1. Focal length
Parallel rays meet at the focus after a convex lens; f is center-to-focus distance.
f = OFO = center, F = focus.
Measuring focal length
Find f by focusing parallel rays or sunlight.
Light phenomenaReflection, intro refraction, mirrors and lenses.
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Measuring focal length
Measure convex-lens focal length on a realistic 3D optical rail: focus parallel rays; lens-to-screen at the sharpest spot is f.
Memory cue: parallel rays → focus · f = lens-to-sharp-spot.
Key formulas
f = OF
How to use
- Recall f and the parallel-ray method.
- On the 3D rail, move the screen until sharp (or snap to focus).
- Read lens-to-screen distance as measured f.
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Related knowledge
Read section by section — and measure f on the 3D optical rail with the parallel-ray method.
2. Parallel-ray method
- Send parallel rays (or a distant scene) along the axis.
- Move the screen until sharp; that distance is f.
3. Sunlight method
Sunlight is nearly parallel. Distance to the smallest spot ≈ f. Never stare at the focus.
4. Pitfalls
- Poor alignment shifts the spot.
- If the object is not far, the value is not exactly f.
Applications
- Instrument design needs measured focal lengths for lenses.
- Classroom optical benches use f and 2f to mark imaging zones.
FAQ
Sunlight method tip?
Sunlight is nearly parallel; find the smallest spot. Never stare at the focus.
Why use parallel light to find f?
Axial parallel rays meet at the focal point; distance to center is f.
Common measurement errors?
Non-parallel source, misalignment, parallax — average repeats.
What does the “Measuring focal length” lab practice?
Tune parameters and match readouts to the “Measuring focal length” relations — useful for prep and demos.