Bench Degree·LASERSchapter
Appendix A: The Bench
The whole volume’s kit in one place. Every experiment in this book is on this list, so you should never have to stop mid-chapter and wait two days for a delivery. Each chapter names what it needs when it needs it and points back here; if you would rather order once and be done, order from this list.
This volume is cheaper to equip than any other in the series. The starter set is about seventy dollars, and a good deal of it is already in the house.
Nothing here is specified by brand or by part number. Every item is described by function, range or grade, so the list stays orderable from anyone, anywhere, for as long as the book exists. Suppliers change and a printed link dies permanently. A specification does not.
Section 1: The starter set, for Chapters 1 to 9
| Item | Specification | Rough cost |
|---|---|---|
| Red laser pointer | Under 1 mW, Class 2, and marked as such on the body rather than only on the box. 650 nm is usual. The three-dollar keychain kind is exactly right. | $4 |
| Green laser pointer | Under 1 mW, Class 2, 532 nm. Buy two if you can: one to use and one to take apart in Chapter 8. | $9 each |
| Flashlight | Any. Called a torch in Britain. You almost certainly own one. | owned |
| Audio CD | Any, scratched is fine, label side irrelevant. The track pitch is a manufacturing standard of 1.6 µm (0.000063 in) on every audio CD ever pressed, which is the ruler Chapter 1 measures light with. | owned |
| DVD | For comparison. Its track pitch is 0.74 µm (0.000029 in), less than half the CD’s. | owned |
| Tape measure or metre rule | Ordinary. Chapter 1 and Chapter 6 both need two lengths measured to a millimetre or so. | owned |
| Diffraction grating | Optional, because a CD does the job. A cheap film grating is easier to hold to your eye. | $8 |
| Visible-blocking filter | For the infrared test in Chapter 2. A welding filter of shade 5 or darker, a piece of exposed and developed colour negative, or the magnetic film out of a 3.5 in floppy disk. Any of the three works. | $6 or free |
| Phone | With a camera. Most have some sensitivity past 700 nm, which is what makes the infrared leak test possible. | owned |
| Small screwdriver set, tweezers, magnifier | For the Chapter 8 teardown. A phone macro lens will do for the magnifier. | $12 |
| Laser safety goggles | Rated for 532 nm AND 1064 nm, with the wavelength range printed on the lens itself, not just the packaging. Chapter 2 Section 6 explains why a pair rated only for the green is worse than none at all. | $25 |
| Table salt | For the flame spectrum in Chapter 3. | owned |
Starter set total: about $70, or under $50 if you own a flashlight, a rule and some discs.
The goggles are the one line not to economise on, and they are the one item whose specification you must actually read. A green pointer emits 1064 nm as well as 532, per Chapter 8, and eyewear that blocks only what you can see removes your caution without removing the hazard.
A welding helmet is not laser eye protection, and this is worth spelling out because a lot of us own one and nobody wants to buy a second thing. A welding filter is built and rated for the broadband glare of an arc, under a different standard from laser eyewear, and it carries no attenuation figure at any particular wavelength. Laser eyewear states an optical density at a stated wavelength, because a laser puts all of its energy into one line and what matters is the attenuation at exactly that line and nowhere else. A filter that is very dark to your eye may be nearly transparent in the near infrared, which is the part of a green pointer’s output you cannot see and cannot blink away.
And an auto-darkening helmet is worse than a fixed one here, not better. It darkens because a sensor detects an arc and drives the shutter, which takes milliseconds. A Q-switched pulse from Chapter 12 is finished in nanoseconds. The shutter will still be clear when the pulse arrives, and all you have is whatever passive ultraviolet and infrared filtering the lens does in its light state.
Where a welding helmet genuinely does the job is an arc, which is why the plasma volume asks for one and this one does not.
Section 2: Added for Chapters 10 to 16
| Item | Specification | Rough cost |
|---|---|---|
| Microscope slides | A box. One is the beamsplitter for Chapter 11; the rest are spares, because you will break some. | $6 |
| Two small flat mirrors | Flat, and glass rather than plastic. A shaving mirror is far too curved. Small glass craft mirrors 25 to 50 mm (1 to 2 in) square are ideal. | $6 |
| A rigid base | An offcut of 18 mm (0.7 in) plywood or worktop, roughly 300 by 400 mm (12 by 16 in). Rigidity matters more than anything else in Chapter 11, and a table top is not rigid enough. | free |
| Modelling clay or hot glue | To mount optics. Clay is better because it is adjustable. | $5 |
| Clear acrylic rod or tubing | 200 mm (8 in) or so, for the total internal reflection experiment in Chapter 12. A clear plastic bottle also works. | $5 |
| Photodiode | Any small silicon photodiode, or one salvaged from a dead device. With a multimeter it gives relative readings. Chapter 16 explains at length what it will and will not tell you. | $3 |
| Multimeter | The single most useful twenty dollars an engineering mind can spend, and it appears in every volume of this series. | owned, or $20 |
| Graph paper and a sharp pencil | Chapter 6 asks you to plot readings. Do it on paper; the shape is the point. | $4 |
Second set total: about $50.
Section 3: The one expensive thing, and whether you need it
A thermal laser power meter costs a few hundred dollars for a usable head and reads any wavelength honestly, which no cheap method does.
You do not need one for any experiment in this book. Every measurement asked for here is a wavelength, a length, an angle or a relative comparison, and none of those needs it.
You would want one the moment you owned a laser whose output you did not trust, which Chapter 15 suggests is most of what is sold online as a medical device, and Chapter 8 suggests is a fair share of green pointers. If you ever find yourself wanting to know what a beam actually is rather than what its label claims, this is the instrument that answers, and Chapter 16 is about why the cheap substitutes lie.
Borrow one before buying one. A university lab, a maker space, an eye clinic or a sign shop will have one, and most will let you put a pointer on it out of curiosity.
Section 4: What you already own more of than you think
A CD is a precision diffraction grating. Its track pitch was standardised in 1982 and every disc pressed since holds it. That is the whole basis of Chapter 1’s measurement, and it cost nothing.
A phone camera is an infrared detector. Not a good one, and good enough to answer the only question that matters about a cheap green pointer.
A floppy disk contains a usable infrared filter. So does a strip of developed colour negative, which is why photographers have known this trick for decades.
And a dead laser pointer is four devices, which Chapter 8 takes apart. Do not throw one away.
Section 5: The experiments, and where they are
Nine, all of them at Class 2, and the whole list fits on the kit above.
| Experiment | Chapter | What it costs |
|---|---|---|
| Flashlight against pointer: spread, colour, and speckle | 1 | nothing |
| Measure the wavelength of light with a CD and a rule | 1 | nothing |
| One atom, one colour: flame spectra through a grating | 3 | nothing |
| Find out whether your pointer leaks infrared | 2 | nothing |
| Watch a laser cross its threshold, using a freezer | 5 | nothing |
| Measure divergence, then compute the beam waist | 6 | nothing |
| Take a green pointer apart and find all four devices | 8 | $9 |
| Light round a corner: total internal reflection | 12 | $5 |
| Build a Michelson interferometer | 11 | $40 |
The two in bold are the ones to do if you only do two. The first measures a fundamental quantity with a tape measure. The second builds an instrument that reports someone walking across the room, and whose refined version detected two black holes colliding.
Section 6: Kits
A single collection covering everything above, ordered once, is the obvious convenience and it is being worked on. When it exists it will be at benchdegree.com/kit/lasers, and that address is deliberately the only place it will be published: suppliers and product listings change, and a link printed in a book cannot be corrected. The specifications in this chapter can be taken to any supplier and will still make sense in twenty years.
Bench Degree
Get the degree without the diploma.
Learn the material, not how to pass the exam.