Thora sends a flashlight beam, spread wide and straightened into parallel rays, through a concave lens. On the far wall the patch of light is wider than the lens. The rays never meet anywhere behind the glass.
A concave lens spreads parallel rays apart, so they have no meeting point beyond the lens. But the spread is not shapeless: the beam fans out in a precise pattern, and that pattern hides a point.
Straighten a flashlight beam into parallel rays and send it through a concave lens.
After the lens, the rays spread apart. They never cross on the far side.
Now take a ruler to the drawing. Extend each spread-out ray straight backward, through the lens, toward the incoming side. Draw those extensions dashed.
The dashed extensions all meet at one point on the incoming side of the lens.
It is virtual because no light actually travels through it: only the dashed traced-back lines meet there. The dashes say so.
The distance from the lens to the virtual focal point is the lens's focal length, written in words as always: a focal length of 10 cm, say.
You met this grammar on the convex mirror: parallel rays reflecting as if they spread from a point behind it. The concave lens plays the same trick on the incoming side.
A thin concave lens has a matching point on each side; the one the trace-back finds sits on the side the light comes from.
Parallel rays pass through a concave lens and spread. Quill traces the spread rays straight back and finds the dashed lines meeting 10 cm in front of the lens. What has Quill found?
The spread rays behave as if they all came from one point.
The traced-back dashed lines meet at that point, 10 cm in front of the lens, on the incoming side.
The meeting point of the trace-back is the virtual focal point, and the lens-to-point distance is the focal length.
Quill has found the virtual focal point: the lens has a focal length of 10 cm.
Romilly looks for a bright spot on a card behind a concave lens held in sunlight, the way she would with a convex lens. Will she find one?
A bright spot needs the rays to actually meet on the card.
A concave lens spreads the parallel rays apart, so they never meet beyond the lens.
The only meeting is on the traced-back dashed lines, in front of the lens, where no light actually travels.
No. The card catches a widening patch, never a spot; the focal point of a concave lens is virtual.
On which side of a concave lens does its virtual focal point sit?
A finished concave-lens drawing shows the solid spread rays and their dashed extensions meeting in front of the lens. Along which drawn lines does light actually travel?
Quill says: 'Parallel rays leaving a concave lens spread out exactly as if they all came from one point in front of the lens.' Is Quill correct?
Which statement describes the virtual focal point of a concave lens?
Why is the concave lens's focal point called virtual?
Stellan holds a convex lens in sunlight, then swaps in a concave lens of the same size. Which statement tells the two apart?