8 votes
Accepted

Sunscreen protection, SPF, and amount applied: relationship?

A sunscreen's sun protection factor (SPF) is defined as $$\frac{\text{minimal erythemal dose in sunscreen-protected skin}}{\text{minimal erythemal dose in non-sunscreen-protected skin}},$$ so its ...
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  • 542
5 votes

Why is it so much harder to keep the same room cool than to keep it warm?

I am asking why the cool room warms up faster (loses the cold after you turn off the AC) then the warm room loosing cool down (lose the heat after you turn off the heater). That's not a universal ...
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  • 33.6k
3 votes
Accepted

Why is a firm, inflated stand up paddle board at atmospheric?

for the purposes of inflating tires, rafts, basketballs, etc. the ambient (atmospheric) pressure is taken to be zero: a tire pressure gauge will read zero pressure when it is actually sensing ambient ...
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1 vote

What is this colored ring effect on my computer screen?

Your colored rings are part of a bullseyes pattern that can be seen in some smartphones and tablets with a birefringent layer in the screen, if the screen is viewed through a linear polarizing filter (...
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  • 2,188

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