Katla points a convex mirror at the Sun. No hot spot forms anywhere in front of it. The reflected light spreads out instead.
Parallel beams gave the concave mirror its focal point. This lesson finds the convex mirror's version, and it is not in front of the mirror.
Parallel beams arrive at the convex mirror.
After reflection the beams spread apart. They never meet in front of the mirror.
Trace each reflected beam straight back behind the mirror. The traced lines all cross at one point on the principal axis.
The reflected beams spread out exactly as if they came from that point behind the mirror.
It is virtual: the reflected light stays in front of the mirror, and only the traced-back lines meet behind it.
On the drawing, the traced-back lines are dashed, the same convention the plane mirror lessons used: a dashed line marks where light only appears to come from.
The distance from the mirror to the virtual focal point is the convex mirror's focal length, again written in words.
Jorun draws parallel beams striking a convex mirror and their spread-out reflections. How does she find the virtual focal point?
She traces each reflected beam straight back behind the mirror, drawing the traced lines dashed.
The dashed lines cross at one point on the principal axis behind the mirror.
That crossing is the virtual focal point.
The virtual focal point is the crossing of the dashed traced-back lines, behind the mirror.
Ivor's convex mirror has a focal length of 4 cm. Where is its virtual focal point?
A convex mirror's virtual focal point sits behind the mirror, on the principal axis.
The focal length is the distance from the mirror to that point.
The virtual focal point sits 4 cm behind the mirror, on the principal axis.
Parallel beams strike a convex mirror. What happens to them after reflection?
On Katla's convex mirror diagram, the lines behind the mirror are dashed. What do the dashed lines show?
Ivor says: 'A convex mirror's virtual focal point sits behind the mirror, on its principal axis.' Is Ivor correct?
Why is a convex mirror's focal point called virtual?
Parallel beams arrive at Jorun's concave mirror and at Lennart's convex mirror, each along the mirror's principal axis. Which option says where each mirror's reflected beams meet or appear to come from?
Katla traces the reflected beams from her convex mirror straight back behind it. Where do the traced lines cross?