Learning Outcomes:


Candidates should be able to:

(i) describe the action of a thin converging lens on a beam of light


(j) define the term focal length


(k) *draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens


(l) define the term linear magnification


(m) *draw scale diagrams to deduce the focal length needed for particular values of magnification (converging lens only)


(n) describe the use of a single lens as a magnifying glass and in a projector and draw ray diagrams to show how each forms an image

 

Thin Converging lens

Action of a thin converging lens on a beam of light

Focal length, f, of the lens is the distance between the focal point, F, and the optical centre, C, of the lens

 

Ray Diagram

Rules for Drawing Rays

    Rule 1: The ray leaving the tip of the object traveling parallel to the optical axis will pass through the focal point F2 after passing through the lens.

    Rule 2: The ray leaving the tip of the object and passing through the focal point F1 will emerge from the lens traveling parallel to the optical axis.

    Rule 3: The ray leaving the tip of the object and passing through the center of the lens will emerge from the lens undeflected.

Click here to understand a bit more about using ray diagram to locate an image. Try the example 2.1 and 2.2 and click here for step by step leading to the answers.

Alternatively, you can view the following java applets to have a pictorial representation of the use of ray diagram.

Click on the screen capture of the java applet in action to access.

For the more adventurous one, you can visit here to download a ray tracing program to learn more about ray diagrams. Look for the file lens1455.exe for a colour version of this program. Have fun.

 

Linear Magnification

The magnification is defined as the ratio of the image size to the size of the object being imaged.  When the image is upside down, the magnification is negative.  If the image is upright, that is the same orientation as the object, the magnification is positive.  From the ray diagram for either a positive or negative lens the magnification (sometimes called the transverse magnification), m, can be shown to be equal to -di/do, where do and di are the object and image distances respectively.

 

Use of ray diagram to find focal length

The use of a ray diagram is not always use to locate the position and the size of image. It can also be used to locate the focal length. Refer to the diagrams below to see for yourself how to find the focal point, F and the focal length, f.

Ray diagrams to illustrate the formation of real and virtual images of an object by a lens and its uses

Click here to see for yourself the formation of real & virtual images by adjusting the position of the object. The java applet by Prof Fu is pretty useful in learning this.

Of course, you can refer to your textbook for the table "Images formed by a thin converging lens". Please ask your teacher for the page if you cannot find it.

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