When exposed to ultraviolet light from the sun (or other long-wave UV source), UV nail polish changes color! The change is reversible. When the UV Nail Polish is removed from sunlight, it returns to its original color! Although the product is great for nails, it can just as easily be painted on an index card or acetate to make an ultraviolet light detector that can be used for testing the effectiveness of sunscreen lotion or any other UV filter. 0.25 oz bottle