UChicago Instructional Physics Laboratories

Air currents are made visible through the use of a single mirror schlieren optical setup, which provides a qualitative method to observe minute variations in the index of refraction of air. These variations can be due to the presence of a different gas or to the presence of temperature or pressure gradients, which affect the indices of refraction of materials. This demonstration can be a good way for students to visualize convective air currents, turbulent and laminar flow, standing sound waves, etc.

The setup consists of a pinhole light source, a concave spherical mirror of 1 m focal length, a razor blade and a video camera connected to a monitor. The pinhole source is placed at the center of curvature of the mirror, slightly off axis, and aimed to the surface of the mirror. A razor blade is placed at the center of curvature such that the light from the pinhole focuses just at the edge of the blade, which partially blocks the beam. The light is then sent to a video camera that is coaxial with the mirror, which allows one to see variations in the index of refraction through the monitor.

Given that at the center of curvature the light beam is essentially point-like, by partially blocking its path, any deviation in the beam path caused by a varying index of refraction will cause light that would have otherwise hit the camera sensor undisturbed to bend towards or away from the blade. Light that bends toward it will be blocked and will appear as a dark spot on the sensor; light that bends away from it will hit a position on the sensor where other undisturbed light is already hitting, thus creating a bright spot. Therefore, the image formed on the sensor will be a 2D representation of the spatial distribution of the index of refraction of the air in the testing region.

As it is currently, the setup consists of two main components: a 1 m focal length spherical mirror mounted on a kinematic telescope primary mirror mount attached to an aluminum base, and a camera mount and optical breadboard attached to an optical rail. On the breadboard, a razor blade is mounted on two perpendicularly installed translation mounts that allow for x and z displacement, z being the axial direction of the mirror and beam path, and x being the horizontal and perpendicular direction to it. By placing the razor blade such that the edge is perpendicular to the table as shown in the picture, having y adjustment is not crucial as long as the beam hits the edge. The pinhole source is placed around 2 inches to the side of the razor blade, ensuring that the output is roughly at the same z position as the razor edge. Behind the razor blade, I placed a mount for installing a laser for alignment purposes. Finally, behind the breadboard, a camera mount with fine x and y adjustment is placed, so that we can align the sensor to be coaxial with the beam. In this case z adjustment is not too crucial, though we want it to be as close as possible to the focus point such that the greatest amount of light hits the sensor before it diverges.

Given that there are only two main components to the setup, the alignment process is relatively simple as long as the distance between the mirror and the blade is precisely 2 m. For this reason, a metal spacer was made after initial alignment; it runs from the front edge of the aluminum mirror mount to the front edge of the optical rail.

The mirror mount, clamped to the edge of the table. This is needed if you want to get the whole setup to fit on a single table. The sparkly block is a lead brick; that and the come-along are being used to secure the optical rail to the table

Provided the position of the breadboard on the optical rail is not changed, placing the mirror and rail at each end of the spacer ensures that the blade is within a few millimeters from the center of curvature of the mirror; one can then correct for any deviation from 2 m with the z translation mount for the blade.

The other end of the setup. Note the 2m stick shown here has its other end at the base of the mirror. The optics are most straightforwards when both the light source (bottom) and razor (middle) are 2m away from the mirror.

To align the setup, a laser is installed behind the razor blade and aimed toward the center of the mirror. Using the kinematic mirror mount, the beam is then aimed toward the pinhole opening such that the center of the beam (which is about 2-3 mm in width) is on top of the pinhole (≤0.5 mm wide). This ensures that the light from the pinhole that hits the mirror will be focused at the center of curvature.

The laser spot, approximately centered in the mirror.	Shims used on the far side of the optical rail. These may or may not be needed, but if you find you're having trouble making the mirror focus with the tension knobs then a larger displacement like this can help.

A piece of paper can be helpful for initially locating the laser spot. Sometimes it'll be nowhere close to the pinhole.	The laser beam reflected onto the pinhole. It is hard to capture with a camera, but not too bad to see.

One can ensure that this is the case by turning off the lights and placing a white sheet of paper at the center of curvature when the pinhole source is on. The razor blade edge position is then adjusted such that it is at the point where the beam is smallest.

Testing for alignment. In this case the paper was mounted in the same holder as the razor blade. It currently is too high to be adjusted via mirror screws alone, so the aforementioned shims were needed.

An example of a properly aligned spot, with poor focus from my camera. When adjusting the vertical ($z$) position of the laser, you can see a definite transition as the spot moves from the paper to the edge of the metal.

Finally, the camera position can be adjusted until one sees the light coming from the mirror (with the room lights off and pinhole source on, this will look like a white disk), and its lens focused at around 8-12 inches in front of the mirror. In order to see the Schlieren, minute adjustments of the razor edge position need to be made while looking at the video output. When the edge is placed right at the center of curvature, a significant fraction of the light from the mirror will be blocked; any vibrations coming from the table will be visible as the blade will also move back and forth, causing more or less light to be blocked. At this point, one should be able to see warm air rising from one's hand when it's placed in the testing region.