What are the rules for the formation of image by a concave lens?

When light rays pass through a concave lens, they diverge, leading to the formation of specific types of images. The rules for image formation by a concave lens primarily describe the characteristics and location of the image based on the object's position.

General Characteristics of Images Formed by a Concave Lens

A fundamental rule for concave lenses is that the image they form always exhibits consistent characteristics, regardless of the object's position:

• Virtual: The image is formed by the apparent intersection of diverging light rays, meaning it cannot be projected onto a screen.
• Erect: The image appears upright, in the same orientation as the object.
• Diminished: The image is always smaller than the actual object.

Specific Cases of Image Formation

The position of the object relative to the concave lens dictates the precise location of the image. Here are the two primary cases outlining the rules for image placement:

Case 1: Object Placed Anywhere Between Optical Centre and Infinity

• Object Position: When an object is positioned anywhere between the optical centre (the geometric center of the lens) and infinity. This includes positions like beyond the principal focus (F) or at 2F.
• Image Formation Rule: The image formed is always located between the optical centre and the principal focus (F) of the lens.
• Image Characteristics: As per the general rule, the image will be virtual, erect, and diminished.

Case 2: Object Placed at Infinity

• Object Position: When an object is placed at an extremely large distance from the concave lens, effectively considered at infinity.
• Image Formation Rule: The image formed by the concave lens will be precisely at its principal focus (F).
• Image Characteristics: Consistent with the general rule, this image will also be virtual, erect, and highly diminished (often appearing as a point image).

To summarize the image formation rules for a concave lens: