If they can see a significant effect is a small number of participants, that is good - it means they didn't need to search for the effect of UV light, it was ob

If they can see a significant effect is a small number of participants, that is good - it means they didn't need to search for the effect of UV light, it was obvious. The reason to have a big sample is usually to control for side-effects in drug testing. Often, the big sample size will mean that even very, very small effects are statistically significant.