Lens & Ray Diagrams

There are two main types of lens:

Shape of Lens	Convex	Concave
Ray Diagram
Affect on rays	Converging	Diverging

The thickness of the lens affects how much it bends light and therefore the power of the lens. This can be measured using the focal length (f) of the lens.

• For a convex lens this is the distance from the centre of the lens to the point at which parallel rays of light converge (meet). The shorter the distance the more powerful the lens, the light has been refracted more.
• For a concave lens this is the distance from the centre of the lens to a where rays appear to have come from when parallel rays entered the lens.

However is is quite rare that we will be dealing with parallel rays as they are from a distance source. The lenses will will bring the rays of light to a focus at points other than the focal distance. To understand this we need to be able to construct ray diagrams.

To draw a ray diagram, we draw 3 rays front the top of the object:

• Horizontally to the axis of the lens and the through the far focal point
• Through the centre of the lens which is no refracted ie straight through
• Through the near focal point to the axis of the lens and then horizontally out the otherside

Where the rays converge (meet/cross) or appear to have come from converging is were the image is formed.

Convex Lens

You can move and change the size of the object (black arrow) using your mouse. See what it does to the rays and where the image (blue arrow) is formed. The green lines represent where the rays appear to have come from
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Thin Lens demonstration by Fu-Kwun Hwang

The nature of the image produced depends on where the object is relative to the focal length:

• If the image is larger than the object, we say 'the image has been magnified'.
  If it is smaller then we say 'the image has been diminished'.
• It the image is formed on the other side of the axis to the object, then we say 'the image is inverted'.
  If the images is formed on the same side then we say 'the image is upright'.
• If the rays that form the image actually cross then we say 'the image is real'. This type of image can be projected on to a screen.
  If the image is formed where rays appeared to come from then we say 'the image is virtual'. In order for the images to be seen, it takes the extra lens in the eye to converge the rays.

Use either the applet above, or construct your own ray diagrams on paper, to complete the table below:

Position of object	Magnification	Orientation	Type of image
Beyond 2f
Between 2f and f
Inside f

To calculate the magnification produced by a lens or mirror use the formula:

Concave Lens

The same can be done for concave rays.

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Thin Lens demonstration by Fu-Kwun Hwang

Position of object	Magnification	Orientation	Type of image
Anywhere

Uses of lens

Cameras make use of lens to produce an image of an object on a detecting device - film or sensor. The image needs to be so that a focused image is formed on the detector's surface. The image normally needs to be than the object so that it will fit on to the sensor (the exception to this is macro photography).

Because of all of this a lens with a shape is used with the object positioned further than focal lengths away from the lens.

Magnifying glasses are used to make objects appear larger. Therefore the image needs to than the object and ideally the images should be for easy viewing.

Because of this a lens with a shape is used with the object positioned between and focal lengths away from the lens. images are produced which require the eye to bring them into focus.