📚 Refraction of Light | 光的折射
When light travels from one transparent material to another, its speed changes. If it enters at an angle, the light bends. This bending is called refraction. Understanding refraction helps explain how lenses, eyes and prisms work.
当光从一种透明材料进入另一种透明材料时，速度会发生变化。如果光以一定角度入射，就会发生弯曲。这种弯曲现象叫做折射。理解折射有助于解释透镜、眼睛和棱镜的工作原理。
1. What is Refraction? | 什么是折射？
Refraction is the bending of light when it travels from one transparent medium into another at an angle. The bending occurs because the speed of light changes as it enters the new medium.
折射是光从一种透明介质斜射入另一种透明介质时发生的弯曲现象。光进入新介质时速度发生变化，因此传播方向发生偏折。
If light enters a new medium along the normal, meaning at 0 degrees to the normal, it does not bend. It only changes speed and wavelength.
如果光沿法线方向进入新介质，也就是与法线夹角为0度，光不会弯曲，只会改变速度和波长。
A common example is a straw placed in a glass of water. The straw looks bent or broken at the water surface because light from the submerged part changes direction as it leaves the water.
一个常见例子是放在水杯中的吸管。吸管在水面处看起来弯曲或折断，这是因为来自水下部分的光离开水时改变了方向。
2. The Normal Line and Angles | 法线与角度
To describe refraction accurately, we draw a dashed line called the normal. The normal is perpendicular to the boundary between the two media at the point where the light ray hits.
为了准确描述折射，我们画一条虚线，叫做法线。法线在光线入射点处垂直于两种介质的分界面。
The angle of incidence i is the angle between the incident ray and the normal. The angle of refraction r is the angle between the refracted ray and the normal. Both angles are always measured from the normal, never from the surface.
入射角 i 是入射光线与法线之间的夹角。折射角 r 是折射光线与法线之间的夹角。两个角都必须从法线量起，绝不能从界面量起。
- Incident ray: the incoming light ray. | 入射光线：射向界面的光线。
- Refracted ray: the light ray after it has changed direction. | 折射光线：改变方向后的光线。
- Normal: the reference line at 90 degrees to the surface. | 法线：与界面成90度的参考线。
3. Speed of Light in Different Media | 光在不同介质中的速度
m/s in a vacuum. In air its speed is almost the same, but in water and glass it is slower.
光传播得非常快：在真空中约为 3.00 × 10⁸ m/s。在空气中速度几乎相同，但在水和玻璃中会更慢。
| Medium | 介质 | Approximate speed | 近似速度 |
|---|---|
| Vacuum | 真空 | 3.00 × 10⁸ m/s |
| Air | 空气 | Very close to 3.00 × 10⁸ m/s |
| Water | 水 | About 2.25 × 10⁸ m/s |
| Glass | 玻璃 | About 2.00 × 10⁸ m/s |
The greater the change in speed, the greater the change in direction. The frequency of light does not change during refraction, but the wavelength does.
速度变化越大，方向变化也越大。光的频率在折射过程中不变，但波长会改变。
4. Refraction from Air to Glass or Water | 从空气进入玻璃或水
When light travels from air into glass or water, it slows down. If it enters at an angle, the ray bends towards the normal.
当光从空气进入玻璃或水时，速度减慢。如果斜射入，光线会向法线偏折。
This means the angle of refraction r is smaller than the angle of incidence i. We can write this relationship in the centre below.
这意味着折射角 r 小于入射角 i。我们可以把这个关系写在下面居中位置。
r < i
A swimming pool appears shallower than it really is because light from the bottom bends away from the normal as it leaves the water, making the bottom look higher.
游泳池看起来比实际浅，因为来自池底的光离开水面时偏离法线，使池底看起来升高了。
5. Refraction from Glass or Water to Air | 从玻璃或水进入空气
When light travels from glass or water into air, it speeds up. The ray bends away from the normal.
当光从玻璃或水进入空气时，速度加快。光线会偏离法线。
In this case the angle of refraction r is larger than the angle of incidence i.
在这种情况下，折射角 r 大于入射角 i。
r > i
This is why a fish seen from above the water is not exactly where it appears. The light rays coming from the fish bend away from the normal when they leave the water, so the fish appears closer to the surface.
这就是为什么从水面上方看到的鱼并不在它看起来的位置。来自鱼的光线离开水面时偏离法线，所以鱼看起来更接近水面。
6. Optically Dense vs Optically Less Dense | 光密与光疏介质
Materials can be described as optically dense or optically less dense. An optically dense medium is one in which light travels more slowly.
材料可以描述为光密或光疏。光密介质是光在其中传播较慢的介质。
Glass is optically denser than water, and water is optically denser than air. Optical density is not the same as physical density, although the two are sometimes related.
玻璃的光密程度大于水，水的光密程度大于空气。光密程度与物理密度并不相同，尽管两者有时相关。
A simple rule: from optically less dense to optically more dense, light bends towards the normal; from optically more dense to optically less dense, light bends away from the normal.
简单规律：从光疏到光密，光向法线偏折；从光密到光疏，光偏离法线。
7. Apparent Depth and Everyday Effects | 视深与日常现象
Refraction creates several everyday effects. An object underwater appears at a shallower depth than its real depth. This is called apparent depth.
折射会产生许多日常现象。水下的物体看起来比实际深度浅，这叫做视深。
A pencil placed in a glass of water looks bent at the water surface. A straw in a drink shows the same effect.
放在水杯里的铅笔在水面处看起来弯曲。饮料中的吸管也有同样的现象。
During sunrise and sunset, the Sun is actually below the horizon, but atmospheric refraction bends its light around the Earth, so we can see it before it has truly risen or after it has truly set.
在日出和日落时，太阳实际上位于地平线以下，但大气折射使太阳光绕地球弯曲，因此我们能在太阳真正升起之前或真正落下之后看到它。
8. Refraction in Lenses | 透镜中的折射
Lenses use refraction to change the direction of light rays. A converging lens, also called a convex lens, is thicker in the middle and brings parallel rays together at a focus.
透镜利用折射改变光线的方向。凸透镜中间较厚，能把平行光线会聚到焦点。
A diverging lens, also called a concave lens, is thinner in the middle and spreads parallel rays apart.
凹透镜中间较薄，能把平行光线发散开来。
The human eye contains a lens that refracts light to form a clear image on the retina. Glasses and contact lenses use refraction to correct focusing problems.
人眼包含一个晶状体，它通过折射光线在视网膜上形成清晰的像。眼镜和隐形眼镜利用折射来矫正聚焦问题。
9. Dispersion and the Prism | 色散与棱镜
White light is a mixture of colours. When white light passes through a triangular glass prism, it is refracted twice and splits into a spectrum of colours.
白光是多种颜色的混合。当白光通过三角形玻璃棱镜时，经过两次折射后分散成光谱。
The spectrum colours are red, orange, yellow, green, blue, indigo and violet. Red light is refracted the least, while violet light is refracted the most.
光谱颜色为红、橙、黄、绿、蓝、靛、紫。红光折射最少，紫光折射最多。
This separation of white light into colours is called dispersion. Rainbows are formed when sunlight is refracted, reflected and dispersed by water droplets in the air.
白光分散成各种颜色的现象叫做色散。彩虹是太阳光被空气中的水滴折射、反射和色散后形成的。
10. Total Internal Reflection: An Introduction | 全内反射：简介
When light travels from an optically denser medium into an optically less dense medium, such as from glass into air, there is a special angle called the critical angle.
当光从光密介质进入光疏介质时，例如从玻璃进入空气，存在一个特殊的角度，叫做临界角。
If the angle of incidence is greater than the critical angle, all the light is reflected back into the denser medium. This is called total internal reflection.
如果入射角大于临界角，所有光线都会被反射回光密介质。这叫做全内反射。
Total internal reflection is used in optical fibres to carry light signals over long distances. It also makes diamonds sparkle because light bounces inside them many times before escaping.
全内反射用于光纤中长距离传输光信号。它也使钻石闪闪发光，因为光在钻石内部多次反射后才逃出。
11. Key Points and Exam Tips | 考点与应试技巧
When drawing refraction diagrams, always include the boundary, the normal, arrows on the rays, and clearly label the angles i and r. Measure angles from the normal.
画折射光路图时，必须画出分界面、法线、光线箭头，并清楚标出角度 i 和 r。角度要从法线量起。
Use key words such as refraction, normal, incident ray, refracted ray, optically dense and optically less dense in written answers. Avoid saying light bounces unless you mean reflection.
在书面作答中要用关键词，如折射、法线、入射光线、折射光线、光密和光疏。除非指反射，否则避免说光弹回。
Remember the quick rule: air to glass or water - towards the normal; glass or water to air - away from the normal.
记住快速口诀：空气到玻璃或水——向法线偏折；玻璃或水到空气——偏离法线。
Published by TutorHao | Physics Revision Series | aleveler.com
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