Lens glass must be free of bubbles, free of striae (compositional streaks) and free of internal strain, and it takes three stages to make one: melt optical-quality glass, press a blank, then grind and polish it. For common soda-lime glass with a refractive index near 1.5, the focal length of a single convex lens is about the radius of curvature divided by 0.5, so a 250 mm radius gives roughly a 500 mm focal length. A single convex lens is a magnifying glass, and two lenses make a telescope or microscope.
Part of Glass Blowing and Forming
What distinguishes lens glass from container glass and the route from melt to finished lens.
Lens Glass Fails On Defects Container Glass Hides
A bottle can hold a bubble, a streak or a patch of strain and still work. A lens cannot: a bubble scatters light in every direction, a streak bends it unevenly, and strain makes the glass split under the grinding tool or bend light differently in different directions. The corpus’s Bubble-Free Glass article sets out why bubbles scatter light and ruin contrast.
| Stage | What it removes | Article |
|---|---|---|
| Optical melt | Bubbles and streaks | Optical Quality Glass |
| Pressing a blank | Gives a disc near the final size | Pressing Lens Blanks |
| Slow anneal | Strain | Annealing Oven |
| Grinding and polishing | Rough surface | Basic Lens Grinding |
What You Need
- Pure sand and fresh flux; the corpus’s Silica Sources article says iron contamination turns glass green or brown.
- A furnace that can hold a melt near its maximum temperature for hours.
- An annealing oven that can cool very slowly.
- Grinding tools and abrasives.
Method: From Glass To Lens
Step 1: Make the glass. See Optical Quality Glass.
Step 2: Make the blank. See Pressing Lens Blanks.
Step 3: Grind and polish. See Basic Lens Grinding and the fuller Lens Grinding article.
Step 4: Test. Check the surface by reflection and the glass between crossed polarising filters.
Worked Example
The corpus’s Lenses article gives focal length ≈ radius of curvature ÷ (n − 1). For n = 1.5, n − 1 = 0.5. A radius of 250 mm gives 250 ÷ 0.5 = 500 mm. In dioptres (1 ÷ focal length in metres), 1 ÷ 0.5 = 2.0 dioptres. A fire-starting lens with a 25-50 mm radius has a focal length of 50-100 mm.
Common Mistakes
- Using window or bottle glass for fine work without checking it. It may carry bubbles or strain.
- Skipping the slow anneal. The blank splits under the tool.
- Grinding dry. See the dust warning below.
Safety
Silica dust from grinding and batch work
Grinding glass with sand, and mixing and sieving a sand batch, raise fine dust, and crystalline silica dust (from sand and quartz) scars the lung permanently. Grind wet, keep the slurry from drying on the bench and floor, never sweep it dry, and mix batch outdoors or wet. See Dust and Respiratory Protection.
Hot glass does not look hot
Pressed discs and melt samples fall below about 600 °C and look cold while burning skin.
What’s Next
- Learn next: Optical Quality Glass — the melt
- Combine with: Optics — what lenses are used for