Schlieren Optics

refraction by air - gradient index of refraction - diffraction around objects - convection currents

What it shows:
Changes in the index of refraction of air are made visible by Schlieren Optics. This special optics technique is extremely sensitive to deviations of any kind that cause the light to travel a different path. In this demonstration we "see" the difference in the index of refraction of room-temperature air and the slightly warmer air that envelopes your hand, for example. With a video camera and monitor, the audience can see warm convection currents rising from your hand or, alternatively, cold air sinking from a glass of ice water. SF6  gas can be visibly poured from a bottle into a glass (until overflowing) and subsequently poured out of the glass.  Additionally, the bending of light around objects (diffraction by edges) can also be seen with this technique.

How it works:
A sphere, by definition, is a surface which is everywhere perpendicular to a radius drawn from its center of curvature. Hence, all light emerging from a given point on the object will be reflected to a corresponding point symmetric about the center of curvature. This produces a real image. Here the object is a point light source and it is imaged onto a razor blade; in particular, it is imaged as close to the edge of the razor blade as possible. A person (or a camera) looks over the razor blade at an object in front of the mirror. If, for any reason, the path of the light rays is perturbed so that the image is shifted upwards, then the light previously blocked by the razor blade reaches the eye (or camera). This results in an illusion of brightness in the region that is the cause of the perturbation. The effect is visually quite remarkable.

One can also demonstrate the diffraction of light around objects. This is done by turning down most of the lights so that there is very little ambient illumination - basically, it's very dark. An object placed in front of the mirror is seen as dark (not surprisingly) but surrounded by a thin, bright, outline of light. This light, whose origin is the point light source, is being diffracted by the edges of the object into the eye of the observer - another striking effect.

Setting it up:
The setup must be precisely aligned; this means the spherical mirror with respect to the optics rail which holds the light source, razor blade, and video camera.

(a) The "point light source" is an automotive light bulb 1   positioned behind a 400 micron pinhole. 2   The light bulb should be as close as physically possible to the pinhole and rotated for maximum intensity (this aligns the filament with the pinhole). The razor blade is mounted on an x-y adjustable optics post which is oriented so that the blade can be moved vertically as well as longitudinally in the direction of the mirror.

(b) The spherical mirror 3   is positioned twice the focal length (246" or 624.8 cm) away from the optics rail so that the point light source and razor blade assembly is as close to the center of curvature of the mirror as possible.

(c) Since the mirror is so far away, the video camera should be equipped with a long focal length zoom lens; 100 to 150 mm works best for tight cropping of the image. The lens is attached (with a specially made mount) to the optics rail and the small CCD camera is supported by the lens. The camera (lens) should be 15-30 cm behind the razor blade.

The most convenient support for the telescope mirror is the sturdy angle-iron stand normally used for the Cavendish experiment. The Spindler & Hoyer optics rail (on the tripod mount) is also nice and sturdy and fully adjustable. To speed up the alignment procedure, an adapter has been made to mount a HeNe laser onto the C-mount thread of the zoom lens (the CCD camera of course needs to be removed). This allows us to laser-align the entire setup, working backwards (from the laser to the pinhole point-light source). The procedure is as follows: (start with the razor blade lowered out of the way)

(1) Level the optics rail on which the point light source, razor blade, and zoom lens/laser is mounted. Position the telescope mirror 246" (624.8 cm) from the razor blade, lock the mirror stand into place with the leveling screws, and adjust the height of the optics rail so that the laser beam hits the telescope mirror in the middle.

(2) Rotate the mirror/mirror holder to direct the laser beam back towards the pinhole. Fine-adjust the mirror (using the horizontal and vertical adjustment screws) to precisely hit the pinhole with the beam. Remove the laser from the lens and mount the video camera.

(3) Stop the lens way down (typically f/22). This produces a hexagonal vignetting pattern (from lens iris). Re-adjust the telescope mirror (with the fine adjustment screws) so that this hexagonal pattern is centered in the mirror (this part is crucial for best results!).

(4) Raise the razor blade and slide its mount back and forth on the optics rail to coarsely focus the point light source on the blade; fine tune the focusing with the longitudinal adjustment screw (on the razor blade mount). Lower the razor blade so that its edge just barely cuts off the point source image. Open up lens to maximum aperture and fine tune razor edge height. The lens is focused on whatever object is held in front of the mirror.

Comments:
The most dramatic effects are seen when the object that disturbs the optical path is right in front of the mirror. Take care not to touch the mirror! It is extremely sensitive when properly aligned; an occasional draft will look like a passing cloud. Heat from your hand rises up in wisps while air, chilled by a glass of ice water, sinks rapidly. A heated soldering iron looks like it's on fire. SF6 gas appears to be a liquid pouring out of a container.  

Although a bit of a bear to set up, the demonstration is very rewarding with stunningly beautiful geometric and physical optics effects. Rating ****





1 GE 1184 (6 volt, 7 amp) operated at 3.5 VAC using a 6.3 VAC CT transformer (rated at 6 amps).
2 Melles Griot
3 12.5" (31.8 cm) diameter, 123" (312 cm) focal length, f/10, protected aluminum mirror from Edmund Scientific; the mirror mount is on loan from Costas Papaliolios.