Duduli

It consists of two plano-convex lenses of equal focal lengths. The distance between the two lenses is equal to the focal length of either lens. The convex surfaces of both the lenses face the incident light.

The optical instruments can be broadly divided into two groups.One group forms real images of an object, which is projected onto  screen or photographic plate.The image can be viewed simultaneously by many observers.Projectors belong to this category.The other group forms virtual images of an object and only one observer can see the image.The virtual image formed by instrument is transformed by the eye into a real image.Therefore, the optical instrument forming a virtual image are often called optical aids.

Answer:- These are used to project on the screen a real, inverted and magnified image of an opaque or transparent object so as to be viewed by a large audience.The object is, however, so fitted that its image is seen in erect from.