Lesson overview
Reflection in mirrors: Where is the image you see in a mirror?
Draw the normal on a mirror line, aim a ray box at a plane mirror at 20°, 40°, 60° and along the normal, predict where each ray goes and read both angles on a protractor. Then draw an arrow in front of the mirror, send two rays through each end, extend the reflected rays back behind the mirror to find the image, hold a white card there, and check the image with a search pin and no parallax.
Suggested sequence
- Lesson 1 · The law of reflection. Rays, the normal, the angle of incidence and the angle of reflection; the law of reflection measured with a ray box, a plane mirror and a protractor; Supplement: the angle between the rays. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Images in a plane mirror. The image in a plane mirror found with rays and checked with a white card and a search pin (no parallax): same size, as far behind as the object is in front, virtual; Supplement: constructing an image and the periscope. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: The law of reflection
- Define and use the terms normal, angle of incidence and angle of reflection (3.2.1.1)
- State that for reflection, the angle of incidence is equal to the angle of reflection; recall and use this relationship (3.2.1.3)
- Supplement: Use simple constructions, measurements and calculations for reflection by plane mirrors: angles to the normal, to the surface and between the rays (3.2.1.4)
Part 2: Images in a plane mirror
- Describe the formation of an optical image by a plane mirror and give its characteristics, i.e. same size, same distance from mirror, virtual (3.2.1.2)
- Supplement: Use simple constructions, measurements and calculations for reflection by plane mirrors: locating an image, the ray that reaches the eye, two mirrors (3.2.1.4)
Misconceptions to listen for
- “Line 1 is the normal, because it is at 90° to line 2.” — The normal is drawn at 90° to the mirror, and it is dashed. Line 1, with its back hatched, is the mirror itself.
- “Line 4 is the incident ray, because it is on the right.” — Follow the arrows: the incident ray points towards the mirror and the reflected ray away from it.
- “w, because it is the angle between the incoming ray and the mirror.” — The angles of incidence and reflection are measured from the normal, not from the surface of the mirror.
- “y, because it is the other angle next to the normal.” — y is between the normal and the reflected ray: it is the angle of reflection.
- “z, because it is the angle at the mirror on the side the light goes to.” — z is between the reflected ray and the surface of the mirror. The angle of incidence belongs to the incident ray and is measured from the normal.
- “The normal is a line along the mirror’s surface.” — The normal is at 90° to the mirror, not along it.
- “The normal is a line at 45° to the mirror’s surface.” — The normal makes a right angle with the mirror: 90°, not 45°.
- “The normal is the path of the ray towards the mirror.” — That is the incident ray. The normal is a construction line at 90° to the mirror; it is not a ray at all.
- “Row A.” — These are the scale readings. Each angle is measured from the normal, which is on the 90° mark, so take the difference between each reading and 90°.
- “Row B.” — These are the angles between each ray and the mirror, which lies along the straight edge of the protractor. Measure from the 90° mark instead.
Scope
The normal, the angle of incidence and the angle of reflection (3.2.1.1); the image in a plane mirror: same size, as far behind the mirror as the object is in front, and virtual (3.2.1.2); the angle of incidence equals the angle of reflection (3.2.1.3); Supplement: simple constructions, measurements and calculations for reflection by plane mirrors (3.2.1.4). Reflection of water waves is in Waves in a ripple tank, refraction in Refraction and total internal reflection; curved mirrors are not in this syllabus.