Your question has been answered by Sarah Newnham who is a young female scientist and a Doctor of Microbiology. She works mainly with bacteria and spends a lot of time looking down a microscope!
This question involves explanation of a number of different things. So I have separated this into a number of little questions...
Why do we see things?
We don’t actually see things, as such, we see the light . This can be the light they produce (sun, stars, light bulbs), the light reflected off of the object (you and me, the moon, cinema screens, books) or the light is changed when passing through it (water, glass). This is why you cannot see well at night, when there is only a little light, or nothing at all when it is pitch black but can see well in the day when there is a lot of light.
What is light?
Light is energy. We use the term to refer to the visible light we can see but light is a physics term that can be applied to all electromagnetic radiation. Electromagnetic radiation travels in wavelengths, of varying electric and magnetic fields. These wavelengths go in straight lines, their direction can be changed by contact with molecules, like visible light being reflected that we can then see. The spectrum of electromagnetic radiation includes; gamma rays, X-rays, ultraviolet (which can be seen by other animals, e.g. Bees, and can cause glowing/ fluorescing), VISIBLE LIGHT (including all the colours of the rainbow violet to red) infrared (which can be seen as heat with special glasses), microwaves and radiowaves (in order of shortest to longest wavelengths).
How can we feel air, in the wind, if we don’t see it?
Air is made up of a mixture of molecules and there is approximately 26,700,000,000,000,000,000,000 molecules in one litre of air. When we move through the air or the air moves around us, as wind, we can feel it as all the molecules collide with us. Wind occurs due to changes in pressure and these differences are what we feel as wind.
When can we see air or wind?
Air is ultimately transparent. This is because light can travel through the air without being affected this is because molecules are spread out , the molecules in solid objects are touching each other so there is no path through for the light, so it is changed and we can see it.
Here are some times when the air does change the path of light and we can see it:
- Tornado: Some of what you see is debris and dirt that is picks up but also you are seeing the formation of water (liquid) from the water vapour (gas) as the temperature and pressure drops fast inside the funnel of air.
- Blue sky: over the distance of the atmosphere molecules do affect the light from the sun. This is called light scattering. During the day when there is a lot of light from the sun more blue light is scattered than red light, from space the atmosphere also gives off a blue glow. However, at sunset we see red/orange colours as the blue light has been scattered further away and out of our line of sight.
- Mirage: Looking along a road (over a long distance) on a hot day wavy lines, that look a bit like water, can be seen just above the road. These are caused by the hot air rising and the bending of the light as it passes from colder to hotter air, this bending can also occur in the change between matter (for example a straw in water looks bent as it enters the water).
- Stars twinkling: The atmosphere has changing air pressures, low pressure has a lower density then high pressure and therefore reflects less light (or has a lower refractive index) as there are less molecules to alter the lights path. These changes in atmospheric pressure altering the light path cause stars to twinkle, the star is still producing the same amount of light we are seeing the change in air.