PHY016 · Optics · Classes 10–12
Convex lens focal length
Objective: To find the focal length of a convex lens by locating images for different object distances.
🧰 Apparatus
- Convex lens on a holder
- Optical bench or metre scale
- Candle or illuminated object
- White screen
📐 Formulas and SI units
1/f = 1/v − 1/u (u, v, f in cm)
m = v/u
P = 1/f (f in m, P in dioptres)
💡 Theory
A convex lens converges parallel rays to its principal focus. For an object at distance u the image distance v obeys 1/f = 1/v − 1/u (Cartesian sign convention). Beyond F the image is real and inverted; inside F it is virtual, upright and magnified.
🪜 Procedure
- Focus a distant object (a window) on the screen to get a rough f.
- Place the candle at about 2f in front of the lens.
- Move the screen until a sharp image forms; record u and v with signs.
- Repeat for four other object distances.
- Calculate f = uv/(u − v) for each and take the mean.
- Plot 1/v against 1/u: the intercepts give 1/f.
📋 Observations Simulated values
✅ Result and conclusion
With u = −30 cm and f = 20 cm, the image forms at v = 60 cm, real, inverted and twice the size; all readings give the same f.
🔬 In a real lab
Sharp focus is a judgement: v readings vary by a few millimetres, so f is found to about ±0.5 cm. Thick lenses and the lens holder offset add small errors.
⚠️ Safety
Keep candle flames away from hair, sleeves and paper; never use the lens to focus sunlight on skin or eyes.
❓ Quick check
1. An object at 2F of a convex lens forms an image…
2. Where should the object be for a magnifying glass?
3. A convex lens has f = 50 cm. Its power is…
🏋️ Practice
Easyu = −20 cm, f = 10 cm. v?
Answer: +20 cm
Mediumu = −15 cm, f = 10 cm. v and m?
Answer: v = +30 cm, m = −2
Challengeu = −5 cm, f = 10 cm. v and nature?
Answer: v = −10 cm, virtual, upright, ×2