How Lenses Form Images
Suppose we place an arrow, A
, in front of a convex lens. The ray AC
, parallel to the principal axis, will pass through the lens and emerge as DE
. The ray is always bent toward the thick portion of the lens, both at its entrance into the lens and its emergence from the lens.
We know that two rays determine the position of any point of our image; hence in order to locate the image of the top of the arrow, we need to consider but one more ray from the top of the object. The most convenient ray to choose would be one passing through O
, the optical center of the lens, because such a ray passes through the lens unchanged in direction, as is clear from Figure. The point where AC
meet after refraction will be the position of the top of the arrow. Similarly it can be shown that the center of the arrow will be at the point T
, and we see that the image is larger than the object. This can be easily proved experimentally. Let a convex lens be placed near a candle; move a paper screen back and forth behind the lens; for some position of the screen a clear, enlarged image of the candle will be made.
If the candle or arrow is placed in a new position, say at MA
, the image formed is smaller than the object, and is nearer to the lens than it was before. Move the lens so that its distance from the candle is increased, and then find the image on a piece of paper. The size and position of the image depend upon the distance of the object from the lens. By means of a lens one can easily get on a visiting card a picture of a distant church steeple.
FIG. - The image is larger than the object. By means of a lens, a watchmaker gets an enlarged image of the dust which clogs the wheels of his watch.
FIG. - Rays above O are bent downward, those below O are bent upward, and rays through O emerge from the lens unchanged in direction.
FIG. - The lens is held in such a position that the image of the candle is larger than the object.
FIG. - The image is smaller than the object.
FIG. - The lens is placed in such a position that the image is about the same size as the object.