📚 KS3 CIE Light Revision Questions | KS3 CIE 光学复习题
This revision guide covers the main KS3 CIE light topic with concise explanations and exam-style quick questions. Work through each section in order, attempt every question before looking at the answer, and use the tables and ray-diagram rules as a checklist before your test.
本复习指南涵盖 KS3 CIE 光学主题的主要知识点，配有简洁的解析和考试型快速问题。请按顺序逐节学习，先独立尝试每一道题，再看答案，并把文中的表格与光路图规则当作考前清单使用。
1. How Light Travels and Shadows | 光的传播与影子
Light travels in straight lines called rays. We draw rays with arrows to show the direction of travel, and this straight-line behaviour is called rectilinear propagation. A shadow forms when an opaque object blocks light, because light cannot bend around the object in normal conditions.
光沿直线传播，我们把带箭头的线称为光线，箭头表示传播方向，这种直线传播的性质叫做光的直线传播。当不透明物体挡住光时就会形成影子，因为光在通常情况下不能绕到物体后面。
A shadow has two parts when the source is large. The umbra is the fully dark inner region where no light reaches, and the penumbra is the lighter outer region where some light from the extended source still arrives.
当光源较大时，影子分为两部分。本影是完全黑暗的内部区域，没有任何光到达；半影是较亮的外部区域，仍有来自扩展光源的部分光到达。
Quick question: Why is a shadow sharper when the light source is very small?
快速问题：为什么光源很小时影子边缘更清晰？
Answer: A small source behaves almost like a point source, so rays spread out less and the edge between light and dark is sharper than with a large extended source, which produces a wide penumbra.
答案：小光源近似点光源，光线发散较少，明暗边界比大光源时更清晰；大光源会产生较宽的半影，使边缘变模糊。
2. Transparent, Translucent and Opaque Materials | 透明、半透明与不透明材料
Materials are grouped by how much light they let through. Transparent materials let most light pass through, so objects behind them can be seen clearly. Translucent materials scatter light, so objects behind them look blurred. Opaque materials do not let light pass through at all, so they form shadows.
材料按透光程度分类。透明材料让大部分光通过，所以能清楚看到后面的物体。半透明材料会散射光，所以后面的物体看起来模糊。不透明材料完全不让光通过，因此会形成影子。
| Type | 类型 | Behaviour of light | 光的表现 | Examples | 例子 |
| Transparent 透明 | Most light passes straight through 大部分光直接通过 | Clear glass, clean water 透明玻璃、清水 |
| Translucent 半透明 | Light passes but is scattered 光能通过但被散射 | Frosted glass, tissue paper 毛玻璃、薄纸 |
| Opaque 不透明 | No light passes through 光不能通过 | Wood, metal, stone 木头、金属、石头 |
Quick question: A sheet of tracing paper is placed between a lamp and a wall. Is the paper transparent, translucent or opaque?
快速问题：把一张描图纸放在灯和墙之间，这张纸是透明、半透明还是不透明？
Answer: It is translucent, because light passes through but is scattered, so a blurred patch of light appears on the wall instead of a clear image.
答案：它是半透明的，因为光能通过但被散射，所以墙上出现的是模糊光斑，而不是清晰图像。
3. The Law of Reflection | 光的反射定律
The law of reflection states that the angle of incidence equals the angle of reflection. Both angles are measured from the normal, which is an imaginary line drawn at 90° to the mirror surface at the point where the ray hits.
反射定律指出入射角等于反射角。两个角都从法线量起，法线是画在光线入射点、与镜面成90°的一条假想线。
i = r
In this equation, i is the angle of incidence and r is the angle of reflection. The incident ray, the reflected ray and the normal all lie in the same plane.
在这个公式中，i 是入射角，r 是反射角。入射光线、反射光线和法线都在同一平面内。
Quick question: A ray hits a plane mirror at 30° to the mirror surface. What is the angle of incidence?
快速问题：一条光线以与镜面成30°角射向平面镜，入射角是多少？
s 90° − 30° = 60°. The angle of incidence is 60°, and the angle of reflection is also 60°.
答案：与镜面的夹角是30°，所以与法线的夹角是 90° − 30° = 60°。入射角为60°，反射角也是60°。
A common exam trap is measuring the angle from the mirror surface. Always measure from the normal, or your answer will be the complement of the correct value.
常见失分点是把角从镜面量起。一定要从法线量起，否则你的答案会是正确答案的余角。
4. Regular and Diffuse Reflection | 镜面反射与漫反射
Regular reflection happens on a smooth surface such as a plane mirror. Parallel rays stay parallel after reflection, so a clear image is formed. Diffuse reflection happens on a rough surface such as paper or a wall. Parallel rays reflect in many different directions because the normal points in a different direction at every point on the surface.
镜面反射发生在平面镜等光滑表面上。平行光反射后仍然平行，所以能形成清晰像。漫反射发生在纸、墙等粗糙表面上。由于表面上每一点的法线方向都不同，平行光反射后射向各个方向。
Diffuse reflection is why we can see non-luminous objects such as books and tables. Light from a lamp or the Sun bounces off the rough surface and into our eyes from every direction.
漫反射正是我们能看见书本、桌子等不发光的物体的原因。来自灯或太阳的光被粗糙表面反射，从各个方向进入我们的眼睛。
Quick question: A polished metal sheet and a piece of white paper are both lit by the same lamp. Which one shows a clear reflection of the lamp?
快速问题：一块抛光金属片和一张白纸被同一盏灯照亮，哪一个能映出清晰的灯像？
Answer: The polished metal sheet, because it gives regular reflection. The paper gives diffuse reflection, so light is scattered and no clear image forms.
答案：抛光金属片，因为它发生镜面反射。白纸发生漫反射，光被散射，因此不能形成清晰像。
5. Ray Diagrams and Plane Mirror Images | 光路图与平面镜成像
A ray diagram for a plane mirror is built from two rules. First, the image is as far behind the mirror as the object is in front of it. Second, the image is virtual, the same size as the object, and laterally inverted, which means left and right are swapped.
平面镜的光路图依据两条规则绘制。第一，像在镜后的距离等于物在镜前的距离。第二，像是虚像，与物等大，并且左右相反。
hen extend the reflected rays backwards behind the mirror with dashed lines. The point where the dashed lines meet is the image position.
作图时，在物体上取一点，向镜面画两条光线。用 i = r 作出每一条反射光线，再把反射光线用虚线向镜后延长。两条虚线相交的位置就是像的位置。
Quick question: A student stands 2.0 m in front of a plane mirror. How far is the image from the student?
快速问题：一名学生站在平面镜前2.0 m处，像与学生相距多远？
behind the mirror, so the distance between the student and the image is 2.0 + 2.0 = 4.0 m.
答案：像在镜后2.0 m处，所以学生与像之间的距离是 2.0 + 2.0 = 4.0 m。
Because the image is virtual, it cannot be projected onto a screen. Light does not actually pass through the image point; our eyes only trace the reflected rays backwards.
由于是虚像，它不能呈现在光屏上。光并没有真正通过像点，我们的眼睛只是把反射光线反向延长来定位像。
6. Pinhole Cameras and Shadow Size | 针孔相机与影子大小
A pinhole camera is a dark box with a tiny hole at one end and a screen at the other. Light from the top of the object passes straight through the hole and lands at the bottom of the screen, so the image is inverted. The smaller the hole, the sharper but dimmer the image.
针孔相机是一个暗箱，一端有一个小孔，另一端是光屏。来自物体顶部的光沿直线穿过小孔，落在屏的底部，所以像是倒立的。孔越小，像越清晰，但也越暗。
The size of the image follows the similar-triangle rule, which can be written as a ratio:
像的大小遵循相似三角形规律，可以写成一个比例式：
hᵢ ÷ hₒ = v ÷ u
Here hᵢ is the image height, hₒ is the object height, v is the distance from the hole to the screen, and u is the distance from the object to the hole.
其中 hᵢ 是像高，hₒ 是物高，v 是小孔到屏的距离，u 是物体到小孔的距离。
Quick question: An object 10 cm tall is 50 cm from a pinhole, and the screen is 20 cm behind the hole. What is the image height?
快速问题：一个高10 cm的物体距小孔50 cm，屏在小孔后20 cm处，像高是多少？
Answer: hᵢ = hₒ × v ÷ u = 10 × 20 ÷ 50 = 4 cm. The image is 4 cm tall and inverted.
答案：hᵢ = hₒ × v ÷ u = 10 × 20 ÷ 50 = 4 cm。像高4 cm，并且是倒立的。
7. Refraction of Light | 光的折射
Refraction is the change in direction of light when it crosses the boundary between two materials. It happens because the speed of light is different in different materials. Light slows down when it enters a denser medium such as glass or water, and it speeds up again when it leaves.
折射是光穿过两种材料的分界面时传播方向发生改变的现象。它产生的原因是光在不同材料中的速度不同。光进入玻璃、水等较密介质时会变慢，离开时会再次变快。
When light goes from air into glass, it bends towards the normal, so the angle of refraction is smaller than the angle of incidence. When light goes from glass into air, it bends away from the normal.
光从空气射入玻璃时向法线偏折，所以折射角小于入射角。光从玻璃射入空气时偏离法线。
The relationship is described by Snell's law:
这一关系由斯涅尔定律描述：
n₁ sin θ₁ = n₂ sin θ₂
In this equation, n₁ and n₂ are the refractive indices of the two materials, and θ₁ and θ₂ are the angles measured from the normal.
式中 n₁ 和 n₂ 是两种材料的折射率，θ₁ 和 θ₂ 是从法线量起的角。
Quick question: A pencil looks bent when it is placed in a glass of water. Explain why.
快速问题：把铅笔放进一杯水中，它看起来是弯的，请解释原因。
Answer: Light reflected from the underwater part of the pencil refracts as it leaves the water and enters the air, bending away from the normal. Our brain assumes the light travelled in a straight line, so the pencil appears shifted and bent.
答案：从水下部分反射的光在离开水进入空气时发生折射，偏离法线。大脑默认光是沿直线传播的，所以铅笔看起来位置偏移并且弯折。
8. Total Internal Reflection and Optical Fibres | 全反射与光纤
Total internal reflection happens when light travels from a denser medium to a less dense medium, such as glass to air, and the angle of incidence is greater than the critical angle. At angles larger than the critical angle, no light escapes and all of it is reflected back inside the material.
全反射发生在光从较密介质射向较疏介质（如从玻璃到空气）时，且入射角大于临界角的情况。当入射角大于临界角时，没有光射出，全部光被反射回材料内部。
Two conditions must both be satisfied: light must go from denser to less dense, and the angle of incidence must exceed the critical angle. The critical angle can be found from:
必须同时满足两个条件：光从较密介质射向较疏介质，且入射角大于临界角。临界角可由下式求出：
sin C = 1 ÷ n
Optical fibres use total internal reflection to carry signals over long distances with very little loss. Diamonds sparkle for the same reason, because their critical angle is small, so light is reflected many times inside before it leaves.
光纤利用全反射把信号远距离传输，损耗非常小。钻石闪闪发光也是同样原因，因为它的临界角很小，光在射出前会在内部多次反射。
Quick question: Why does a light ray travelling along an optical fibre not escape through the sides of the fibre?
快速问题：为什么沿光纤传播的光不会从光纤侧面漏出？
Answer: The ray hits the fibre wall at an angle greater than the critical angle, so total internal reflection occurs and the ray stays inside the fibre.
答案：光线以大于临界角的角度射到光纤壁上，发生全反射，因此光线被保留在光纤内部。
9. Dispersion, Colour and Filters | 色散、颜色与滤光片
White light is a mixture of colours. When it passes through a glass prism, each colour refracts by a different amount, so the colours spread out into a spectrum. Red bends least and violet bends most, giving the order red, orange, yellow, green, blue, indigo, violet.
白光是由多种色光混合而成的。当白光通过玻璃棱镜时，各色光的折射程度不同，于是分散成光谱。红光偏折最小，紫光偏折最大，因此顺序是红、橙、黄、绿、蓝、靛、紫。
A coloured filter absorbs some colours and transmits others. A red filter transmits red light and absorbs the other colours. A coloured object looks red because it reflects red light and absorbs the rest.
彩色滤光片吸收部分色光，透过其他色光。红色滤光片透过红光，吸收其他色光。物体看起来是红色，是因为它反射红光而吸收其余色光。
The primary colours of light are red, green and blue. Mixing all three gives white light, which is why screens use these three colours.
光的三原色是红、绿、蓝。三者混合得到白光，这也是显示屏使用这三种颜色的原因。
Quick question: A red object is viewed through a green filter. What colour does it appear?
快速问题：透过绿色滤光片观察一个红色物体，它会呈现什么颜色？
Answer: It appears black, because the green filter transmits only green light, and the red object reflects only red light, so no light reaches the eye from that object.
答案：看起来是黑色，因为绿色滤光片只透过绿光，而红色物体只反射红光，所以没有光从该物体进入眼睛。
10. The Eye and Vision | 眼睛与视觉
The eye is a light detector. Light enters through the cornea and the pupil, and the lens focuses it onto the retina at the back of the eye. The retina contains rod cells, which work in dim light, and cone cells, which detect colour in bright light.
眼睛是一个光的探测器。光通过角膜和瞳孔进入，晶状体把光聚焦到眼球后部的视网膜上。视网膜上有视杆细胞和视锥细胞，视杆细胞在弱光下工作，视锥细胞在强光下辨别颜色。
The iris controls the size of the pupil, so the amount of light entering the eye can be adjusted. In bright light the pupil becomes smaller, and in dim light it becomes larger.
虹膜控制瞳孔的大小，从而调节进入眼睛的光量。强光下瞳孔变小，弱光下瞳孔变大。
Short sight means distant objects look blurred because the image forms in front of the retina, and it is corrected with a concave lens. Long sight means near objects look blurred because the image forms behind the retina, and it is corrected with a convex lens.
近视看远处模糊，因为像成在视网膜前方，用凹透镜矫正。远视看近处模糊，因为像成在视网膜后方，用凸透镜矫正。
Quick question: Why does a person see colours less clearly at night?
快速问题：为什么人在夜晚对颜色的辨别能力变差？
Answer: Cone cells, which detect colour, need bright light. In dim light only rod cells work well, and they do not distinguish colour.
答案：辨别颜色的视锥细胞需要强光。弱光下只有视杆细胞正常工作，而它们不能分辨颜色。
11. The Speed of Light | 光速
/s, which is 300 000 km every second. In glass and water it travels more slowly, and that change of speed is what causes refraction.
光是我们已知最快的物质。在真空中它约以 3.0 × 10⁸ m/s 传播，也就是每秒30万千米。在玻璃和水中传播得更慢，正是这种速度变化导致了折射。
c = 3.0 × 10⁸ m/s
Because light is so fast, we see lightning before we hear thunder. Sound travels at only about 340 m/s in air, roughly a million times more slowly than light.
由于光非常快，我们总是先看到闪电再听到雷声。声音在空气中传播速度只有约340 m/s，比光慢大约一百万倍。
Astronomers also use the light year, which is the distance light travels in one year:
天文学家还使用光年，它是光在一年中走过的距离：
1 light year ≈ 9.46 × 10¹⁵ m
Quick question: A flash of lightning is seen 6 s before the thunder is heard. Approximately how far away was the strike?
快速问题：闪电出现6秒后才听到雷声，闪电大约发生在多远的地方？
swer: Using distance = speed × time = 340 × 6 ≈ 2040 m, so the strike was about 2 km away.
答案：用距离 = 速度 × 时间 = 340 × 6 ≈ 2040 m，所以雷击大约在2 km远处。
12. Mixed Exam-Style Questions and Answers | 综合考试型练习与答案
Question 1: State the law of reflection and name the line from which the angles are measured.
第1题：写出反射定律，并说明角是从哪条线量起的。
= r, and both angles are measured from the normal, which is at 90° to the mirror surface.
答案：入射角等于反射角，即 i = r，两个角都从法线量起，法线与镜面成90°。
Question 2: A ray of light enters a glass block at an angle of incidence of 40°. Does it bend towards or away from the normal, and why?
第2题：一束光以40°入射角进入玻璃砖。它向法线还是远离法线偏折？为什么？
Answer: It bends towards the normal, because glass is denser than air and light travels more slowly inside the glass.
答案：向法线偏折，因为玻璃比空气密，光在玻璃中传播得更慢。
Question 3: Explain the difference between the umbra and the penumbra of a shadow.
第3题：解释影子中本影与半影的区别。
Answer: The umbra receives no light at all and is completely dark. The penumbra receives light from part of an extended source and is only partly dark.
答案：本影完全接收不到光，是全黑的。半影能收到扩展光源一部分发出的光，只是部分变暗。
Question 4: A light ray inside a glass block hits the glass-air boundary at 50°. The critical angle for the glass is 42°. Describe what happens to the ray.
第4题：玻璃砖内的一束光以50°射到玻璃与空气的分界面，该玻璃的临界角是42°。请描述光线的去向。
Answer: The angle of incidence is greater than the critical angle, and the light is travelling from a denser to a less dense medium, so total internal reflection occurs and the ray is reflected back into the glass.
答案：入射角大于临界角，且光从较密介质射向较疏介质，因此发生全反射，光线被反射回玻璃中。
Question 5: Why does a red shirt look black in green light?
第5题：为什么红色衬衫在绿光下看起来是黑色的？
Answer: The shirt reflects only red light and absorbs other colours. Green light contains no red light, so almost no light is reflected into the eye and the shirt appears black.
答案：这件衬衫只反射红光，吸收其他色光。绿光中不含红光，所以几乎没有光反射进眼睛，衬衫看起来是黑色。
Final tip: In KS3 CIE light questions, always draw the normal as a dashed line, label the angles clearly, and write the equation before substituting numbers. Marks are often given for the correct method even when the final value is wrong.
最后提示：在 KS3 CIE 光学题中，一定要用虚线画出法线，清楚标注角度，并先写公式再代入数据。即使最后结果算错，正确的解题方法通常也能得分。
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