For a while I thought that the reason I felt warmth from the sun was because my skin was being hit by photons, but then I realized that photons also hit me when I take an X-ray, but I don't feel any heat from that. So, why is it that you feel warmth from the sun, or any hot object for that matter?
[Physics] Why does the sun make me feel warm
energykinematicsphotons
Best Answer
X-rays do warm you up. It's just that the X-rays are more dangerous per photon (they can do major damage to cells and DNA, and are known to cause tumors and cancer), so they limit the amount of time you're exposed to the bare minimum needed for a clear image.
The total energy from standing in the sunlight for several seconds is much higher than the energy from all the X-rays you're likely to take in your life, which is why you feel it but don't feel the X-ray.
Addendum:
Total Energy from an X-ray. I found this page with radiation doses from common radiology treatments. The maximum radiation dose listed is 10 millisieverts (mSv). 1 Sv is defined as one joule (J) of energy per kilogram (kg) body mass. Assuming an average person is around 70 kg, 10 mSv corresponds to 0.7 J of energy absorbed. Note that the actual energy is probably a bit lower, because the sievert definitions also account for the biological effects (so a dose to an internal organ is weighted higher per actual joule than a dose to a finger or something). But this should be a decent approximation.
Total Energy from Sunlight. The sun outputs about 1300 watts per square meter (W/m²) in space near the earth, which gets reduced to around 650 W/m² in the middle of the day after going through the atmosphere. 1 watt is defined as 1 joule per second (J/s). So that's about 650 joules of energy per second per square meter.
According to this site, the surface area of a human body is between 1 and 2 m². At least half of that is on the side of your body not facing the sun, and you'll get less radiation if the sun is shining on the end of your body (like your head/shoulders) instead of the front or back of your body. If we assume the sun is right overhead, and you're laying on your back facing up at the sun, then you've got 0.5 to 1 m² in the sunlight (it's a little less because some of that surface area is parallel to the sunlight, but we're in the ballpark).
Ok, combining all of the above, sunlight shines down with 650 J/(s*m²) * [0.5 to 1] m² = 325 to 650 J/s. One second of direct sunlight is therefore 465 to 930 times more energy than one X-ray image, hence why it feels so much hotter.