• What happens when parallel rays of light fall on a lens making some angle with the principal axis?

Let us observe the following figures

When parallel rays, making an angle with principal axis, fall on a lens, as shown in figures 12(a) and 12(b), the rays converge at a point or appear to diverge from a point lying on the focal plane. Focal plane is the plane perpendicular to the principal axis at the focus.

Rules to draw ray diagrams for formation of lenses

Let us learn a few basic rules to draw ray diagrams to locate the position of images.

For drawing a ray diagram to find position and size of the image formed by lens for any position of object on the principal axis you need to follow the rules mentioned below:

For locating position and to find the size of image, we need two rays out of four rays that were mentioned in the situations I to IV.

• Select a point on the object placed at a point on the principal axis.
• Draw two rays that were chosen by you from rays mentioned in situations I to IV
• Extend both rays to intersect at a point. This point gives position of the image
• Draw a normal from point of intersection to the principal axis.
• Length of this normal represents the size of the image.

Observe the following diagrams (situations 1-6). They represent image formation by a convex lens for various positions of the object.

1.Object is at infinity

• What do you mean by an object at infinity?
• What type of rays fall on the lens?

You know that the rays falling on the lens from an object at infinity are parallel to the principal axis.