Overhead projectors are used to show transparency on a screen. What type of lens does an overhead projector use?

Hint: The focal length is a measure of how strongly the system can converge or diverge rays of light and the focal length of concave lens and convex lens are different. The focal length is half of the radius of curvature in the case of a mirror. A person with clear vision can see objects clearly at distances ranging from $25cm$ to essentially to infinity.

Complete step by step solution:
Lens formula which gives a relationship between object distance, image distance and focal length. If the focal length is positive then the system will converge rays of light and if it is negative then the system will diverge rays of light.
Lens formula is given by:
 $\dfrac{1}{v}=\dfrac{1}{u}=\dfrac{1}{F}$
Where $u$ =object distance
$v$ =image distance
$F$ =focal length
Magnification (M) it is defined as the ratio of height of image $({{h}_{i}})$ to the height of object $({{h}_{o}})$
Magnification formula is given by:
 $M=\dfrac{{{h}_{i}}}{{{h}_{o}}}$
An overhead projector is a device that projects an enlarged image of an acetate or other transparency placed on it onto a screen by means of an overhead mirror.
The image formed by a convex lens is a real and inverted image.
An overhead projector uses a convex lens and concave lenses cannot form real images that can be produced onto a screen.

Note: Students the polarity of focal length is negative for the concave lens and it is positive for the convex lens. Concave lens always forms a virtual and erect image while the convex lens forms a real and inverted image. By using the lens maker formula we can build a lens with required focal length and there is standard value for focal length.