StellarTypes of Lenses
Lenses come in two primary types. A diverging (concave) lens causes light rays to diverge, making them appear to originate from a point behind the lens. Due to this, they will always form virtual images. Also, the focal length of a diverging lens is always considered to be negative, which is important to understand when calculating the image distance.
A diverging (convex) lens causes parallel rays of light to converge at a focal point, and can produce real images when the object is beyond the focal length and virtual images when the object is within the focal length. For example, think of magnifying glasses. In these lenses, the focal length will always be positive, which is important to remember in calculations.