Lennart starts drawing a curved mirror diagram. He has the mirror arc. But he has nowhere to measure from: no line fixes where anything in the drawing sits.
Before any beam is drawn, every mirror diagram gets one reference line. This lesson names that line.
Here is a concave mirror drawn from the side, with one straight line added through it.
The line passes through the center of the mirror, and it meets the mirror's surface at a right angle.
That line is called the principal axis.
It is the reference line of a ray diagram: the rays and points of the diagram are located from it.
The special points of every mirror diagram sit on the principal axis. The next lesson places the first of them.
A tilted line, a line along the surface, or a line through an edge is not the principal axis. Only the through-the-center, right-angle line is.
Jorun draws a convex mirror seen from the side. Where does its principal axis go?
The principal axis passes through the center of the mirror.
It meets the mirror's surface at a right angle at that point.
Jorun draws the straight line through the mirror's center, perpendicular to the surface there.
Katla draws a line that touches her concave mirror only at its top edge. Is that line the mirror's principal axis?
The principal axis passes through the center of the mirror, not through an edge.
Katla's line passes through the top edge.
No. Katla's line is not the principal axis.
Ivor draws a concave mirror from the side and adds three lines. Which line is the mirror's principal axis?
What job does the principal axis do in a mirror diagram?
Katla says: 'A lens has a principal axis too: the straight line through its center, perpendicular to it.' Is Katla correct?
Jorun draws a convex mirror from the side and adds four lines. Which is the principal axis?
A principal axis is drawn correctly on a curved mirror diagram. Which of the following must be true of it?
Which of the following states what the principal axis of a curved mirror or lens is?