6. Object placed between focal point and optic centre

If we place an object between focus and optic centre, we will get an image which is virtual, erect and magnified.

From the ray diagram shown in fig.- 17, you will notice that the image formed is a virtual, erect and is formed on the same side of the lens where the object is placed. The size of the image is larger than that of the object. It is a magnified image.

1. As the image formed is virtual, we can see it with our eyes. In all other cases the image is real which we can’t see with our eyes but can be viewed if the image is captured on a screen.
2. A magnified virtual image is formed on the same side of the lens where the object is placed. Thus the image you are seeing through a lens is not real, it is a virtual image of the object.

This particular behaviour of convex lens helps to construct a microscope, which gives a magnified image. You might remember that the magnification of the virtual image is possible only when the object is at the distance less than the focal length of the lens.

Till now we have drawn ray diagrams for various positions of object placed on principal axis using convex lens. Draw ray diagram for an object placed between C₁ and F₁ for a concave lens.

• What do you notice?

Verify your ray diagram with the ray diagram we have drawn for a convex lens. See fig.- 18. Try to draw ray diagrams for other positions of an object. You will notice that irrespective of the position of object, on the principal axis, you will get an erect, virtual image, diminished in size in between the focal point and optic centre for concave lens.

Let us see a few examples of ray diagrams.

Example-4

Draw a ray diagram to locate the position of image when a point source (S) is placed on optical axis MN of a convex lens, in such a way that it is beyond focal point (F₂). See fig.- E(4).

Solution