Learning outcomes
- Define normal, angle of incidence and angle of reflection.
- Recall and apply i = r.
- Describe an experiment to verify the law of reflection.
- Construct and describe the image formed by a plane mirror.
6.1 Rays, normals and angles
Light rays are straight lines used to represent the direction in which light energy travels. At the point where a ray meets a surface, the normal is an imaginary line drawn at 90° to the surface. The angle of incidence i is between the incident ray and the normal. The angle of reflection r is between the reflected ray and the normal.
Angles are never measured from the mirror surface unless a question explicitly asks for that angle. If a ray makes 25° with the mirror, it makes 65° with the normal. The law of reflection states i = r for each incident ray.
6.2 Verifying the law of reflection
Place a mirror on a line drawn on paper. Direct a narrow ray box beam toward the mirror and mark two points on the incident ray and two on the reflected ray. Remove the apparatus, join the points, draw the normal at the point of incidence and measure i and r with a protractor.
Repeat for several incidence angles. A fair conclusion is that the measured values agree within experimental uncertainty. Use thin pencil lines, widely separated points and a narrow beam to reduce angular uncertainty. Keep the mirror exactly on the reference line.
6.3 Regular and diffuse reflection
A smooth surface reflects parallel incident rays in parallel directions, producing regular or specular reflection and a clear image. A rough surface has many differently oriented microscopic normals, so reflected rays leave in many directions. This is diffuse reflection.
Each ray still obeys i = r at its own local surface. Diffuse reflection explains why paper and walls can be seen from many positions, while a mirror gives a strong reflection only in particular directions.
6.4 Plane-mirror image
A plane mirror forms a virtual image. Rays from the object reflect into the eye and appear to come from a point behind the mirror. The rays do not actually meet behind the mirror, so the image cannot be projected onto a screen.
The image is upright, the same size as the object and the same perpendicular distance behind the mirror as the object is in front. It is laterally inverted: left and right appear reversed. A ray diagram locates the image by extending at least two reflected rays backward with dashed lines.
6.5 Finding image position experimentally
Place a mirror upright on a line and an object pin in front. Looking into the mirror, position a second pin behind the mirror so that it appears to coincide with the image. Move the eye sideways; correct alignment is reached when there is no parallax between the image and the second pin.
Measure object and image distances perpendicular to the mirror. Repeating for several positions should show equality. The no-parallax method is important because apparent coincidence from one viewing position alone may be misleading.
Worked examples
Angle measured from the surface
A ray makes 35° with the mirror surface. It makes 55° with the normal, so i = 55° and r = 55°.
Mirror movement
An object is 18 cm in front of a plane mirror. The image is 18 cm behind it, so object-to-image separation is 36 cm.
Practical focus
Investigation
Use pins to trace an incident and reflected ray. Place the pins far apart, keep them vertical and align them carefully. Calculate the difference i − r for several readings and discuss whether zero lies within the likely measurement uncertainty.
Examination guidance
- Draw the normal at the point of incidence, not at an arbitrary point.
- Use dashed lines for virtual backward extensions.
- “Virtual” means rays appear to originate there; it does not mean the image is imaginary or invisible.
Check your understanding
- State four characteristics of a plane-mirror image.
- A ray has i = 42°. Find r.
- Why can a virtual image not be caught on a screen?
Answers
- Upright, same size, same distance behind the mirror, laterally inverted and virtual.
- 42°.
- The actual rays do not converge at the image position.
