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Convex and Concave Lenses Ground as a Pair
Rub one glass disc over another with wet grit and both change shape: the one on top goes hollow, the one beneath goes domed, and they stay a perfect fit. Grind both faces of a pair this way and the bottom disc becomes a double-convex lens, the top one a double-concave lens of the same curve. Longer work gives deeper curves and shorter focal lengths.
The picture: a double-convex lens held at arm's length shows the scene small and upside down.
Advanced
Several days per pair
Instructions
1
1
Cut two discs
Cut two discs
Score two equal circles on the glass sheet, snap them out and nibble the edges round with grozing pliers. Fix a short wooden handle to the back of each with warm pitch.
Materials for this step:
Glass Sheet1 piece
Pine Pitch Glue20 g
Hardwood Strip1 pieceTools needed:
Glass Cutter
Grozing Pliers
Safety Glasses2
2
Rough grind the first faces
Rough grind the first faces
Lay one disc face up on a firm table, wet it and sprinkle coarse grit. Push the other disc over it, face down, in short strokes, turning both a little all the time. The top one becomes concave, the bottom convex. Rinse and recharge as the grit wears.
Materials for this step:
Silicon Carbide Grit50 g
Water2 litersTools needed:
Ruler3
3
Check the curve
Check the curve
Lay a ruler across the hollow disc and measure the depth at the centre with the caliper. Keep the top disc on top to deepen the curve; swap them to flatten it.
Tools needed:
Ruler
Digital Caliper 6-Inch4
4
Fine grind and polish
Fine grind and polish
Go through finer grits, washing everything clean between grits. Polish each face on a pitch lap pressed to the other disc's curve, with tin oxide (putty powder) and water, until clear.
Materials for this step:
Silicon Carbide Grit30 g
Tin Oxide (Stannic Oxide)30 gPine Pitch Glue100 g5
5
Turn them over, do the second faces
Turn them over, do the second faces
Move the handles to the finished faces and grind the other sides the same way, the same disc on top, to the same depth. The bottom disc is now a double-convex lens, the top one double-concave.
Materials for this step:
Silicon Carbide Grit60 gTin Oxide (Stannic Oxide)20 gTools needed:
Digital Caliper 6-Inch6
6
Find the focal length
Find the focal length
Hold the convex lens in sunlight over paper and move it until the spot is smallest; that distance is its focal length. Grind more pairs to different depths for a set of focal lengths.
Tools needed:
Ruler7
7
Focal length from the curve
Focal length from the curve
Loading Jupyter Notebook...
8
8
Limits
Limits
Which disc goes hollow is the mirror makers' rule (Sky & Telescope, Stellafane); the lens formula is from Wikipedia. Window glass is not optical glass: expect some colour fringes and bubbles.
Materials
6- 1 piecePlaceholder
- 120 gPlaceholder
- 1 piecePlaceholder
- Placeholder
- 2 litersPlaceholder
- Placeholder
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