The shape of the lesson
By the end of the lesson, learners can
- Bounce a ray off the mirror: Change the angle of the incoming ray and watch the reflected ray and the angles from the normal.
- The image behind the glass: The law fixes the angles; the image in a plane mirror always has the same set of properties.
- Angles from the normal. If a question gives an angle "to the surface" or "to the mirror", subtract it from 90 before doing anything else.
- Exact image words. A plane mirror image is virtual, upright, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front. Memorise that sentence word for word.
One picture every idea returns to
The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal. The image in a plane mirror is virtual, upright, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Angle in, angle out.. A mirror does something wonderfully predictable: whatever angle a ray of light arrives at, it leaves at the very same angle on the other side of an imaginary line. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Bounce a ray off the mirror | Change the angle of the incoming ray and watch the reflected ray and the angles from the normal. Project tab 1 of the lab (Reflection) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: The image behind the glass | The law fixes the angles; the image in a plane mirror always has the same set of properties. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (7 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The image behind the glass, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Always measure the angles from the normal, the line perpendicular to the mirror, not from the mirror surface. | Measuring from the surface is the single most common reason ray diagrams lose marks. |
Reflection of light: worksheet
7 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
State the law of reflection.
A ray of light strikes a plane mirror at an angle of 32° to the mirror surface. Calculate the angle of reflection, measured from the normal.
State four characteristics of the image formed by an object placed in front of a plane mirror.
A student writes that the image in a plane mirror is "reversed and the same size." Explain why this answer would not gain full marks, and give the correct wording.
A person stands 1.5 m in front of a large plane mirror.
(a) How far behind the mirror does their image appear? [1] (b) What is the total distance between the person and their image? [1]The diagram shows a ray of light striking a plane mirror. The normal is drawn at the point where the ray hits. The angle between the incident ray and the normal is 50°.
The surface of a calm lake acts like a mirror, giving a clear reflection of the trees on the far bank. On a windy day, the same lake gives only a blurred, broken reflection.
Explain, in terms of reflection, why the reflection is clear when the water is calm but blurred when it is rough.
Answer key and marking notes
The angle of incidence is equal to the angle of reflection, both measured from the normal. ✓
"From the normal" is part of the law. Leaving it out can cost the mark.
Convert to the normal:
i = 90° − 32° = 58°Law of reflection:
r = i = 58°angle of reflection = 58°
The 32° is to the surface. Always convert first. Writing 32° is the classic error.
- Virtual (cannot be formed on a screen)
- Upright (the same way up as the object)
- Same size as the object
- Laterally inverted
- "Reversed" is too vague. The examiner needs the exact term laterally inverted, which means left and right are swapped.
- "Same size" is incomplete. It should read same size as the object.
(a) The image is as far behind as the object is in front, so 1.5 m behind. ✓
(b) 1.5 m + 1.5 m = 3.0 m ✓
Image distance behind = object distance in front. Total separation is double the object distance.
(a) Angle of reflection = 50° (equal to the angle of incidence). ✓
(b) The reflected ray leaves the point on the other side of the normal, at 50° to the normal, travelling up and to the right. ✓✓
The reflected ray is the mirror image of the incident ray across the normal. Draw it with a ruler and an arrow pointing away from the mirror.
- When the water is calm, the surface is smooth and flat, so parallel rays reflect in the same direction. This is regular reflection and gives a clear image.
- When the water is rough, the surface is uneven. The normal points in different directions at different points, so parallel rays reflect in many different directions. This is diffuse (scattered) reflection and the image is broken up.
1 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A ray of light strikes a plane mirror. The angle of incidence, measured from the normal, is 30°. What is the angle of reflection?
- A15°
- B30°
- C60°
- D90°
Quiz key
By the law of reflection the angle of reflection equals the angle of incidence, so it is 30°. Option C (60°) measures from the mirror surface instead of from the normal.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.