Ch 2 · 3.2.1 Reflection of Light · O Level Physics 5054
Topic 2 of 7 in Waves — O Level Physics 5054 — 40 lessons.
Introduction
When you look in a mirror, your reflection seems to stand behind the glass, exactly as far back as you are in front. No light actually goes behind the mirror. This lesson explains the simple rule light follows when it reflects, and how it makes the image you see. (Syllabus section 3.2.1.)
Key ideas — The words you need
Always measure angles from the normal, not from the mirror's surface.
🧑🏫 Teacher's explanation
Imagine you are standing in a dark room with a single flashlight, pointing it at a large, flat mirror. The light hits the mirror and bounces off in a specific direction. If you were to draw a line perfectly perpendicular (at 90^°) to the mirror's surface at the exact spot where the light hits, that line is called the 'normal'.
📖 Definitions
🔍 Worked example — Easy
A ray of light strikes a plane mirror such that the angle between the incident ray and the mirror surface is 35^°. Calculate the angle of reflection.
🔍 Worked example — Standard
A student performs an experiment with a plane mirror. The incident ray makes an angle of 42^° with the normal. What is the total angle between the incident ray and the reflected ray?
🔍 Worked example — Exam-style
An object is placed 12.0 cm in front of a plane mirror. Describe the characteristics of the image formed and state its distance from the mirror.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the distance of the image from the mirror. [1] (b) Describe how the student could use a pin and a piece of paper to confirm the image is virtual. [3] [4 marks]
Key ideas — The law of reflection
angle of incidence = angle of reflection (i = r)
🧑🏫 Teacher's explanation
Imagine you are standing in a dark room with a single flashlight, pointing it at a large, flat mirror. When the beam hits the mirror, it doesn't just scatter everywhere; it bounces off at a very specific, predictable angle. This 'bounce' is what we call reflection.
📖 Definitions
🔍 Worked example — Easy
A ray of light strikes a plane mirror such that the angle between the incident ray and the mirror surface is 35^°. Calculate the angle of reflection.
🔍 Worked example — Standard
A light ray is incident on a plane mirror. The angle of incidence is 42^°. Find the total angle between the incident ray and the reflected ray.
🔍 Worked example — Exam-style
An object is placed 12.0 cm in front of a plane mirror. Describe the characteristics of the image formed and state its distance from the mirror.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The image in a plane mirror
The image in a flat (plane) mirror is:
🧑🏫 Teacher's explanation
Imagine you are standing in front of a mirror. When you raise your right hand, your reflection seems to raise its left hand. This is called lateral inversion. The mirror doesn't actually 'flip' the image; rather, it reflects the light rays back along a path that your brain interprets as coming from a point behind the glass. Because those rays never actually travel through the mirror, we call the image 'virtual'.
📖 Definitions
🔍 Worked example — Easy
A ray of light strikes a plane mirror at an angle of 35^° to the mirror surface. Calculate the angle of reflection.
🔍 Worked example — Standard
An object is placed 12.0 cm in front of a plane mirror. How far is the image from the object?
🔍 Worked example — Exam-style
A ray of light is incident on a plane mirror. The angle between the incident ray and the reflected ray is 80^°. Calculate the angle of incidence.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Practical notes
Worked example
Example 1. A ray hits a plane mirror and makes an angle of 35° with the mirror's surface. Find the angle of reflection.
🔍 Worked example — Easy
A ray of light strikes a plane mirror and makes an angle of 35^° with the mirror's surface. Calculate the angle of reflection.
🔍 Worked example — Standard
An object is placed 1.5 m in front of a plane mirror. Determine the distance between the object and its image.
🔍 Worked example — Exam-style
A ray of light is incident on a plane mirror such that the angle between the incident ray and the reflected ray is 80^°. Calculate the angle of incidence.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The angle between the incident ray and the normal. (2) 28°; 90° - 28° = 62°. (3) Any three: same size; same distance behind the mirror as the object is in front; virtual; upright; laterally inverted. (4) Draw a normal to a mirror line; aim a ray at the mirror at a measured angle to the normal; mark and draw the reflected ray; measure the angle of reflection; repeat for several angles; the two angles are equal each time.