I have read about schlieren photography, which uses the ability of non-uniform air to create shadows. Is it really possible that air makes shadows?
[Physics] Can air make shadows
airopticsrefractionshadowvisible-light
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There are multiple ways in which you can in principle disguise the source of light, but photons still must be emitted in order to illuminate something. So a shadow can only really be generated if there is a source of light, although the specific location of the source can be easily masked.
The simplest example is the case of knife-edge diffraction, which is used to send signals over obstructions, particularly in radio wave propagation. Essentially, the transmitter is behind a relatively sharp edge object (like a mountain in the case of radio waves), and the light will actually "bend" around the object to a receiver on the other side.
More advanced cases look at "bending light" from a laser around an object. Experimentalists now claim to be able to "bend light" an arbitrary number of degrees.
Much of the research into metamaterials is focused on different applications involving refracting light using materials with negative refractive indexes. These materials play key roles in the concepts of modern "cloaking devices" where the idea there is to mask an object in a cocoon of meta material well into the visible spectrum.
If you get into the metamaterial realm, the idea might be that one could not only bend the light around the object, but amplify the light in some way so that in some arbitrary direction light passing through the metamaterial exits with greater intensity than the background ambient light such that an object in the path of the amplified light would cast a shadow. I would imagine that the impression of the observer would be that some region they are looking at somehow appear brighter, although the exact location of the light source might not be identified exactly.
So in general it is possible to mask the specific location of a source, but you simply shouldn't be able to generate a shadow without there being some change in intensity between the front and back of the object casting the shadow.
Working out how much force is required to move through a fluid, whether it's air or water, is surprisingly difficult because there are two effects you need to take into account. However neither of those effects is pressure (or at least only indirectly related to pressure).
The first effect is viscosity, which is simply how thick the fluid is. Obviously it's harder to move through a viscous fluid like treacle than through a thin fluid like water. Non-zero viscosity means that as you move you feel a viscous force.
The second effect is inertia. If you move your hand through water then the water in front of your hand has to start moving so it can flow out of the way. This means you have to accelerate the water, and because water has a mass Newton's first law tells us you need to apply a force to accelerate it. You feel this force as an inertial force.
However neither of these forces directly involve pressure. There is an indirect effect for compressible fluids like air because as you increase the pressure the air becomes more dense. As the air density increases its viscosity increases so you feel an increased viscous force, and its density also increases so you feel an increased inertial force. So all that pressure does indeed make the air harder to move through, but only indirectly.
If you now consider water, this is almost incompressible and its density doesn't change much over the pressure range humans can tolerate. This means the force required to move through water changes hardly at all with pressure. You say it's harder to swim the deeper you go, but then scuba divers can swim down to 300m depth where the pressure is about 30 times the pressure at the surface - admittedly 30m is a more sensible limit for recreational diving, but that's still 3 times the pressure at the surface.
Best Answer
Taken from this site:
Yes, air can indeed make shadows. A shadow occurs when an object in a light beam prevents some of the light from continuing on in the forward direction. When the light beam hits a wall or the ground, a darker shape is visible where less light is hitting the surface. Both the light and the shadow, which is just the absence of light, travel to the surface at the speed of light. There are three ways that an object can prevent light from continuing on in the forward direction:
Absorption: The light that hits the object is absorbed and converted to heat. A black table creates a shadow on the wall mostly by absorbing the light that hits it.
Reflection: The light that hits the object is reflected off the front surface and redirected to another part of the room. A silvery bowl creates a shadow on the wall by reflecting away the light that hits its front surface.
Refraction The light that hits the object passes through, but the light's direction is bent by the object. If the direction is bent enough, the light that passes through the object will be angled out of the forward-traveling beam. As a result, the beam will have a dark spot; a shadow. Consider completely transparent objects such as glass cups, bottles of water, or the lenses of eyeglasses. Even though such transparent objects do not absorb or reflect very much light, they still interact with light through refraction. Refraction is what makes transparent cups visible to our eyes. Refraction also enables clear objects to cast shadows. Take off your eyeglasses and place them on the table at night under the illumination of a single lamp and you will see a distinct shadow caused by the transparent lenses.
Although air is almost perfectly transparent, it can still cast shadows via refraction. The key principle regarding refraction is that light is bent when the index of refraction differs from one location to the next. Air and glass are different materials and have different indices of refraction. Light therefore bends when it goes from air into glass, such as at the surface of a glass lens. Refraction does not happen inside a glass lens because the material inside the lens is uniform. Refraction happens at the surface of a glass lens because that is the only place where the index of refraction differs. Uniform air itself cannot refract light and create shadows because the index of refraction does not differ anywhere. But, when different regions of air have different indices of refraction, the air can indeed bend light away from the forward direction and create a shadow.
The most common way to get a changing index of refraction in different regions of air is to heat the air. As air heats up, it expands and its index of refraction changes. A pocket of warm air sitting next to a pocket of cold air will therefore constitute regions with different indices of refraction. The interface between the cold air and the warm air will therefore bend light and cause shadows. This effect is most visible when strong direct sunlight is coming in sideways through a window, passes through cold ambient air and then passes through the hot air above a heater. The shadow that this air system creates on the far wall consists of waving, rolling lines mimicking the turbulent motion of the hot air as it rises.
The index of refraction of air also changes as the pressure and composition changes, therefore these effects can also lead to air shadows. For instance, the pressure variations caused by a plane plowing through the air can cause shadows. Also, gases being vented into ambient air creates spatial variations in the air, and therefore shadow-causing variations in the index of refraction.
The ability of non-uniform air to create shadows is used to great advantage in the imaging technique known as schlieren photography. In schlieren photography, the shadows are used to accurately map out the variations in the air.