Katla stands a small cardboard arrow 30 cm from a concave mirror whose focal length is 10 cm. Somewhere in front of the mirror, the reflected light builds an image of the arrow. The three rules can find exactly where, and which way up.
This lesson builds the first full curved mirror diagram, one beam at a time, and reads the image straight off the drawing.
Start with the skeleton: the mirror, the reference line, the focal point 10 cm out, and the arrow standing 30 cm out.
Draw every ray from one point of the object: the arrow's tip.
Ray 1 leaves the tip parallel to the principal axis. It reflects through the focal point.
Ray 2 leaves the tip and passes through the focal point on its way to the mirror. It reflects parallel to the principal axis.
The two reflected rays cross at one point in front of the mirror, below the axis.
The image of the tip sits where the reflected rays cross. The reflected light really passes through that point, so the image is real: a screen there catches it.
The crossing sits below the axis, so the image arrow is drawn upside down: the image is inverted.
The center ray makes a check: drawn from the tip to the point where the axis meets the mirror, it reflects at an equal angle and passes through the same crossing.
An arrow stands 24 cm from a concave mirror with a focal length of 8 cm. Plan the two rays that locate the image of its tip.
Ray 1: from the tip, parallel to the principal axis; it reflects through the focal point.
Ray 2: from the tip, through the focal point; it reflects parallel to the principal axis.
The image of the tip sits where the two reflected rays cross.
Draw the parallel ray and the focal point ray from the tip; the tip's image is at their crossing.
Jorun finishes the diagram for an arrow 25 cm from a concave mirror with a focal length of 10 cm. The reflected rays cross below the axis in front of the mirror. Describe the image she draws.
The reflected rays really cross, so the image is real.
The crossing sits below the axis, so the image is inverted.
Jorun draws the image arrow at the crossing, upside down.
A real, inverted image arrow at the crossing, in front of the mirror.
Ivor starts the diagram for an arrow standing beyond the focal point of a concave mirror. His first ray leaves the arrow's tip parallel to the principal axis. Where does this ray go after reflection?
Ivor's second ray leaves the tip and passes through the focal point on its way to the mirror. Where does it go after reflection?
The two reflected rays cross in front of the mirror, below the principal axis. Where does Ivor draw the image of the arrow's tip?
Lennart completes the ray diagram for an arrow 27 cm from a concave mirror with a focal length of 9 cm. Which description fits the correct finished diagram?
In a finished concave mirror diagram, the image is drawn where the reflected rays cross. Why there?
Katla's finished diagram shows an arrow 12 cm from a concave mirror with a focal length of 4 cm. The two reflected rays cross below the axis, 6 cm from the mirror. What does Katla draw at the crossing?