Biomedical Engineering Reference
In-Depth Information
Plus thin lens
F
F
′
Minus thin lens
F
F
′
Figure 4-4.
Determination of the secondary focal points for a plus and minus lens.
A converging lens may form either a real or virtual image. When the
object is located farther from the lens than its focal point, as in Figure 4-6,
the image is always located on the other side of the lens, real and inverted.
It is real because the rays are converging after refraction.
Is this image
smaller or larger than the object? As can be seen in Figure 4-7, the magnification
depends on the object distance. When the object is at twice the focal length, the real
image is the same size as the object (and also twice the focal length from the lens),
but when the object is farther than this distance, the image is minified (and located
farther than the focal length, but less than twice the focal length from the lens).
In comparison, if the object is located closer than twice the focal length (but farther
than the focal length), the image is larger than the object (and located farther than
twice the focal length
from the lens).
As illustrated in Figure 4-8, the situation is very different when the object is
located
closer
than the focal point of a plus lens.
In this case, the rays remain
divergent after refraction necessarily making the image virtual, enlarged, and
erect. It is located on the same side of the lens as the object and farther than
the focal length from the lens.
The image is larger than the object because the
rays that form the image are diverging less than those that emerge from the object.
(You may need to think about this a bit!)
