Thora draws rule 1's ray leaving a convex lens and watches it pass through the far focal point. She wonders about the mirror image of that move: what if a ray ARRIVES through a focal point instead?
Knowing two exit paths in advance already lets you draw a beam's fate. The convex lens's toolkit has one more ray with a known exit, and it starts at the focal point on the object's side.
A convex lens has a focal point on each side. The one on the object's side is the near focal point.
Parallel to the principal axis: the exit path runs level, however steeply the ray slanted on its way in.
Rule 3 is rule 1 run in reverse. Parallel in gives through-the-focal-point out; through-the-focal-point in gives parallel out.
The rule is stated for convex lenses only.
A concave-lens drawing never needs it: the constructions for concave lenses use rule 1 and rule 2 alone.
So the convex toolkit holds three known rays: the parallel ray, the center ray, and the near-focal-point ray. The concave toolkit holds the first two.
With any two of a lens's known rays drawn from one object point, their meeting fixes the image of that point. The constructions begin next lesson.
A ray from an arrow's tip slants down, passes through the near focal point of a convex lens of focal length 10 cm, and enters the glass. State its path after the lens.
The ray passed through the focal point on the object's side before entering, so rule 3 applies.
Rule 3 says that ray leaves the lens parallel to the principal axis.
So the exit path runs level from the point where the ray met the lens.
The ray leaves the lens running level, parallel to the principal axis.
Stellan sees a finished convex-lens drawing in which one refracted ray runs exactly level after the lens. Which path did that ray take on the way in?
A level exit is rule 3's signature.
Rule 3's ray is the one that passes through the near focal point before entering the lens.
Check the drawing: the incoming segment should pass through the focal dot on the object's side.
The ray came in through the near focal point on the object's side.
A ray from an object passes through the near focal point of a convex lens and enters the glass. What is its path after the lens?
Rule 3 names a focal point. Which one?
In a finished convex-lens drawing, one refracted ray runs exactly level after the lens. Which path did it take on the way in?
Which statement is rule 3 of the convex lens's toolkit?
Romilly draws two rays from an arrow's tip at a convex lens: ray 1 arrives parallel to the axis, ray 2 arrives through the near focal point. Which pair of exit paths is correct?
Why does this unit's concave-lens toolkit hold only two principal rays?