Overview
What you’ll learn
- Use the ray model to represent the path of light.
- Describe images in a plane mirror, and state that the angle of reflection equals the angle of incidence, measured from the normal.
- Explain regular reflection at a smooth surface and diffuse reflection at a rough surface.
- Describe uses of plane and curved reflecting surfaces.
- Describe refraction as a change in the speed of light in a different medium, and dispersion of white light by a prism, without calculating angles.
- State one benefit and one harm of visible light or other EM radiation, including light pollution.
In 30 seconds
- 1A ray is a model: a straight line with an arrow showing the direction light travels. Light travels in straight lines in one material.
- 2The normal is a line at 90° to the surface. Angles of incidence and reflection are measured from the normal, not from the mirror.
- 3For a plane mirror, angle of incidence = angle of reflection. The image is virtual, upright, same size, laterally inverted, and as far behind the mirror as the object is in front.
- 4A smooth surface reflects rays in one direction (a clear image). A rough surface scatters them (no clear image).
- 5Refraction happens because light changes speed when it enters a different medium, so the ray bends. No angle calculation is required.
- 6A prism can split white light into a spectrum. Visible light and other EM radiation, such as infrared and ultraviolet, can help or harm.
Notes · 1 of 5
The ray model
We draw light as rays because the real path is invisible and the model is good enough to predict what we see.
A ray is not a photograph of light. It is a model: a straight line with an arrow. In one even material, such as air, the ray is straight. The arrow shows the direction from the source towards the object or the eye.
- Draw rays with a ruler. A freehand wobble is not the model.
- Put an arrow on every ray.
- The normal is drawn as a dotted line at 90° to the surface, at the point where the ray hits.
Notes · 2 of 5
Reflection and the plane mirror
The incoming and outgoing angles match when you measure them from the normal.
The incident ray hits the surface. The reflected ray leaves it. The angle between the incident ray and the normal is the angle of incidence. The angle between the reflected ray and the normal is the angle of reflection. Those two angles are equal.
- Virtual
- The rays do not really meet behind the mirror. They only appear to come from there, so the image cannot be caught on a screen.
- Upright and same size
- The image is the right way up and the same size as the object.
- Laterally inverted
- Left and right appear swapped. A word written on paper looks reversed in a mirror.
- Same distance
- The image is as far behind the mirror as the object is in front.
Notes · 3 of 5
Smooth, rough and curved surfaces
The law still holds at each tiny patch. The surface shape decides where the reflected rays go.
On a smooth surface the normals are parallel, so parallel rays reflect in one direction. That is regular reflection, and you see a clear image. On a rough surface the normals point in different directions, so the rays scatter. That is diffuse reflection. You still see the object, because light reaches your eye, but there is no sharp mirror image.
| Surface | What the rays do | A use |
|---|---|---|
| Plane mirror | Image as far behind as the object is in front | Bathroom mirror, periscope |
| Concave (caves in) | Can bring rays together | Torch reflector, makeup mirror used close up |
| Convex (bulges out) | Spreads rays and shows a wider view | Car wing mirror, shop security mirror |
A concave makeup mirror can make a face look larger when you are close to it. A convex wing mirror shows more of the road, and the image is smaller. You do not need to calculate a focal length in this chapter.
Notes · 4 of 5
Refraction and dispersion
Light changes speed in a new medium, so the ray can bend. A prism spreads white light into colours.
Refraction is the bending of a ray when it passes from one medium into another, because its speed changes. Entering glass or water from air, light slows down and the ray bends towards the normal. Leaving into air, it speeds up and bends away from the normal. If the ray hits along the normal, it does not bend. Angle calculations are not required.
- A straw in a glass of water looks bent at the surface.
- A pool looks shallower than it is, because rays from the bottom bend as they leave the water.
- Lenses bend light to form images in spectacles and magnifying glasses.
A prism can split white light into a spectrum (red, orange, yellow, green, blue, indigo, violet). This is dispersion. Different colours change speed by different amounts, so they bend by different amounts. Red bends the least and violet the most.
Notes · 5 of 5
Uses, harm and light pollution
Visible light is one part of EM radiation. The same technology that helps at night can waste energy and confuse wildlife.
| Radiation | A benefit | A harm |
|---|---|---|
| Visible light | Seeing, photosynthesis | Glare; city lights can waste energy |
| Infrared | Heat lamps, remote controls, thermal imaging | Intense infrared can burn skin |
| Ultraviolet | Used to kill some germs; skin can make vitamin D in sunlight | Sunburn and eye damage; overexposure is a cancer risk |
The syllabus accepts the abbreviation EM. You do not have to spell “electromagnetic” in full. Bright city lights help people see at night, but they also use a lot of electrical energy, wash out the night sky, and can disorient birds. Shielding lamps so light goes downwards is one way to keep the benefit and cut the harm.
Revise
Key terms
The words examiners expect you to use precisely.
- Ray
- A straight line with an arrow used as a model for the path of light.
- Normal
- A line at 90° to a surface, drawn where a ray hits the surface. Angles are measured from it.
- Angle of incidence
- The angle between the incident ray and the normal.
- Angle of reflection
- The angle between the reflected ray and the normal. It equals the angle of incidence.
- Virtual image
- An image that rays only appear to come from. It cannot be formed on a screen.
- Regular reflection
- Reflection from a smooth surface, where parallel rays stay parallel and a clear image is seen.
Show 3 more key termsShow fewer key terms
- Diffuse reflection
- Reflection from a rough surface, where rays scatter and no clear image is seen.
- Refraction
- The bending of a light ray when its speed changes as it enters a different medium.
- Dispersion
- The splitting of white light into a spectrum, for example by a prism.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Measuring the angle of incidence from the mirror surface.
The defined angle is between the ray and the normal.
Instead: Draw the normal at 90° to the mirror, then measure both angles from that line.
Mistake: Saying a plane-mirror image is real because you can see it.
Seeing an image does not mean the rays meet there.
Instead: The image is virtual. Rays only appear to come from behind the mirror.
Show 4 more mistakesShow fewer mistakes
Mistake: Thinking a rough surface does not reflect light.
You can see a rough wall, so light is reflected from it.
Instead: A rough surface reflects diffusely. The rays scatter, so there is no clear image.
Mistake: Saying refraction happens because light “bounces off” the water.
Bouncing is reflection. Refraction is a ray that enters a new medium and changes speed.
Instead: Light slows down in water or glass and bends towards the normal.
Mistake: Swapping concave and convex.
They curve opposite ways and send rays to different places.
Instead: A concave surface caves in and can bring rays together. A convex surface bulges out and gives a wider view.
Mistake: Claiming all colours bend by the same amount in a prism.
Dispersion happens because they bend differently.
Instead: Violet bends more than red. That spread is the spectrum.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Use the ray model
What does a ray in a diagram represent?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 43 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 5 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 5.
Mark scheme
PROChapter 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
- Term 2Practice paperWeighted Assessment 2 (WA2)Open the paper
- Term 3Coming soonWeighted Assessment 3 (WA3)
- Term 4Practice paperEnd-of-Year Examination (EOY)Open the paper
Questions
Frequently asked questions
What is the ray model of light in Sec 1 Science?
It represents the path of light as a straight line with an arrow. You use it to draw reflection and, without calculating angles, to describe refraction.
How do I measure the angle of incidence?
Draw the normal at 90° to the surface where the ray hits. The angle of incidence is between the incoming ray and the normal, not between the ray and the mirror.
What is the difference between reflection and refraction?
Reflection is light bouncing off a surface, with the angle of incidence equal to the angle of reflection. Refraction is light bending because its speed changes when it enters a different medium, such as air into water.
What image does a plane mirror form?
A virtual, upright image, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front. It cannot be formed on a screen.
Is light tested in the Sec 1 EOY?
Many schools teach the ray model in Term 3 and test it in WA3 and the end-of-year exam. Angle-of-refraction calculations are not required.
Up next
Chapter 6: Model of Cells, the Basic Unit of Life