I have a T-shirt that appears to be black and grey in normal light. But when seen through sunglasses, it seems to be a deep red-purple. This also happens when the T-shirt is inside a green tent. It appears normal in the early morning and late evening, so it doesn鈥檛 seem to be a lack of light causing the colour change. What鈥檚 going on?
Lewis O鈥橲haughnessy, By email, no address supplied