Image formation in spherical lenses
Converging and diverging spherical lenses form different types of images and are widely used in the correction of vision problems.
The elements that make up spherical lenses are:
- Optical center (O) : the so-called thin lenses have a thickness much smaller than the size of their faces. This makes the vertices of the faces of these lenses almost occupy the same point, which is defined as the optical center of the lens;
- Focus (f): point where the light is focused;
- Anti-principal point (2f) : point that corresponds to twice the focus in distance from the optical center;
- Principal axis: line containing the anti-principal point, focus, and optical center of a lens.
Properties of spherical lenses
The behavior of spherical lenses when receiving incident light determines the formation of different types of images. Light rays falling on spherical lenses are refracted in three ways:
- Every ray of light that falls parallel to the principal axis is refracted in the direction of the focus;
- Every ray of light that strikes the lens through the focus is refracted parallel to the principal axis;
- Any ray of light incident on the optical center is not deflected.
Converging lens images
As the properties of spherical lenses follow, converging lenses form five distinct types of images:
1. When the object (shown in blue) is positioned before the anti-principal point, the lens forms an image (shown in red) that is real, inverted and smaller than the object.
2. When the object (shown in blue) is positioned over the anti-principal point, the lens forms a real image (shown in red), inverted and equal to the object .
3. When the object (shown in blue) is positioned between the anti-principal point and the lens focus, the image formed (shown in red) is real, inverted and larger than the object .
4. When the object (shown in blue) is positioned over the focus of the lens, an image m is not formed , as the refracted rays are parallel and never cross to form an image of the object.
5. When the object (shown in blue) is positioned between the focus and the optical center of the lens, its image (shown in red) is virtual, straight and larger than the object .
Image of the diverging lens
Diverging lenses are capable of forming only one type of image, because whatever the position of a body (represented in blue) in front of a diverging lens, its image (represented in red) is virtual, straight and smaller . Lenses of this type are used to correct myopia .