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Lesson: Chapter 5 — Optics

5.2.2. Reflection at a concave mirror 1 of 2

Five rays to know

Five rays behave in ways you can predict at a concave mirror. Any two of them are enough to find an image.

1. Along the principal axis

A ray along the principal axis reflected straight back along it.

A ray arriving along the principal axis is reflected straight back along it. In a ray diagram, rays are drawn reflecting at the dashed line through P at right angles to the axis — the mirror's plane.

2. Parallel to the axis

A ray parallel to the principal axis reflected through the focus F.

A ray parallel to the axis passes through the focus after reflection.

3. Through the focus

A ray passing through the focus F on its way to the mirror, reflected parallel to the principal axis.

A ray that passes through the focus on its way to the mirror is reflected parallel to the axis — rule 2 run backwards.

4. Through the centre of curvature

A ray passing through the centre of curvature C, reflected straight back along itself.

A ray through the centre of curvature comes straight back through it, because any line from C to the mirror meets the mirror at right angles.

5. To the pole

A ray striking the pole at an angle to the axis, reflected at an equal angle on the other side.

A ray striking the pole at an angle to the axis is reflected at the same angle on the other side: i = r.

The three used most

  1. Along the axis → back along the axis.
  2. Parallel to the axis → through the focus.
  3. Through the centre of curvature → back through it.