Ultraviolet light and highlighter pens

A few weeks back, we had fun playing with infrared light, remote controls and black pens (some of which absorb infrared as well as visible light, and some of which reflect infrared).  I thought my son might also find it interesting to see what we could do with ultraviolet (UV) light too.

I bought a cheap 'black light' (i.e. UV) torch, and modified a cardboard box to be a UV light box.  I didn't want him to be able to look directly at the UV light for safety reasons, nor to be able to point it at his brother's eyes, so it was mounted pointing downwards into a cardboard box; I cut a cross shape in the box, then pushed the torch up from inside and secured it with an elastic band - very quick and simple. I also cut a large slit on two opposite sides at the bottom so things can be put inside the box and illuminated with the UV light.  As you can see in the picture below, the torch illuminates the inside of the box (it also emits some light of visible wavelengths), and you can see the reflected light off the box, or whatever is inside.

Packaging box turned into a UV light box

We started experimenting with the UV light box after reading a book about colours which includes a rainbow ('Wow said the owl' by Tim Hopgood).  It's ideal because my older son knows the words and the little one likes to hear the names of the colours so I don't actually need to do any 'reading' myself...! 

After we'd gone through the colours of the rainbow at the end of the book, I asked my 3 year old what colour was beyond the violet.  He had remembered from when we talked before about rainbows, and was keen to find out more about ultraviolet.  I explained that we had to be more careful with UV than with infrared as it can hurt our eyes if we look at too much of it, but that I had some UV light we could experiment with.

Highlighter pen drawing in (weak) sunlight

I asked him to draw a picture with some highlighter pens, and - predictably - he wanted to use every colour in the pack.  Highlighter pen ink is fluorescent; the ink absorbs UV light and emits it at a visible wavelength. This happens with the UV light that's in sunlight, but it's really noticeable if you illuminate it in the light box, and the highlighter pen drawing appears to glow really brightly.  This isn't true for all the colours in the (cheap) pack of pens though - the blue, purple and pink aren't fluorescent at all.

Highlighter pen drawing illuminated with UV light
The small boy was quite intrigued, but his baby brother was unimpressed with our activity and we ended up putting the light box away and going to play with something he could be more involved in.  We'll try something else with this soon though!