This is an easy to make laser called the TEA laser that uses parts from around the house or that you can find in local stores. It puts out ultraviolet light, which you can't see. But if you shine it on white paper or a mark made using a highlighter pen, then it get's converted to visible light. You can also send it through a liquid containing fluorescein and it will show up. The T.E.A stands for Transverse Electrical discharge at Atmospheric pressure, though it's sometimes thought of as Transverse Excitation at Atmospheric Pressure.
The wavelength of light that a TEA laser gives off is 337.1nm (nanometers). To give you an idea of how small that is, the average diameter of a human hair is 100um and so it would take 297 of those waves to make up that diameter. It's in the ultraviolet range of wavelengths of light, which humans can't see.
This particular design is one that I find is easy to get working. It's called the Against the Wall design and is by Nyle Steiner.
The following is the circuit diagram for the TEA laser.
Using the TEA laser for other things
Video (demonstrations) - TEA Laser - Easy Homemade/DIY Laser
Demonstrations and experiments of this TEA laser in use. That includes showing it firing a long distance and reflecting in a mirror.
Video - How to Make a TEA Laser - Homemade/DIY Laser
Step-by-step instructions on how to make this TEA laser.
Video - TEA Laser Powered by Wimshurst Machine
It's actually possible to power this TEA laser using a Wimshurst machine, a type of hand-cranked electrostatic machine invented in the 1880s. The following video shows this. And given the simplicity of construction of the TEA laser, that means it would have been possible to make and power one of lasers back then!