is a transparent instrument that uses refraction to bend and
focus light from an object, forming a sharp image. We'll first investigate
how the lens produces an image from a point source of light (a lamp).
at upper right, which makes the object a lamp and gives a
black screen that can be dragged around.
at upper left to see how the lens bends some of the rays
of light from the lamp.
With the lamp positioned far to the left of the lens, you should see that the
rays that go through the lens converge to a point. If the screen is placed
where the beams converge, the image on the screen will be in focus (it will
be a small dot of light since that is what the object looks like in this case).
The screen is then at the focal plane. As the lamp is moved closer to the
lens, the distance between the focal plane and the lens _____.
remains the same.
When the lamp is closer to the lens, the rays going into the lens are
diverging more quickly than when the lamp is further away. Thus, after
going through the lens, they aren't converging as quickly, so it takes a
longer distance for them to focus.
The horizontal blue line through the middle of the lens is called the
. As the lamp is moved above or below the axis, keeping the horizontal
distance to the center of the lens constant, how does the horizontal
distance from the lens to the image change?
It does not change.
This explains why we refer to a focal plane. All objects with the same
horizontal distance to the lens will be focused by the lens on the same
plane. You can see this by selecting
and placing one of the
lamps above original lamp.
Do the rays that go through the lens always converge on the right side of
the lens (forming a real image), regardless of the position of the lamp?
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