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By putting these sunshades in the windows of a car, one can considerably negate the interior heating effect. What is the physics behind this?
I searched for it online but couldn't find a thorough and comprehensive answer. One thing which I got to know that visible light enters through the windows ( without these shades) and is absorbed by the interior of the car. This is given off as infrared light which can't pass back through the windows making the interior of the car very hot.
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This is due to a psychological effect known as Color constancy. Your brain seems to see something based on the context around it, for instance the blue image projected on the red wall may be more black than blue but your brain will compensate for some of that because you also see the red wall.
This is also the source of a number of related optical illusions. For instance consider the image below taken from here. The squares labeled A and B are actually the same color, but one looks dark gray and one looks white because tile B seems to be in a shadow and surrounded by dark tiles and A is not so our brain perceives that B must be lighter.
Here's another optical illusion that may be closer to your question. The center piece in both images is the same color (gray) which you can see here.
The two lights cover different areas of the drivers view.
If there are other cars near, it's important do not aim the light to the eyes of other drivers (or their mirrors). But Also, it's important to cover as much of the street ahead as possible.
One bulb, together with the surrounding reflector of very specific shape, aims the light to the street up to a certain line, and not at all above it, where other drivers eyes are.
The other one just shines lots of light ahead, for situations where you can be sure nobody is irritated.
The more interesting part in terms of physics is how whe mirror works, at least in some cases:
The reflector, that surrounds both bulbs, is shaped in a way such that boths bulbs share part of the reflective area to cover different ranges of angles.
This is done by covering part of the bulbs, together with a rather complex shape of the reflector, (which was derived by trial and error in simulation, I suppose).
When deciding where to put the separation between lit street, and protecting eyes of drivers - the upper edge of the covered range of the first light - there is a problem:
The suspension of the wheels is more or less compressed, depending on load. If the load at the rear end of the car gets increased, the car will tilt up at the front. It's just a few degree at most, barely noticable, but makes a huge difference in terms of where the upper edge of the light ends up.
Because the separation is important to get right - both sides are relevant to safity - there is a mechanism to tilt the lights itself up and down by the driver.
Best Answer
Photo credit: Solar Energy
The sun radiates various types of energy, included in the electromagnetic spectrum.
Without the shade, all of this energy of the sun, except for the wavelengths blocked by the plastic of the windows, enters the car.
By blocking the infrared (heat) and ultraviolet parts of the spectrum, the shades act to reduce the heating of the air inside the car. Less energy is available to raise the temperature, whilst still allowing visible light through.
Solar Radiation is a good source of information on the effects of the various regions of the solar energy spectrum on the Earth.