How do car sunshades work? 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. 
Links


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*http://www.car-sunshade.com/How-do-sunshades-keep-the-inside-of-cars-cool-_n19

*https://www.s2ki.com/forums/car-talk-73/do-sun-shades-really-keep-interior-cooler-394083/
 A: 
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.
A: Those shades don't stop heating - they are only to stop bright light shining directly onto passengers. If the black shade is inside the car and absorbs heat then it heats up the car just as effectively as letting the car seat absorb the light.
The reflective sheets you put across the front windsheild work by reflecting most of the light back out of the car.
A: My guess is that the black shade is absorbing the heat rather than the air in the cabin. Also, if that shade is an adhesive type of sunshade that actually sticks to the glass then perhaps the heat is more readily conducted to the glass and back out of the car.

