I needed to know the power my reading glasses to shop for spares. Problem was, I wasn't sure of the exact figure and of course the original sticker had been peeled off long ago. How to find the dioptre without measuring equipment?
Then I recalled the thin lens formula from high school physics:
For a distant object, that term becomes small so for a convex lens the image comes to focus approximately at the focal length.
So I held the spectacles in front of a wall, covering up one lens to avoid double images, and measured the distance at which distant scenery in the window came into focus. I arrived at 0.4m. The dioptre of a lens is the reciprocal of the focal length in metres so the answer was 2.5. That agreed with my recollection of a sticker reading +250.
This procedure doesn't work for concave lenses used for correcting myopia.
Incidentally optometrists state lens power in dioptres because this measurement has the convenient property that the dioptres of two adjacent lenses add, to a first approximation. Sometimes after an eye exam the optometrist will put a thin correcting lens, e.g. 0.25 dioptres in front of your spectacles to show you it's clearer, and so you need to change your next prescription by that much.
Then I recalled the thin lens formula from high school physics:
For a distant object, that term becomes small so for a convex lens the image comes to focus approximately at the focal length.
So I held the spectacles in front of a wall, covering up one lens to avoid double images, and measured the distance at which distant scenery in the window came into focus. I arrived at 0.4m. The dioptre of a lens is the reciprocal of the focal length in metres so the answer was 2.5. That agreed with my recollection of a sticker reading +250.
This procedure doesn't work for concave lenses used for correcting myopia.
Incidentally optometrists state lens power in dioptres because this measurement has the convenient property that the dioptres of two adjacent lenses add, to a first approximation. Sometimes after an eye exam the optometrist will put a thin correcting lens, e.g. 0.25 dioptres in front of your spectacles to show you it's clearer, and so you need to change your next prescription by that much.
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