Section 1 of 4The Law of Reflection
Normal
- The normal is a construction line drawn at 90° to the mirror at the point where a ray strikes it.
Angle of incidence
- This is the angle between the incident ray and the normal, measured in degrees.
Angle of reflection
- This is the angle between the reflected ray and the normal, and it always equals the angle of incidence.
All waves
- The law of reflection applies to every wave that reflects, including light, sound and water waves.
Section 2 of 4Constructing an Image in a Plane Mirror
1
Draw the mirror as a straight line and draw the object as an arrow in front of it.
2
From the top of the object, draw two rays towards the mirror, and draw the normal at each point of contact.
3
Draw each reflected ray so that its angle to the normal equals the angle of incidence.
4
Extend the reflected rays backwards behind the mirror with dashed lines; where they cross is the position of the image.
Section 3 of 4Properties of a Plane Mirror Image
Properties of a Plane Mirror Image
Size
- The image is the same size as the object.
Distance
- The image is the same distance behind the mirror as the object is in front of it.
Type
- The image is virtual, because the light rays only appear to come from it, so it cannot be projected on a screen.
Orientation
- The image is upright and laterally inverted, so left and right appear to be swapped.
Section 4 of 4Exam Checklist & Tips
3.4.1 – Key Points for the Exam
- Normal A construction line drawn at right angles to a 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 reflected rays appear to meet, which cannot be projected onto a screen.
- Lateral inversion The reversal of left and right in a mirror image.
Common Mistakes
- Measuring the angles from the mirror surface instead of from the normal.
- Drawing reflected rays with angles that are visibly unequal to the incident angles.
- Calling the plane mirror image a real image that can be projected onto a screen.
- Drawing the image in front of the mirror rather than behind it.