16.1Reflection and Refraction
Reflection and Plane Mirrors — American System Physics Revision Notes & Topical Questions
10 min read 7 parts Lesson 3 of 5 1 quiz
Lesson summary
State the law of reflection and define angles of incidence and reflection.Law of reflectionangle of incidence = angle of reflection, i = r (degrees, measured from the normal).Plane mirrorimage distance = object distance, di = do (m), with the image behind the mirror.Magnification…
- The law of reflection has two parts
- Incident ray
- Reflected ray
- Angle of incidence (i)
- Angle of reflection (r)
- Specular (regular) reflection
Section 1 of 7Reflection and Plane Mirrors
Essential knowledge
Ray
Light travels in straight lines in a uniform medium; we represent a narrow beam by a ray with an arrow showing the direction of travel.
When light meets a boundary, some may be reflected, some transmitted (refracted) and some absorbed; a mirror is designed to reflect nearly all of it.
Normal
The normal is drawn perpendicular to the surface at the point of incidence. The angle of incidence and angle of reflection are both measured between the ray and the normal.
The law of reflection has two parts
The law of reflection has two parts i = r, and the incident ray, reflected ray and normal all lie in one plane.
A ray striking a mirror along the normal (i = 0°) reflects straight back along its own path.
In diffuse reflection each tiny part of a rough surface faces a different way, so each part has its own normal; every ray still obeys i = r but the reflected rays spread out. This is why we can see non-luminous objects from any direction.
To locate the image in a plane mirror by construction: draw two rays from a point on the object to the mirror, reflect each with i = r, then extend the reflected rays backwards behind the mirror as dashed lines; the image is where the dashed lines cross.
The line joining a point on the object to its image is perpendicular to the mirror, and the image is the same distance behind the mirror as the object is in front.
Virtual
A plane mirror image is virtual: no light actually passes through the image position, so a screen placed there shows nothing.
Uses of plane mirrors include periscopes (two mirrors at 45°), dressing mirrors, and making a room look larger; lateral inversion explains why AMBULANCE is printed reversed on vehicles.
Reflection happens for all waves (sound echoes, water waves at a barrier); the reflected wave has the same speed, frequency and wavelength as the incident wave because it is in the same medium.
Section 2 of 7Process & sequence
1
Place a plane mirror upright along a straight line drawn on a sheet of paper resting on a soft board.
2
Draw a normal (at 90° to the mirror line) and use a protractor to draw incident lines at 20°, 30°, 40°, 50° and 60° to the normal.
3
In a darkened room, shine a narrow ray from a ray box along each incident line in turn.
4
Mark the reflected ray with two small crosses far apart, remove the mirror and join the crosses to the point of incidence.
5
Measure each angle of reflection from the normal with a protractor and record it in a table next to the angle of incidence.
6
Compare the two columns: within experimental error, i = r; repeat and average to reduce random errors (thin ray and far-apart crosses improve accuracy).
Locating a plane-mirror imagewrite these steps in this order; each step is a marking point.
1
Draw the mirror line and mark the object point O in front of it.
2
Draw two rays from O to different points on the mirror and add a normal at each point.
3
Draw each reflected ray so that angle of reflection equals angle of incidence.
4
Extend both reflected rays backwards behind the mirror as dashed lines.
5
Mark the image I where the dashed lines cross; check it is the same perpendicular distance behind the mirror as O is in front.
Section 3 of 7Compare
| Property | Plane mirror image | Projected (real) image |
|---|---|---|
| Can be caught on a screen? | No (virtual) | Yes (real) |
| Orientation | Upright (erect) | Usually inverted |
| Size | Same size as object (magnification 1) | Can be larger or smaller |
| Position | Same distance behind mirror as object is in front | Where rays actually meet |
| Left/right | Laterally inverted | Depends on system |
Learn the five plane-mirror image properties as a set: virtual, upright, same size, same distance behind, laterally inverted.
| Feature | Specular | Diffuse |
|---|---|---|
| Surface | Smooth (bumps much smaller than wavelength) | Rough |
| Parallel rays after reflection | Stay parallel | Scattered in many directions |
| Image formed? | Yes | No |
| Law of reflection obeyed? | Yes | Yes, at every point |
| Example | Mirror, still water | Paper, wall, clothing |
Section 4 of 7Worked examples
55°
A ray strikes a plane mirror making an angle of 35° with the mirror surface. Find (a) the angle of incidence, (b) the angle of reflection, (c) the angle between the incident and reflected rays.
- (a) Angle with normal = 90° − 35° = 55°.
- (b) By the law of reflection, r = i = 55°.
- (c) Angle between rays = i + r = 55° + 55° = 110°.
4.8 m
A student stands 2.4 m in front of a plane mirror. (a) How far is the student from her image? (b) She walks 0.9 m towards the mirror. What is the new distance between her and her image?
- (a) Image is 2.4 m behind the mirror, so separation = 2.4 + 2.4 = 4.8 m.
- (b) New object distance = 2.4 − 0.9 = 1.5 m; image also 1.5 m behind; separation = 1.5 × 2 = 3.0 m.
24°
A mirror is turned through 12° while a fixed ray strikes it. Through what angle does the reflected ray turn?
- The reflected ray turns through twice the mirror rotation: 2 × 12° = 24°.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Reflection · Incident ray · Reflected ray · Normal · Angle of incidence (i) · Angle of reflection (r) · Law of reflection · Specular (regular) reflection · Diffuse reflection · Virtual image · Real image · Lateral inversion · Object distance (do) and image distance (di)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Words to avoid
1
the angle of incidence is the angle between the ray and the mirrorthe angle between the incident ray and the normal.
2
the image is invertedthe image is upright but laterally inverted (left-right reversed).
3
rough surfaces do not obey the law of reflectioneach ray obeys the law, but the normals point in different directions so rays scatter.
4
the image is on the mirror surfacethe image is the same distance behind the mirror as the object is in front.
5
the image is real because you can see itthe image is virtual because rays only appear to come from it and it cannot be formed on a screen.
Instant feedbackCheck yourself
Q1. A ray hits a plane mirror with an angle of incidence of 40°. What is the angle between the reflected ray and the mirror surface?
- A. 40°
- B. 50°
- C. 80°
- D. 90°
Show answer
B. Angle of reflection = 40° from the normal, so angle with surface = 90° − 40° = 50°.
Q2. Which describes the image in a plane mirror?
1
A. Real, upright, same size
2
B. Virtual, inverted, magnified
3
C. Virtual, upright, same size
4
D. Real, inverted, same size
Show answer
C. Plane mirror images are virtual, upright (erect) and the same size as the object.
Q3. An object is 30 cm in front of a plane mirror. How far is the image from the object?
- A. 15 cm
- B. 30 cm
- C. 45 cm
- D. 60 cm
Show answer
D. Image is 30 cm behind the mirror, so 30 + 30 = 60 cm from the object.
Q4. Why can a sheet of paper be seen from all angles in a lit room?
1
A. It emits its own light
2
B. It absorbs all light
3
C. It gives diffuse reflection
4
D. It refracts light
Show answer
C. The rough surface scatters light in all directions (diffuse reflection).
Q5. A ray strikes a mirror along the normal. The angle of reflection is
- A. 0°
- B. 45°
- C. 90°
- D. 180°
Show answer
A. Along the normal i = 0°, so r = 0°; the ray returns along its own path.
Q6. A mirror rotates by 15°. The reflected ray (fixed incident ray) rotates by
- A. 7.5°
- B. 15°
- C. 30°
- D. 45°
Show answer
C. The reflected ray turns through twice the mirror angle: 2 × 15° = 30°.
Marked practiceQuizzes for this lesson
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