Explain the reason in detail:

• Real images are always formed below the principal axis.
• Virtual images are always formed above the principal axis.

Dear Student

Thre rays which are used form ray diagrams are:

First is the central ray that  goes through the center of the optical axis undeviated.

Next is the parallel ray that travels parallel to the optical axis on the left side, and through the focal point on the right side.

Finally is the focal ray that travels through the focal point on the left side and parallel to the optical axis on the right, opposite the parallel ray.

When a real image is formed, it forms on the right side of the lens (assuming the object is on the left). Well, for two of the three rays defined above (central and focal), the ray is ALWAYS below the optical axis on the right hand side (assuming it is above the axis on the left). This means that the only place these rays can converge on the right hand (real) side is below the principal axis and hence form inverted image.

When virtual image is formed, it forms on the left side of the lens (assuming the object is on the left). Well, for two of the three rays defined above (central and focal), the ray is ALWAYS above the optical axis on the left hand side (assuming it is above the axis on the left). This means that the only place these rays seems to be converge on theleft hand (virtual) side is above the principal axis and hence form erect image.