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.

Natural Curiosity: Stretching Reflections 2

After my earlier post on this topic, I talked to a few people about why they thought these stretching reflections happened. There were a few different ideas, and when I talked to my brother about it he pointed out something in one of the images on my last post that was inconsistent with my explanation.

Sun Reflection | Photo by oboejoe92 on Flickr CC BY-NC 2.0
Sun Reflection | Photo by oboejoe92 on Flickr CC BY-NC 2.0

My hypothesis would have predicted that reflections would stretch down, but not up. However, looking more carefully at this image, the reflection of the Sun is clearly both stretching down and stretching up to the horizon. So it can’t be explained just by the surface appearing to be rougher as it gets closer to the observer.

However, in that discussion we came up with a new hypothesis. As I said in my last post, if we imagine a rough surface as being made up of a lot of small flat mirrors at random angles, some of them will be at the correct angle to reflect light toward you so you’ll see a reflection in those places. The new hypothesis was that the angle required for this would be less extreme above and below the reflection than to the side of it.

In order to test this, I needed 3 things:

  1. A light source
  2. A flat reflective surface
  3. A wedge
  4. A flat surface to rest it all on

Luckily, these things were all readily at hand. For a light source, I used a nearby lamp. My phone’s screen made a good flat reflective surface. I used the alarm remote for my car as the wedge, and rested everything on the floor. I’m sure you could find similar objects to reproduce this experiment for yourself.

First, I lined up the lamp, my phone, and myself so that I could see the lamp’s reflection in the centre of my phone’s screen when it was sitting flat on the floor. Then, using my makeshift wedge I tiled the screen of my phone away from me, then moved the tilted reflective surface towards me until the lamps’ reflection was in the middle of the screen.

I then repeated this for the other directions – away from me, to the left, and to the right. Because my phone isn’t square, I also rotated it so it was landscape when I moved it towards me and away from me, but portrait when moving it left and right. That made it easier to judge when the reflection was in the centre of its screen.

What I found was that I had to move the phone a lot further toward me or away from me than I had to move it left or right in order to see the reflection again. I think this explains, at least in part, why reflections on rough surfaces appear to be stretched towards you.


We can get a rough approximation of the outline of a reflection on a rough surface by assuming it has a maximum roughness, i.e. the maximum angle at which one of those little mirrors that make up its rough surface could be tilted. Then, the approximate outline of the reflection would be along the curve where a mirror at that maximum angle, facing in the right direction, would reflect light toward you.

On a perfectly flat surface, this maximum angle is 0. So the shape of the reflection is exactly as you’d expect, undistorted.

However, as the maximum roughness of the surface increases, the outline moves out from the undistorted reflection. And the reflection doesn’t just get larger, it gets stretched towards you. It’s because the angle required to reflect it at you is less within that outline that reflections on rough surfaces appear to be stretched.

I’ve written a small JavaScript simulation to show this effect. Unfortunately WordPress doesn’t let me embed it in this post, but you can have a play with it by clicking on the image below:

Stretching reflection simulation
Stretching reflection simulation

If you’re interested, you can also take a look at the source on GitHub.

The simulation works by sending out rays from the observer to hit different parts of a horizontal reflective surface. When a ray hits the surface, the simulation calculates the angle that would be required at that point to cause the simulation’s light source (displayed as a red dot) to be reflected there. Places where there would be a reflection are shaded according to the required angle, with brighter yellow areas being flatter, and areas where there would be no reflection are black. The simulation also draws a reflected red dot to show where the reflection would be on a very flat surface.

There are a few numbers you can configure to see how the shape of the shadow changes under various scenarios:

Light source distance
The distance “into the screen” that the light source (the red dot) is from you.
Light source height
How much higher than you the light source is. You’ll want to make sure it’s higher than the reflector.
Reflector height
How much lower (using negative numbers) the reflective surface is than you. The simulation doesn’t look above horizontal for reflections, so this won’t work with positive numbers.
Maximum angle
The maximum amount of roughness the reflective surface can have. Higher numbers are rougher, lower numbers are flatter.
Step size
How far apart the rays are, in degrees. The default setting is 0.1 degrees. Larger step sizes will make the simulation run faster, but it will be less precise.

The simulation shows how reflections can be stretched vertically in this way, depending on the roughness of the reflecting surface and the relative positions of the observer and the light source. If you make the light source very far away and near the horizon, you’ll see that the reflection can stretch all the way up to the horizon just like the Sun’s reflection in that picture.

However, there’s still a decent amount of horizontal spreading so I don’t think this entirely explains the stretched reflections. Yesterday, I saw this beautiful photo on Twitter, taken by Ian Griffin of a sunset in Otago:

In this photo, there is pretty much no horizontal stretching. This can be seen in the black lines in the reflection caused by trees blocking the Sun’s light – if the reflection were stretching sideways then these would be blurred and wouldn’t have such a uniform thickness.

There could be a few things helping in this case. Because this particular example is taken with water being the reflective surface, and the observer was standing at the shore, the waves are mostly perpendicular to the line of sight. That would help minimise horizontal scattering.

It can’t be just that, though, because the same stretching is seen on rough surfaces where the roughness has no direction, such as wet roads:

Wet road reflections | Photo by Thomas Hawk on Flickr CC BY-NC 2.0
Wet road reflections | Photo by Thomas Hawk on Flickr CC BY-NC 2.0

I think the rest of this could possibly be explained by surfaces that reflect the light straight towards you from under the light source appearing larger, because they’re angled towards you. Surfaces to either side of the reflection could also reflect the light towards you, but perspective would cause them to be foreshortened and therefore contribute less to the overall picture.

What do you think?

Natural Curiosity: Stretching Reflections

Auckland reflection at night | Photo by 111 Emergency on Flickr CC BY 2.0
Auckland reflection at night | Photo by 111 Emergency on Flickr CC BY 2.0

On a rough reflective surface like the ocean or a dark wet road, reflections from bright lights like city lights, car brake lights, or the Moon appear stretched vertically. Why is this?

When a surface is perfectly flat, like a regular mirror, the image we see in the reflection isn’t distorted at all. Even if we put a mirror flat on the ground, we wouldn’t see a vertically stretched reflection like this.

Neither the road nor the ocean are perfectly flat though. Their surfaces are rough, and this rough surface scatters light when it’s reflected. If we imagine that each piece of the surface was a little flat mirror, with each piece facing in a random direction, some of these would be at the right angle to reflect light from a source (like the Sun) directly into our eyes, and most would not. We’d only see a reflection in those pieces that are at the correct angle to reflect the light into our eyes.

The further these little mirrors are from the area where we’d see the reflection in a flat mirror, the more extreme an angle they will need in order to still reflect the light at us. If every one of these little surfaces was really really tiny, what we’d expect to see is a blurry reflection. The smaller the pieces get, the less blurry the reflection would get.

We can actually see this in effect when we compare pictures of the Sun reflected off the ocean. When you’re quite near the ocean, all the different reflecting surfaces are relatively large so the reflection is quite blurry and broken (especially if there are lots of waves):

Sun Reflection | Photo by oboejoe92 on Flickr CC BY-NC 2.0
Sun Reflection | Photo by oboejoe92 on Flickr CC BY-NC 2.0

In comparison, if we look at a reflection of the Sun on the ocean that was taken from space, all the waves and ripples that distort the reflection are far too tiny to see, and as a result the reflection is quite clear and crisp:

Sun reflection from space | Photo from NASA
Sun reflection from space | Photo from NASA

Another difference that’s quite apparent between these photos is the vertical stretching that I’ve been wondering about. From up close, it’s very stretched. From a distance? Not so much. This gives me a thought, one that actually hadn’t occurred to me until I got to this point in writing this post and saw those images one after another:

What if it’s important that there’s a significant relative distance between the closest and furthest parts of the surface that are reflecting the light source?

From a long way away, these distances appear quite small. For example, if I’m 1 km away from a surface, then a 1 m distance between two points on that surface is really quite small. If I’m only a metre away myself though, then that’s a very significant distance.

As we just saw, reflections on non-flat surfaces are more blurry when they’re closer to you, so what if this vertical stretching is actually just the reflection getting more blurry towards the bottom, because that part of the road or ocean is closer? As it’s more blurry, this would let the edge of the reflection creep out further, and could look like stretching.

If I’m right, then I should be able to see the same type of stretching if I look at a reflection on a vertical surface, except the stretching would be horizontal in that case. I should also be able to replicate the same stretching effect if I can get a reflecting surface that is smooth on top and gets rougher towards the bottom, and look at a reflection of a light in it like I would a normal mirror (i.e. with the reflecting surface vertical and the light source behind me).

Let me know what you think of this idea in the comments, and if you have any ideas of your own for why we see these stretched reflections. Any ideas about how I could try to disprove my idea would be welcome too! In the meantime, I’ll try to do these experiments, and see if I can find an expert to talk to about this question.

I’ve written more on this topic in another article: Natural Curiosity: Stretching Reflections 2