Lesson focus: Reflection changes a ray’s direction according to a precise angle rule measured from the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it..
Construct reflection ray diagrams and explain plane-mirror images and diffuse reflection.
Build on earlier learning
LightThe previous lesson used reflection to explain vision. This lesson makes the direction change quantitative.
You will be successful when you can…
- Label incident ray, reflected ray and normal.
- Measure incidence and reflection angles from the normal.
- Apply angle of incidence = angle of reflection.
- Explain specular and diffuse reflection and a plane-mirror image.
Words that carry the science
- incident ray
- The incoming ray that reaches a surface.
- reflected ray
- The ray leaving a surface after reflection.
- normal
- An imaginary line perpendicular to the surface at the point of incidence.
- angle of incidence
- The angle between the incident ray and the normal.
- angle of reflection
- The angle between the reflected ray and the normal.
- virtual image
- An image formed where diverging rays appear to originate and which cannot be projected onto a screen.
Measure reflection from the normal
01The law uses the normal
At a surface, the angle of incidence equals the angle of reflection. Both angles are measured between a ray and the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it., which is at 90° to the surface.
Ray arrows matter: the incident rayThe ray of light travelling towards a surface. points toward the surface and the reflected rayThe ray of light that bounces off a surface. points away. Measuring from the mirror gives the complementary angle and is a common error.
02Surface texture changes the reflected pattern
A smooth plane mirror sends parallel incident rays into parallel reflected rays, producing specular reflectionReflection from a very smooth surface, like a mirror, which gives a clear image. and a clear image.
A rough surface has many differently oriented local normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it., so rays scatter in different directions. Each ray still obeys the reflection law at its own point.
03A plane mirror creates a virtual image
Reflected rays reaching the eye diverge, but the brain traces them backward in straight lines. They appear to come from behind the mirror, creating a virtual imageAn image you can see, like your reflection in a mirror, but which cannot be shown on a screen..
The image is upright, the same size as the object and as far behind the mirror as the object is in front. It cannot be captured on a screen at that apparent position.
The law of reflection
Change the angle of incidence. Both angles are measured from the normal.
Controls
Repair an angle measured from the mirror
A ray makes 35° with the mirror surface. Find the angle of incidenceThe angle between a light ray hitting a surface and the normal (a line at right angles to the surface). and reflection.
- The normal is 90° to the mirror.
- Angle of incidence = 90° − 35° = 55°.
- Apply the reflection law: angle of reflectionThe angle between a reflected light ray and the normal (a line at right angles to the surface). = angle of incidence.
- Draw the reflected ray on the opposite side of the normal with an outward arrow.
How to make the reasoning visible: Both the angle of incidence and reflection are 55°.
Reason from four ray situations
Angles in this table are measured from the normal unless the row states otherwise.
| Situation | Given | Required | Result |
|---|---|---|---|
| A | incidence 20° | reflection | 20° |
| B | reflection 47° | incidence | 47° |
| C | ray 30° to mirror | incidence | 60° |
| D | ray along normal | reflection direction | returns along same line |
Explain situation D without saying the ray fails to reflect.
Why does crumpled foil not form the same clear image as smooth foil?
Check your understanding
Answer from memory, then check against the suggested answer.
What is the normal?
State the law of reflection.
Why is a plane-mirror image virtual?
A ray has an incidence angle of 68°. State the reflection angle and the angle between the two rays.
Explain why a white wall can be seen from many positions but usually does not show a clear image.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Draw the normal and the reflected ray on the pad. Label both angles.
(3)Draw your answer in the box below. Then press Show model answer and tick each point your drawing shows.
Draw with a mouse, finger or stylus.
Calculate the angle of incidence.
(1)State the angle of reflection.
(1)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., state the construction sequence and show the angle conversion explicitly.