Ray Diagrams

The Image Formed by a Lens

  • A converging lens is capable of forming two types of images:

    • Real Image:

    • Can be projected on a screen.

    • Light rays from the object actually converge at a point.

    • Virtual Image:

    • Cannot be projected on a screen (e.g., like a magnifying glass).

Rules for Drawing Ray Diagrams

  • The following are the key rules that describe how light rays are bent by a converging lens:

    • Parallel Ray (Red):

    • A light ray that is parallel to the principal axis of the lens refracts through the far focal point (also known as the principal focus).

    • Centre Ray (Blue):

    • A light ray that passes directly through the center of the lens travels in a straight line and is not refracted.

    • Focal Ray (Green):

    • A light ray that travels through the secondary focus refracts to travel parallel to the principal axis after passing through the lens.

Drawing a Ray Diagram

  • To construct a ray diagram for a converging lens, follow these steps:

    • Draw Ray 1: As per the rules for parallel rays.

    • Draw Ray 2: As per the rules for the center ray.

    • Draw Ray 3: As per the rules for the focal ray.

Setting Up the Ray Diagram

  • The essential components for setting up a ray diagram include:

    • Object: The item being viewed or observed.

    • Lens: The converging lens through which the rays are passing.

    • Optical Axis: The straight line that divides the lens into symmetrical halves.

Understanding the Ray Diagram

  • A ray diagram serves to help determine several characteristics of the image formed:

    • Size: Whether the image is bigger, smaller, or the same size as the object.

    • Attitude: Whether the image is upright or inverted.

    • Location: Where the image is formed in relation to the lens:

    • Inside the focal length (f)

    • Between the focal point (f) and double the focal point (2f)

    • Beyond double the focal point (2f)

    • Type: Whether the image is real or virtual.

Describing Images with SALT

  • The acronym SALT helps in describing images created by lenses, where each letter represents a specific quality of the image:

    • S: Size of the image in comparison to the object.

    • A: Attitude of the image (upright or inverted).

    • L: Location of the image (where it is formed relative to the lens: inside f, between f and 2f, or beyond 2f).

    • T: Type of image (real or virtual).

Summary Characteristics for Images Formed by Lenses

  • The nature of the image produced by a lens can be categorized as follows:

    • When an Object is Located Beyond Focal Length:

    • Image Characteristics:

      • Size: Comparable to the object (real image).

      • Type: Real image formed when the object is beyond focal length (2F) and positioned relative to f and 2f.

    • When an Object is Located Inside the Focal Length:

    • Image Characteristics:

      • Size: Larger than the object.

      • Type: Virtual and upright image.

    • Diverging Lens Characteristics:

    • Regardless of object position, the image is always virtual and smaller than the object.

Ray Diagram for Object Located at F

  • If an object is located at distance equal to the focal length (F), then:

    • Outcome: An image is not formed since the rays will refract parallel and not converge to form an image.