Dentist Light

Ok I have heard many unpleasant things about UV light, but I had one of the important issues. I always closed my eyes when the dentist applies the light on my teeth, but when she is out of my mouth, I can see a strong purple light, even with my eyes closed .. .. Is this dangerous exposure usually takes 3-5 seconds

I asked a dentist who is one of my patients the same question and he said it is not harmful.