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# Lenses- focal lengths and thickness Watch

1. I've recently conducted an experiment using 4 different width convex lenses (15mm, 13mm, 8mm and 6mm) and I noticed that the two thickest lenses had focal lengths that were very similar (around 0.2cm difference between the average focal length of the 15mm and 13mm lenses).

However, the two other lenses, had focal lengths that were very far apart; on average around 170cm between the average focal lengths of the 8mm and 6mm lenses.

Does anyone know why this is? I know it's do do with the curvature of the lens, but the two thickest and two thinnest lenses have the same width difference- 2mm- so I thought the distance between each pair would have been similar.
2. (Original post by wegeton)
I've recently conducted an experiment using 4 different width convex lenses (15mm, 13mm, 8mm and 6mm) and I noticed that the two thickest lenses had focal lengths that were very similar (around 0.2cm difference between the average focal length of the 15mm and 13mm lenses).

However, the two other lenses, had focal lengths that were very far apart; on average around 170cm between the average focal lengths of the 8mm and 6mm lenses.

Does anyone know why this is? I know it's do do with the curvature of the lens, but the two thickest and two thinnest lenses have the same width difference- 2mm- so I thought the distance between each pair would have been similar.
The "Lensmaker's Formula" is what you want.

http://en.wikipedia.org/wiki/Lens_(o...r.27s_equation

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