OCR Science: Light – Exam-Focused Revision Notes | OCR 科学：光考点精讲

📚 OCR Science: Light – Exam-Focused Revision Notes | OCR 科学：光考点精讲

Light is a fundamental topic in OCR Science, bridging wave physics, optics and real‑world applications. In your exam you can expect to show understanding of reflection, refraction, total internal reflection, colour formation and lens behaviour. This guide breaks down every key concept with clear explanations, diagrams described in words, and the exact formulas you need to apply, all target-marked for OCR success.

光是 OCR 科学中的核心专题之一，连接着波动物理学、光学和现实应用。考试中你需要准确掌握反射、折射、全内反射、色彩形成以及透镜工作规律。这份考点精讲将所有关键概念拆解为清晰易懂的解释，配合语言描述的光路图与 OCR 要求的必备公式，助你高效备考。

1. What is Light? | 光是什么？

Light is a transverse electromagnetic wave that can travel through a vacuum at a constant speed of 3.0 × 10⁸ m/s. In OCR Science you will also use the wave equation v = fλ when studying its connection with frequency and wavelength. Light travels in straight lines in a uniform medium, which explains how sharp shadows are formed.

光是一种横电磁波，能够在真空中以恒定速度 3.0 × 10⁸ m/s 传播。在 OCR 科学中，你还会用到波动方程 v = fλ 来分析频率和波长的关系。光在均匀介质中沿直线传播，这就是尖锐影子形成的原因。

2. Reflection | 反射

When a ray of light hits a smooth surface, it obeys the law of reflection: the angle of incidence (θᵢ) equals the angle of reflection (θᵣ), both measured from the normal. A plane (flat) mirror produces a virtual, upright and laterally inverted image. The image distance behind the mirror is exactly equal to the object distance in front of it.

当光线射向光滑表面时，遵循反射定律：入射角（θᵢ）等于反射角（θᵣ），两角均从法线量起。平面镜形成的像为虚像、正立且左右颠倒；像在镜后的距离等于物在镜前的距离。

Specular reflection occurs from a polished surface, giving a clear reflection. Diffuse reflection from a rough surface scatters light in many directions, which is why most objects are visible from any viewing angle.

镜面反射发生在抛光表面上，产生清晰的反射图像。粗糙表面发生漫反射，光线向各个方向散射，这就是大多数物体从任意角度都能被看见的原因。

3. Refraction | 折射

Refraction is the change in direction of light when it passes from one transparent medium to another, caused by a change in wave speed. The refractive index n of a material is defined as the ratio of the speed of light in vacuum to the speed in the material. Snell’s law links the angles and refractive indices:

折射是光线从一种透明介质进入另一种介质时方向改变的现象，由波速变化引起。介质的折射率 n 定义为光在真空中的速度与光在介质中速度的比值。斯涅尔定律将入射角、折射角与折射率联系起来：

n₁ sin θ₁ = n₂ sin θ₂

When light enters a denser medium (higher n), it bends towards the normal; when it enters a less dense medium, it bends away from the normal.

当光进入光密介质（n 较大）时，光线向法线偏折；当光进入光疏介质时，光线偏离法线。

4. Total Internal Reflection | 全内反射

Total internal reflection (TIR) occurs when light travelling in a denser medium hits the boundary with a less dense medium at an angle of incidence greater than the critical angle C. At the critical angle the angle of refraction is 90°, leading to the relation:

全内反射（TIR）发生在光从光密介质射向光疏介质、且入射角大于临界角 C 的情况下。在临界角时，折射角为 90°，由此得到关系式：

sin C = 1 / n

(这里 n 为光密介质相对于外部光疏介质的折射率，通常外部是空气，折射率 ≈ 1。)

TIR is used in optical fibres for high‑speed communication and in endoscopes for medical imaging. The critical angle for glass to air is around 42°, which explains why diamonds (high n) sparkle so much.

全内反射广泛应用于光纤高速通信和医用内窥镜成像。玻璃到空气的临界角约为 42°，这也解释了为何钻石（n 很大）格外闪耀。

5. Dispersion of Light | 光的色散

Dispersion is the splitting of white light into its constituent colours when it passes through a glass prism. This happens because the refractive index of glass varies slightly with the wavelength of light: violet light (shorter wavelength) slows down more and bends more than red light. The visible spectrum follows the order red, orange, yellow, green, blue, indigo, violet.

色散是白光通过玻璃棱镜时被分解为其组成色光的现象。这是因为玻璃的折射率随波长不同而略有变化：波长较短的紫光减速更多，偏折也最大。可见光谱按红、橙、黄、绿、蓝、靛、紫的顺序排列。

A rainbow is a natural example of dispersion caused by water droplets. Once separated, a pure spectral colour cannot be further dispersed by a second prism.

彩虹是由水滴引起色散的自然实例。被分离出来的单色光再经过第二个棱镜时不会进一步色散。

6. Colour and Filters | 颜色与滤光片

White light is composed of three additive primary colours: red, green and blue. By combining these in different intensities, all other colours can be produced on screens. This is called additive colour mixing.

白光由三种加色原色组成：红、绿、蓝。用不同强度的光混合，就能在屏幕上产生各种颜色，这就是加色混合。

A filter transmits only certain colours and absorbs the rest. For example, a red filter only lets through red light and absorbs green and blue. The colour an object appears depends on which wavelengths it reflects (for opaque objects) or transmits (for transparent objects) to the eye.

滤光片只允许特定颜色的光透过而吸收其余色光。例如，红色滤光片只让红光通过，吸收绿光和蓝光。不透明物体呈现的颜色取决于它反射哪些波长的光，透明物体则取决于透过的光。

7. Convex Lenses | 凸透镜

A convex (converging) lens is thicker in the middle and brings parallel rays of light to a point called the principal focus. The distance from the lens centre to the focus is the focal length f. A convex lens can form both real and virtual images depending on the object distance u.

凸透镜（会聚透镜）中间厚、边缘薄，能将平行光线汇聚到一点，该点称为主焦点。从透镜中心到焦点的距离就是焦距 f。凸透镜成像性质取决于物距 u，既可成实像，也可成虚像。

Key ray diagram rules for convex lenses: (1) a ray parallel to the principal axis passes through the focus on the other side; (2) a ray passing through the centre of the lens continues straight; (3) a ray passing through the focus on the way to the lens emerges parallel to the axis.

凸透镜光路图的三条特殊光线：（1）平行于主轴的光线经透镜后穿过另一侧焦点；（2）通过透镜中心的光线沿直线前进；（3）朝向焦点的光线经透镜后平行于主轴射出。

8. Lens Equation and Magnification (Higher Tier) | 透镜方程与放大率（高阶内容）

For OCR Higher Tier, you need to use the lens equation and magnification formula. With object distance u, image distance v and focal length f, the lens equation is:

OCR 高阶考生需要掌握透镜方程和放大率公式。设物距为 u，像距为 v，焦距为 f，透镜方程为：

1/f = 1/u + 1/v

Sign convention: real images have positive v, virtual images have negative v. The magnification m is given by:

符号约定：实像 v 取正，虚像 v 取负。放大率 m 由下式给出：

m = image height / object height = v / u

Knowing how to calculate image distance and magnification allows you to predict whether the image is enlarged, diminished or the same size, and whether it is upright or inverted.

能够计算像距和放大率，你就能预测像是放大、缩小还是等大，以及是正立还是倒立。

9. The Eye and Vision Defects | 眼睛与视力缺陷

The eye contains a convex lens system (cornea and crystalline lens) that focuses light onto the retina, forming a real, inverted image. The eye adjusts for different object distances by changing the shape of the lens, a process called accommodation.

眼睛包含凸透镜系统（角膜和晶状体），将光线聚焦到视网膜上，形成倒立实像。眼睛通过改变晶状体形状来调节对不同距离物体的聚焦，这一过程称为调节。

Short sight (myopia) occurs when the eyeball is too long or the lens is too powerful, causing the image to focus in front of the retina. It is corrected with a concave (diverging) lens. Long sight (hyperopia) occurs when the eyeball is too short or the lens too weak, and is corrected with a convex lens.

近视（myopia）是由于眼球过长或晶状体屈光力过强导致像落在视网膜前方，需用凹透镜矫正。远视（hyperopia）是由于眼球过短或晶状体屈光力过弱，需用凸透镜矫正。

10. Applications and Exam Tips | 应用与考试技巧

Light principles appear in many exam contexts. TIR is vital for fibre‑optic communications: the signal travels by repeated total internal reflections, minimising loss. Endoscopes use bundles of fibres to produce images inside the body. Camera lenses and projectors form real images on film or screens using convex lenses; magnifying glasses produce enlarged virtual images when the object is placed inside the focal point.

光的原理在许多考题情境中出现。全内反射是光纤通信的关键：信号通过多次全内反射传播，损耗极小。内窥镜用光纤束传像。照相机和投影仪利用凸透镜在胶片或屏幕上生成实像；放大镜则是在物体放在焦点以内时形成放大的虚像。

For your OCR exam, practise drawing ray diagrams neatly with a ruler, label the normal clearly, and show all angles. Memorise the formulas for reflection, refraction, TIR and lenses, and be ready to describe colour mixing and filter effects using the terms ‘absorb’ and ‘transmit’.

备考 OCR 时，务必用尺规整齐绘制光路图，清晰标出法线并注明角度。熟记反射、折射、全内反射和透镜的公式，并学会用“吸收”和“透过”等关键词准确描述颜色混合及滤光片效果。

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OCR Science: Light – Exam-Focused Revision Notes | OCR 科学：光 考点精讲