Fun with Light

Fun with Light

There are lots of cool science activities you can do at home with light.

Like I’ve done almost every year of my life, I spent my summer break at my family bach at Oakura. Last summer I wrote a post about a trip to the rocks and what could be found living there. This summer, on the relatively few sunny days we had, I had fun playing with light.

Here are three easy, fun, and cheap activities you can try yourself.

  1. Make a Telescope
  2. See Shadows Jump
  3. Wave at the International Space Station

Make a Telescope

The previous year, I made a simple telescope out of a $2 set of two magnifying glasses. Playing with trial and error and a piece of soft wood, I ended up with something that had a zoom of about 2x. However, because it only used two lenses the resulting image was inverted.

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This summer, I came prepared with an extra set of magnifying glasses, making four in total. I raided the recycling bin and used some ginger beer bottles to hold them in place, facing an island in the bay. Then I moved them back and forth until the zoom and focus seemed as good as I could get it.

Once I had the placement right, I marked off the distances on a long piece of wood, then taped the magnifying glasses to it. What I ended up with wasn’t the strongest or most portable telescope in the world, but all it took to make was $4 and a fun afternoon.

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See Shadows Jump

My brother Jeremy is a concept artist for Weta Workshop, which has left him with a good understanding of light and colour. One evening up at the beach he started talking about some interesting things that shadows do.

Watching shadows of leaves dance on the ground, he wondered if they often form natural pinholes. When we had a partial solar eclipse in Auckland in 2012, my mum (who also has a great artistic understanding of light and colour) mentioned to me how the shadows in her garden looked strange when she went outside during the eclipse. This would have been due to the pinhole effect, and it’s why some of the recommended ways of viewing an eclipse are to make a pinhole in a piece of paper or use a colander.

You’ve probably seen diagrams showing the basics of how a pinhole camera works. Even without a lens, when light passes through a small hole it can project a sharp image on a surface opposite that hole. However, that image will be inverted (like in my first attempt at making a telescope).

Pinhole-camera

I often collect pāua shells from my trips to the rocks when I’m at the beach. A pāua shell has a row of holes along one side. When I held it a certain distance away from a wall, with the Sun low on the horizon, we found it made a row of pinholes. But because a projection of the Sun looks the same inverted as it does normally, in order to tell if the image really was inverted I moved a cardboard roll behind the pāua and watched at the holes “filled up” with shadow backwards – just as we’d expected.

But something else happened which I definitely didn’t expect. Watch this video we took to see the shadow of the pāua shell reach out to touch the cardboard roll’s shadow as they get close together:

If instead the pāua shell was held closer to the Sun and the cardboard roll was closer to the wall, then we found it would be the shadow of the cardboard roll that bulged out as they got close.

We immediately took to pen and paper to try to draw out diagrams that would explain how this worked. My initial idea was that we were seeing the area of intersection between the penumbras – the hazy edge of the shadows where the Sun was only partially obscured. But this wouldn’t explain why the bulge would change depending on which object was in front of the other.

Before too long, one of Jeremy’s ray diagrams seemed to explain what was happening. I’ve tried to reproduce them here (I hope you’re all suitably awed by my skills with MS Paint):

Shadow Single

This diagram shows a light source on the left casting a shadow from the object in the middle onto the surface on the right. It shows how a non-point light source such as the Sun produces a shadow with an umbra (where none of its light reaches) and a penumbra (where part of its light reaches). The darkest part of the shadow, the umbra, is the middle section between the lines on the right.

Now, what would happen if I insert another object partly between the light source and the first object?

Shadow Overlap

The new object blocks some of the light from reaching the original object. As this ray diagram shows with the red line – where the light is partially blocked – the result of inserting this second object is that the umbra of the first object’s shadow is extended toward the new object. This is the cause of the bulge you can see in the video above.

It turns out this shadow jumping effect is called the shadow blister effect. You can observe it easily for yourself on any sunny day.


Wave at the International Space Station

The sky at Oakura is lovely and dark, with the nearest city being nearly 50 km away. Before the Moon rose one night after Christmas a few of us went up a nearby hill to stare up at the night sky.

With a clear dark sky, you can see the band of the Milky Way galaxy arc across the sky like a pale cloud, as well as the fuzzy blobs that are the Large Magellanic Cloud and Small Magellanic Cloud. These are dwarf galaxies which orbit the Milky Way.

We also saw many meteors, and a surprisingly high number of satellites. From Earth satellites look just like stars, except they move steadily across the sky in a straight line. Usually they appear quite dim, but there is one satellite in particular which can shine brighter than any star in the sky, and even brighter than any of the planets. That is the largest artificial satellite of them all: the International Space Station (ISS).

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The ISS orbits the Earth about once every 90 minutes, and although it doesn’t pass over New Zealand each time it does fly over us more often than you might think. But we can’t always see it in the sky; the conditions have to be right first.

Before we can see the ISS the sky needs to be dark enough for it to stand out. Also, it needs to be in the right position for sunlight to reflect down at us off its massive arrays of solar panels. This means that you’ll only be able to see it in the hours after sunset and before sunrise.

It generally takes 1-6 minutes for the ISS to pass visibly overhead. This will usually end with it appearing to fade into darkness as it stops reflecting sunlight back at us – you won’t see it set over the horizon like you would with the Sun or Moon.

NASA has a great online service, which you can subscribe to and get email alerts, that can tell you when and where to look to spot the ISS. It’s called Spot The Station. It lets you enter a city, and will tell you when the next few ISS sightings will be as well as how long they will last, and how it will travel across the sky.

ISS sightings often come in clusters – there will be sightings around a similar time in the morning or evening for several days in a row, followed by a period of no sightings. If you’re extra lucky, you might get to see it twice in one evening as it comes back round an hour and a half later.


I’d be remiss if I didn’t also mention that you can rent our bach if you ever want to see Oakura with your own eyes.

What Fake Astrophotography Can Teach Us

What Fake Astrophotography Can Teach Us

The New Zealand Herald has an article today about a cool and very popular image of the Moon positioned perfectly within a radio satellite, produced by astrophotographer Chris Pegman: Supermoon image goes into media orbit

Image by Chris Pegman
Image by Chris Pegman

The article talks about how there has been debate online about whether or not this could be taken without resorting to Photoshop. It concludes that “the verdict was that it might be, but it would require an incredible amount of planning” but this isn’t strictly correct.

The apparent rotation of the Moon changes as it travels through the sky. When it rises, it will appear to be “on its side” relative to when it is at its zenith, and when it sets it will have rotated further still.

This is most obvious with a crescent Moon. Depending on if it’s waxing or waning, the Moon will rise with the crescent facing either down or up, then when it’s at its zenith the crescent will be facing sideways, and as it sets it will have rotated around further. Of course, the lit side of the Moon always faces the Sun. It’s the fact that the Earth rotates beneath us that makes it look like the Moon is rotating as it travels across the sky.

Here’s an example of this which I took with my phone in July, showing a waning crescent Moon shortly before sunset:

We can see from the lunar maria (the dark areas) that the Moon in Chris Pegman’s picture is rotated how it would be if (when viewed from the southern hemisphere) it were near its peak, not near the horizon, so his picture couldn’t be produced without artificial manipulation.

Mark Gee is a fantastic astrophotographer from Wellington. In October he captured a time lapse of a full moon rising, in which you can clearly see that angle of the Moon is not the same as in Chris Pegman’s image when it rises: Supermoon rises over New Zealand timelapse.

There’s a Twitter account called Fake Astropix, which tweets fake astronomical images with the reasons why they are recognised as fake (well, as much as can be given within a tweet).

I find these reasons can be very educational and thought provoking. For example, it’s impossible to take a photo from Earth where the Sun and Moon don’t appear to be roughly the same apparent size. Also, the full moon can’t appear next to the Sun in the sky (remember the lit side faces the Sun). So “debunking” these fake astronomical images can be a good educational exercise that makes you think a bit more carefully about how things work in our solar system.

Do you have any fake astronomical images that you can share, along with the reason why you can tell it must have been faked?

Have you seen any astronomical images that you think might be fake but you’re not sure? Share them here and let’s investigate, and see if we can learn something.

Natural Curiosity

Blue Sky | Photo by Mohammed Tawsif Salam - CC BY-SA 3.0
Blue Sky | Photo by Mohammed Tawsif Salam – CC BY-SA 3.0

On the second Wednesday of every month, there’s a great Twitter chat on science communication in New Zealand: #SciCommNZ

Unfortunately I’m always busy on Wednesday evenings while this is going on, but I’ve tried to participate as much as I can by joining in late and reading through each discussion. The questions that have been asked have made me think about the things I write about on this blog, and some of the things I’d like to write about:

After having these thoughts churn around in my head for a few weeks, I’ve come up with something I’d like to try.

There are a lot of “everyday science” questions that I see asked and answered fairly often. Common examples include “why is the sky blue?” (which is not quite as simple as you might think) and “how do rainbows work?”. I really like these questions, but I feel sometimes like they’ve all been done many times already.

Of course, they haven’t all been done many times already. But I do feel like I see the same “everyday science” questions over and over again. I think they’re great and really interesting the first time you encounter them, so I want more.

As a remedy to this, and as an attempt to do something different and (hopefully) interesting with my science communication, I’m going to start asking some of my own everyday science questions. This might be a bit grandiose, but I’m calling this little project of mine Natural Curiousity

The format may change as I get into it, but the way I see this happening is to take every question in two parts:

First, I’ll write a post framing the question and some of my own thoughts (as a non-scientist) on what the potential answers might be, and what some problems with those potential answers might be. I want to try to do this without any Googling, but I might try a few homemade experiments. My hope would be that posts like these could get some interesting discussion going, but I guess we’ll see.

After that, I’d like to talk with someone who is an expert in a relevant topic and get their thoughts on the question, both on the potential answers brought up in the first post and on what they think the answer probably is and why. This isn’t something I’ve done before, so I hope I’ll be able to find some experts who’ll be happy to find some time to talk to me about this.

If you have any everyday science questions that you’ve been wondering about, let me know in the comments. I’ll update this post with links to posts using this format as I publish them.

  1. Stretching Reflections
  2. Stretching Reflections 2