (low f → high f) | Red → Orange → Yellow → Green → Blue → Indigo → Violet |
Top 3 Exam Uses | Microwaves: cooking/satellite; X-rays: medical imaging; Gamma: sterilisation |
Top 3 Exam Hazards | UV: skin damage/blindness; Gamma: DNA mutation/cancer; Microwaves: internal heating |
7. 常见问题
所有电磁波在空气中的传播速度都相同吗？
从考试答题角度来说是肯定的：电磁波在空气中的传播速度与在真空中的速度（约3 × 10⁸ m/s）几乎完全一致，除非题目额外说明，否则二者视为相等。
电磁波谱的能量沿哪个方向递增？
能量随频率升高而增大，因此从无线电波（能量最低）到伽马射线（能量最高）能量依次递增。
我需要记住每一类电磁波的精确波长数值吗？
不需要：你只需要牢记不同波段按波长/频率排列的顺序，无需记忆每个波段对应的具体数值。
深入阅读
下一步
Now that you have mastered the core properties, order, uses and hazards of the electromagnetic spectrum, you are ready to move on to related wave topics in the Edexcel IGCSE Physics specification. You can apply your knowledge of EM waves to the next subtopic on light and total internal reflection, which covers how visible light is used in optical fibres in more detail. You should also revisit general wave properties to practice applying the wave equation v = fλ to EM wave calculations, which are commonly tested alongside spectrum questions. Finally, make sure you practice past paper questions on this topic to familiarise yourself with exam phrasing, as questions often ask you to link properties to uses or hazards for 2-3 mark explain questions.