Stellan watches sunlight pass through a convex lens and meet at the focal point. The middle ray runs straight. But the ray entering near the top edge ends up steeply tilted downward. Something turned it.
You know WHAT a convex lens does: it converges parallel rays at the focal point. This lesson explains WHY. Follow one beam through the glass, one surface at a time.
Take the ray that enters near the top edge of the lens.
At the front surface, the ray crosses from air into glass, a new medium, so it refracts: its direction changes at the crossing.
The front surface near the top edge tilts the crossing so the ray bends downward, toward the principal axis.
At the back surface, the ray crosses from glass back into air, so it refracts again.
The back surface near the top edge tilts that crossing the same way: the ray bends downward again.
The same happens below the axis, mirrored: a ray entering near the bottom edge is bent upward at each surface, toward the axis.
Rays entering nearer the middle meet surfaces that tilt less, so they bend less; the exact middle ray meets the two faces square-on and runs straight.
Every ray is turned toward the principal axis, so the whole parallel set closes in on one point: the focal point.
A ray enters a convex lens near its bottom edge, parallel to the principal axis. Describe its direction after each surface.
At the front surface the ray crosses into the glass and refracts: near the bottom edge, the surface's tilt bends it upward, toward the axis.
At the back surface the ray crosses back into the air and refracts again: the tilt there bends it upward a second time.
Both bends point the ray toward the principal axis.
The ray leaves the lens tilted upward, heading for the focal point on the axis.
Romilly asks why the ray through the exact center of the lens is not turned. What is the reason?
A ray bends at a surface because the surface meets it at a tilt.
At the exact center, the lens's two faces meet the ray square-on, like a flat pane of glass.
With no tilt at either crossing, neither surface turns the ray.
The center ray passes through without changing direction.
A ray enters a convex lens near its top edge, parallel to the principal axis. Which way is it bent?
Why does a ray change direction at the front surface of a glass lens?
Thora says: 'A ray through the top edge of a convex lens refracts twice: once entering the glass and once leaving it.' Is Thora correct?
Which statement explains why a convex lens brings parallel rays together?
A ray travels inside the glass of a convex lens and reaches the back surface. What happens there?
A ray enters a convex lens near its bottom edge, parallel to the principal axis. After both surfaces, where is it heading?