Investigating Reflection

Reflection is when light bounces off a surface, like a mirror. We can measure and predict how it bounces using the law of reflection.

Key Ideas and Words

• Normal: an imaginary line drawn at right angles (90°) to the mirror surface at the point where the light hits.
• Angle of incidence (i): the angle between the incoming ray and the normal.
• Angle of reflection (r): the angle between the reflected ray and the normal.
• Law of reflection: \,$$i = r$$ The two angles are always equal.

Simple Investigation with a Plane Mirror

Equipment: plane mirror, ray box (or torch with a narrow slit), protractor, ruler, paper, pencil.

1. Place the mirror on a sheet of paper and draw its straight edge (the mirror line).
2. Pick a point on the mirror line. Draw a normal (a line at 90° to the mirror) at that point.
3. Shine a thin ray so it meets the mirror at the normal point. Trace the incoming ray.
4. Trace the reflected ray. Remove the mirror and use a protractor to measure $i$ and $r$ from the normal.
5. Repeat for different incident angles (e.g. 20°, 30°, 40°). Compare $i$ and $r$.

You should find $i = r$ each time (small differences can be due to measurement error).

Constructing the Image in a Plane Mirror

1. Draw the mirror line.
2. Mark the object point (e.g. the tip of an arrow) in front of the mirror.
3. Measure the perpendicular distance from the object to the mirror. Mark the image the same distance behind the mirror along a perpendicular line.
4. Draw at least one incident ray from the object to the mirror, then the reflected ray so that $i = r$. Extend the reflected ray backward with a dashed line to meet the image point. Dashed lines show the rays only appear to come from behind the mirror.

What a Plane Mirror Image is Like

• Same size as the object
• Upright
• Same distance behind the mirror as the object is in front
• Laterally inverted (left and right are swapped)
• Virtual (cannot be formed on a screen)

Real-World Connections

• Periscopes use two plane mirrors to let you see over obstacles.
• Bathroom mirrors show lateral inversion (text looks reversed).
• Smooth surfaces give clear reflections; rough surfaces scatter light and do not show a clear image.

Common Mistakes

• Measuring angles to the mirror surface instead of to the normal. Always measure from the normal.
• Forgetting to draw the image the same perpendicular distance behind the mirror.
• Not using dashed lines for virtual rays when constructing images.