Peoples Physics Concepts

16.7. Lens http://www.ck12.org

diverging rays, you should use dotted lines for the extrapolated lines in front of a lens in order to produce the

virtual image. When comparing measured distances and heights to calculated distances and heights, values within

10 % are considered “good.” Use theTable(16.5) as your guide.

TABLE16.5:

Mirror type Ray tracings

Converging lenses

(convex)

Ray #1: Leaves tip, travels parallel to optic axis,

refracts and travels through to the focus.

Ray #2: Leaves tip, travels through focus on same side,

travels through lens, and exits lens parallel to optic axis

on opposite side.

Ray #3: Leaves tip, passes straight through center of

lens and exits without bending.

Diverging lenses

(concave)

Ray #1: Leaves tip, travels parallel to optic axis,

refracts OUTWARD by lining up with focus on the

SAME side as the candle.

Ray #2: Leaves tip, heads toward the focus on the

OPPOSITE side, and emerges parallel from the lens.

Ray #3: Leaves tip, passes straight through the center

of lens and exits without bending.

Example 1

You have a converging lens of focal length 2 units. If you place an object 5 units away from the lens, (a) draw a ray

diagram of the situation to estimate where the image will be and (b) list the charactatistics of the image. Finally (c)

calculate the position of the image. A diagram of the situation is shown below.

Solution

(a): To draw the ray diagram, we’ll follow the steps laid out above for converging lenses.
First we draw the a ray that travels parallel to the principle axis and refracts through the focus on the other side (the
red ray). Next we draw a ray through the focus on the same side that refracts out parallel (the green ray). Finally we
draw the ray that travels straight through the center of the lens without refracting (the blue ray). The result is shown
below.