Read a ray diagram

Romilly finds a finished ray diagram in her notes with no caption: a lens, two rays, a crossing. Nothing says what kind of image it shows. The diagram itself has to be read.

Every completed mirror or lens diagram answers the same questions about the beam it draws, read off the same marks. This lesson is the reading routine, and it works on both elements.

A completed diagram tells you an image's location and its three attributes. Read them with three questions.

Question 1, the kind: do the solid rays actually cross, or do only dashed trace-backs meet?

A lens read: ring 1 the meeting, ring 2 the side, ring 3 the heights.

Solid rays actually crossing mean a REAL image; a dashed-only meeting means a VIRTUAL one. The meeting's position is the image's location.

Question 2, the orientation: does the meeting sit on the other side of the principal axis from the object's tip?

The other side means INVERTED; the same side means UPRIGHT.

Question 3, the size: compare the image arrow's height with the object arrow's.

A taller image arrow means ENLARGED; a shorter one means REDUCED.

The register case in one read: rays crossing below the principal axis on the far side of a lens show a real, inverted image.

The same three questions read a mirror diagram: solid reflected rays crossing in front mean real; dashed meetings behind the mirror mean virtual.

A mirror read with the same rings: the questions do not change with the element.

So one routine covers every element: find the meeting, read its side, compare the heights.

The whole routine: three questions, three marks, six possible words.

A completed lens diagram shows the two refracted rays actually crossing below the principal axis on the far side, with the image arrow shorter than the object arrow. Read the full image description.

Question 1: the solid rays actually cross, so the image is real, located at the crossing on the far side.

Question 2: the crossing sits on the other side of the principal axis from the tip, so the image is inverted.

Question 3: the image arrow is shorter than the object arrow, so the image is reduced.

Real, inverted, reduced, on the far side of the lens.

A completed concave-mirror diagram shows dashed trace-backs meeting behind the mirror, the image arrow taller than the object and on the tip's side of the principal axis. Read the full image description.

Question 1: only dashed trace-backs meet, so the image is virtual, located behind the mirror.

Question 2: the meeting sits on the same side of the principal axis as the tip, so the image is upright.

Question 3: the image arrow is taller, so the image is enlarged.

Virtual, upright, enlarged, behind the mirror.

In a finished lens diagram, the two refracted rays actually cross, below the principal axis on the far side. Which pair of words does question 1 and question 2 give?

In a finished mirror diagram, only dashed trace-backs meet, behind the mirror, on the tip's side of the principal axis, at an arrow taller than the object. Read the full description.

A finished diagram's image arrow is shorter than its object arrow. What does question 3 read?

Your turn

A finished lens diagram shows dashed trace-backs meeting on the object's side, on the tip's side of the principal axis, at an arrow taller than the object. Which full description is correct?

A finished concave-mirror diagram shows the reflected rays actually crossing in front of the mirror, below the principal axis, at an arrow shorter than the object. Which full description is correct?

Which drawn feature of a finished diagram tells you its image is real?