WHEN YOU PUT SUN TAN OIL ON, MAKE SURE YOU RUB IT WELL IN
WRONG!!!
The Sun radiates a huge range of different wavelengths of "ELECTROMAGNETIC WAVES" (light is one category under this heading). Another category of electromagnetic waves is
ultraViolet or UV. The longer wavelength UV rays give us a suntan but the shorter (and more energetic) UV wavelengths give us sunBURN (and maybe ultimately skin cancer).
So.....we want to allow the tanning rays through whist stopping the burning rays so we use a filter...that is what the suntan oil/sunscreen is. A thin layer on the skin acts just like a filter allowing the tanning rays through whilst stopping the burning rays.
If you rub the oil in too much is is absorbed by the skin and does not sit on top acting as a filter so is therefore very much less efficient.
To think of the oil as a filter here is a comparison: on the Stage we may need a red spotlight, so we take an ordinary spotlight which puts out white light (comprising all the colours of the rainbow). We put a piece of "red" plastic infront of the light and it absorbs all the wavelengths (colours) from the white light except the red which it lets through. This is how the sunscreen acts with UV.