The Fizeau Wedge and the Optical Flat

Newton's rings happen between a curve and a flat. Put two nearly FLAT surfaces together instead, with a hair's tilt between them, and the fringes become straight lines — one for every half wavelength of gap. Hippolyte Fizeau formalised this in 1862, and it turned interference from a philosophical demonstration into a shop-floor instrument. What makes it useful is that a fringe is a CONTOUR LINE. The pattern is a height map of the gap, drawn in steps of about three hundred nanometres, and you read the surface directly: straight fringes mean flat, curved fringes mean the surface bows, and the amount they curve measured against their own spacing IS the departure from flat. That last point is the whole reason it survives. You are not measuring against a ruler that could be wrong — you are measuring the fringes against each other. The tilt you introduced cancels out, and what remains is the surface. Every gauge block, seal face, mirror blank and lens surface made anywhere is checked this way, and the apparatus is two pieces of glass and a monochromatic lamp. It is the cheapest thing in metrology that produces a NUMBER rather than an opinion.

初学者

2 hours

说明

Make the wedge

Clean both flats with isopropyl alcohol. Rest the reference flat on the test surface and press one edge very lightly to introduce a slight tilt. Illuminate with the sodium lamp or a filtered LED from directly above and look straight down. A set of dark bands appears. Press harder and they widen; press elsewhere and they rotate. That responsiveness is how you know you are seeing interference and not a reflection.

此步骤所需材料：

Optical Flat (Reference)1 个

Isopropyl Alcohol 99%30 毫升

Paper Towel1 包

所需工具：

LED Light Source

Hand Lens (10x)

Nitrile Gloves

Read the bow, not the tilt

Photograph the fringes square-on. Lay a straightedge along one dark band in the image and measure two things: the spacing between adjacent bands, and the greatest departure of a band from the straightedge. Divide the second by the first. That ratio, in fringe widths, is your surface error — multiply by half a wavelength for nanometres. The tilt cancels because it affects both numbers equally.

所需工具：

Steel Rule

Desktop Computer

Convert to nanometres, and pick your instrument

正在加载 Jupyter 笔记本…

所需工具：

Desktop Computer

Compendium: wringing, and what a flat is worth

WRINGING, AND WHY IT IS NOT GLUE. Two surfaces flat to a fraction of a wavelength and scrupulously clean will ADHERE when slid together, strongly enough to hang a weight from. Nobody is entirely certain of the mechanism — molecular attraction, a residual film of moisture, and air pressure all contribute — but the effect is what makes a stack of gauge blocks behave as one block of the summed length. It is also why a flat left resting on a surface overnight can be difficult and expensive to separate. THE LIMIT THAT SENDS YOU TO BLUEPRINT 7. Fringes of equal thickness need the two surfaces nearly in contact, so a Fizeau tests flats against flats and cannot look at a lens, a prism or an assembled telescope at all. Putting the test optic into one ARM of an interferometer removes the constraint entirely — that is Twyman and Green's 1916 change, and it costs about two orders of magnitude more money.