Refractive Optical Invisibility & Index Matching
A challenging, hands-on discovery and STEM project designed for curious young minds aged 10 to 14.
Materials Needed
- • Borosilicate (Pyrex) glass test tube or small Pyrex measuring cup (1 unit)
- • Pure vegetable glycerin (or baby oil / mineral oil) (2 cups)
- • Clear glass beaker or wide drinking glass (2 units)
- • Water (2 cups)
Pro Tips
- → Regular water has a refractive index of n=1.33, while borosilicate (Pyrex) glass is n=1.47. In water, the glass edges remain visible.
- → Pure glycerin or baby oil has a refractive index of approximately n=1.47, matching the Pyrex glass.
- → When light passes through materials with identical refractive indices, it neither bends nor reflects at the boundary, making the glass visually disappear.
Safety First
Handle glass vessels with care; keep oils away from heat and wipe oil spills promptly.
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Step-by-Step Instructions
Step 1
Fill one clear glass tumbler with tap water and a second identical tumbler with pure vegetable glycerin.
Step 2
Lower a small, clean Pyrex glass test tube into the water and observe how light bends at the surface, highlighting the tube’s contours.
Step 3
Lower the Pyrex tube into the glycerin glass, allowing the viscous liquid to fill the interior of the tube.
Step 4
Observe from all angles: as light travels through the matching refractive index materials, the submerged glass boundaries vanish entirely.