The Focal Point: Where Images Escape to Infinity

1 · Predict

As an object approaches a converging lens's focal length from beyond it, what happens to the image distance?

• The image distance shrinks toward zero.
• The image distance stays roughly constant.
• The image distance grows without bound, diverging to infinity right as the object reaches the focal point.

2 · Set Up

1. Open the focal-point preset and press Reset. The lens's focal length is fixed at 10 cm.
2. Enable the image-distance readout.
3. Set the object distance for each trial (approaching the focal length from beyond it) and record the image distance.

3 · Collect Data

Plot image distance d_i (y-axis) against object distance d_o (x-axis) for your three trials. Does it shoot upward as d_o approaches 10 cm?

4 · Analyze

1. For one trial, compute d_i = d_o·f/(d_o − f) using f = 10 cm. Compare to the table.
2. Explain, using the thin-lens equation, why d_o − f approaching zero (from the positive side) sends d_i toward positive infinity.

5 · Extend

1. When the object sits exactly at the focal point, rays leaving the lens emerge perfectly parallel — an image 'at infinity.' Explain why a lighthouse or searchlight places its bulb exactly at the lens's (or mirror's) focal point.
2. This exact setup — a source at the focal point producing parallel output rays — is called a collimator, used to create parallel beams for lab instruments. Why might scientists need a beam of perfectly parallel light for certain experiments?

The Physics Behind This Experiment

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