Stellan draws the parallel ray through a magnifying lens and reaches for the focal point to aim it. Then he notices the drawing's middle ray: it crosses the lens dead center, and it has not bent at all.
Rule 1 needed a focal point to aim at. The toolkit's second ray is simpler: it needs no focal point, because nothing happens to it.
Rule 2 of the toolkit uses one special place on the lens: its exact center.
That holds whatever angle the ray arrives at: slant it steeply or gently, the center ray still runs straight.
And it holds for both lens kinds: convex or concave, the center ray passes straight through.
The reason sits in the lens's middle: at the exact center, the two faces are parallel to each other, like a flat pane of glass.
A thin lens is thin enough that this pane-like middle leaves the ray's direction unchanged.
The other principal rays need a focal point to aim at; the center ray needs none, which makes it the easiest ray to draw.
This is rule 2 of the lens toolkit: through the exact center, straight through.
A ray from the top of a candle slants down through the exact center of a magnifying lens. State its path after the lens.
The ray passes through the exact center of the lens, so rule 2 applies.
The center ray continues straight through, without changing direction.
So the exit path continues the entry path's slant, as one unbroken straight line.
The ray carries on in its original direction, straight through the center and onward.
Quill swaps the magnifying lens for a concave lens of the same size and sends the same slanting ray through the exact center. Does the exit path change?
Rule 2 holds for both lens kinds.
At the exact center, both lenses present two parallel faces, like a flat pane.
So the concave lens also leaves the center ray's direction unchanged.
No. The ray runs straight through the concave lens's center exactly as before.
A ray passes through the exact center of a thin convex lens. What happens to its direction?
A ray arrives at a steep slant and passes through the exact center of a thin lens. What happens?
Romilly says: 'The center ray behaves the same through a convex lens and a concave lens: straight through.' Is Romilly correct?
Which statement is rule 2 of the lens toolkit?
Why does the center ray pass through a thin lens without changing direction?
In a finished lens drawing, one ray runs as a single unbroken straight line from the object's tip, through the lens, and onward. Which principal ray is it?