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Physics · 3.2.4 Thin converging lens · Paper 6 practical

Convex Lens. Focus it.

Determine the focal length f of a thin converging lens using an illuminated object on an optical bench. For each object distance u, find the image distance v that gives a sharp image; then apply 1/f = 1/u + 1/v.

0625 Topic 3.2.4 — Thin lenses Paper 6 — ATP
Setup — set the object distance, then drag the screen until the image is sharp.

Drag the screen along the optical bench. Shortcuts Space auto-focus · Enter record · R reset.

Variables

25.0
40.0
10.0
⚠ u < f — the image is virtual and cannot be projected onto a screen. Increase u.

Live readouts

Object distance u
25.0 cm
Screen distance v
40.0 cm
Sharp image position vimage
— cm
Computed f = uv/(u+v)
— cm
Image height
×1.0
Sharpness
Out of focus

Trial data — vary u, measure v

Try u = 15, 20, 25, 30, 40 cm. Auto-focus the screen, then record.

1/v vs 1/u — intercepts give 1/f

📋 Method (Cambridge ATP procedure)
  1. Set up the illuminated object (e.g. an LED with a crosswire mask) at one end of the optical bench.
  2. Place the convex lens at a distance u from the object, where u > f.
  3. Move the white screen back and forth until the projected image of the crosswire appears sharp on the screen.
  4. Measure the image distance v from the lens to the screen using a metre rule.
  5. Repeat for five different object distances u from 15 cm to 40 cm.
  6. For each pair (u, v) compute f = uv / (u + v).

Analytical control: plot a graph of 1/v (y) against 1/u (x). The line has gradient −1; both intercepts equal 1/f.

⚠ Sources of error & precautions
  • Finding the sharp image — move the screen slowly forward and back across the sharpest position to bracket it; record the midpoint.
  • Parallax when reading distances — view the metre rule perpendicular to the scale.
  • Position of the lens centre — measure to the centre of the lens, not the holder.
  • Ambient light — darken the room so the image contrast is high.
  • Lens chromatic aberration — different colours focus slightly differently; use a single-colour LED if possible.
🧪 Apparatus list
  • Optical bench with metre-rule track
  • Illuminated object (LED with crosswire mask)
  • Thin convex (converging) lens on a holder
  • Matte white screen on a holder
  • Metre rule (resolution ± 1 mm)
🎯 Syllabus reference (0625)
  • 3.2.4 Thin converging lens — describe an experiment to find the focal length f of a thin converging lens by the method of locating a real image on a screen using an illuminated object; describe the characteristics of a real image and contrast them with those of a virtual image.
  • Paper 6 — plan and carry out an experiment; record results in a table; plot graphs with sensible scales and a best-fit line; calculate a quantity from a graph using intercepts or gradients.

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