Question: do the suns tanning rays penetrate water and to what degree

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  1. Sun tanning rays are called ultraviolet light (UV) and yes they can penetrate water. Within UV light, there is a spectrum of colour called the electromagnetic spectrum. The different colours within UV light can penetrate different depths, but generally the colours at the higher end of the spectrum, penetrate deepest. The blue wavelength of UV light can penetrate down to depths of 200 feet. This is said to by why deep water appears bluish.


  2. Sun tanning is caused by ultraviolet (UV) light. This is split into two broad ranges (called UVA and UVB), which have slightly different effects. The wavelengths of these (in nm = nanometres) are:

    UVA = 320 to 400 nm (which includes part of the violet and blue visible light)
    UVB = 250 to 320 nm

    The absorption (in units of 1-per-cm) are:

    250nm = 0.0019
    300nm = 0.0004
    320nm = 0.0002
    350nm = 0.00012
    400nm = 6.00E-05

    Reference: K. S. Shifrin, “Physical Optics of Ocean Water” (1988).

    As you can see these are small numbers. That means that UV can certainly penetrate water (especially UVA). Blue light (around 380 – 400 nm) very easily goes through water, as Sarah says. However the colour of deep water can also be affected by other things, such as marine life, dissolved chemicals and other factors.


  3. Further to the other answers: yes, you have to be very careful when you are in water, as you can still easily get sunburnt — that’s why most suncreams are water resistant!


  4. As above