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.


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

  2. https://www.s2ki.com/forums/car-talk-73/do-sun-shades-really-keep-interior-cooler-394083/

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    $\begingroup$ Honestly, your description you found seemed fairly reasonable to me. $\endgroup$ – JMac Jun 28 '17 at 17:47
  • $\begingroup$ What do you find unsatisfactory about that explanation? $\endgroup$ – user154420 Jun 28 '17 at 17:52
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    $\begingroup$ That explains why the interior gets hot. It doesnt say anything about how these shades work. $\endgroup$ – cobra121 Jun 28 '17 at 17:53
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    $\begingroup$ @cou The fact that these sunshades are inside the car so they can't reflect the heat effectively $\endgroup$ – LLlAMnYP Jun 28 '17 at 18:02
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    $\begingroup$ @OP despite my comment, the shades keep all the heating localized to the very skin of the car, so although radiative transfer is still hindered, the other modes of heating are less effective. Also trapped ir light is by far not the only thing that makes the car hot. Absence of mixing of air with the outside is very significant. $\endgroup$ – LLlAMnYP Jun 28 '17 at 18:12

enter image description here

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.

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  • $\begingroup$ How do these shades block the infrared and ultraviolet parts ? The shades I usually see are made up of some fabric stitched together. Is this because of the properties of the fabric? $\endgroup$ – cobra121 Jun 29 '17 at 9:09
  • $\begingroup$ Also your answer still allows trapped IR heat. The visible light is absorbed by the interior and reflected off as IR which is trapped. Link- car-sunshade.com/… $\endgroup$ – cobra121 Jun 29 '17 at 9:13
  • $\begingroup$ If the material is black,it works by absorbing the IR. If it is,like on my car, metallic (just for parking up), it reflects it. Yes, the car will still heat up because the material is not very efficient. I am guessing that if it is "tuned" to block incoming IR, then the IR that does get in will bounce off of the car interior and change wavelength, so the sunshade cannot deal with that new wavelength, the sunshade makers can just try to concentrate on reducing the incoming IR. $\endgroup$ – user154420 Jun 29 '17 at 10:01

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.

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