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Anti-Reflection Coating
Katharine Blodgett at General Electric, working with Irving Langmuir, learned to build soap-like films one molecule-thick layer at a time. Stacking just enough layers onto glass, she made it nearly invisible: light reflected from the top of the film and from the glass beneath cancelled out.
**US 2,220,861**, "Reduction of surface reflection", Blodgett, assigned to General Electric, filed 16. June 1938, granted 5. November 1940: a coating whose "optical thickness … corresponds at least approximately to a quarter wave length of visible light" and whose refractive index is "intermediate" between air and the glass.
**You measure the reflection a coating removes**, design one in a notebook, and watch quarter-wave interference happen in a soap film.
Beginner
90 minutes
Instructions
1
1
Measure bare glass
Measure bare glass
Clip the solar cell to the multimeter (DC mA). In a dim room shine the laser straight onto the cell and note the current. Then bounce the beam off a clean microscope slide, close to perpendicular, onto the cell. Note that current. Never look along the beam.
Materials for this step:
Microscope Slides with Coverslips1 pack
Solar Cell Mini - 350mV1 pieceTools needed:
Laser Pointer
Multimeter
Alligator to Alligator Cables2
2
Mix a film solution
Mix a film solution
Stir 1 part detergent and 1 part glycerin into 10 parts water in a jar. Bend copper wire into a 60 mm loop with a handle.
Materials for this step:
Mild Detergent20 ml
Glycerin (Vegetable)20 ml
Copper Wire1 meterTools needed:
Glass Jar
Pliers Set3
3
Watch a film thin
Watch a film thin
Dip the loop and hold it upright in front of black card, lit by a window behind you. Colour bands slide down as the film drains; at the top a black band appears - a film too thin to reflect.
Materials for this step:
Black Card Stock1 sheet4
4
Fresnel, coating design, soap film
Fresnel, coating design, soap film
Loading Jupyter Notebook...
5
5
History & Context
History & Context
**What the patent specifies:** a surface layer of quarter-wave optical thickness with a refractive index between those of the surrounding medium and the glass. **Modern build (derived):** measurement and design only - production coatings are evaporated in vacuum.
Blodgett's stearate films were too soft to wipe clean. Alexander Smakula at Zeiss had developed coatings evaporated in vacuum in 1935; that durable route won, and today almost every camera lens and eyeglass is coated, often with many layers. Their limits: a single layer is perfect at only one wavelength, and coatings scratch.
Materials
6- Placeholder
- Solar Cell Mini - 350mV10% commission1 piece$2.22
- 20 mlPlaceholder
- 20 mlPlaceholder
- 1 meterPlaceholder
- 1 sheetPlaceholder
Tools Required
5- Placeholder
- Placeholder
- Alligator to Alligator Cables10% commission$3.58
- Placeholder
- Placeholder
Estimated Total
What the maker bought. Materials shown without a price are sourced wherever you buy them.
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