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Episode 4 - Ice

Ice Lens

What is an ice lens?
How does an ice lens work?
What shape should the ice lens be?
What you need to make an ice lens
Why does the ice need to be clear?
Making clear ice
Shaping the ice
Creating fire
Related websites

Challenge
To make heat on the glacier - enough to set kindling alight.

What is an ice lens?
An ice lens is a lens, made out of ice, which is meant to focus light onto one point. The idea is to focus sunlight onto dry bits of wood, straw, or paper and start a fire.

Some lenses have sides that bulge outwards (convex sides) which bring light rays closer together and focus the light. Lenses with sides that cave inwards (concave sides) spread light out.

convex lens concave lens
Sketch of convex and concave lenses
focal point (place where all parallel
light rays are focussed by the lens)

This diagram shows the ideal situation for a perfect lens. These ice lens will have imperfections, and won't have a completely smooth surface. Consequently, the focal 'point' will be more of a focal 'blur', spread out a bit in space. The more this blur resembles a point of light, the more perfect your lens will be.

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How does an ice lens work?

Imagine a ray of light passing through a piece of glass or water. When a light ray passes from air into glass or water, it is 'bent'(refracted), and emerges from the other side at a less steep angle than it went in with.

Diagram of refracted light

Imagine two prisms with curved sides being placed together and attached at the bases. They would make a shape almost like a convex lens. Looking at how prisms bend light, you may be able to see how a lens would bend light.

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Sketch of prism bending light rays

What shape should the ice lens be?

Fat lenses have short focal lengths (the distance from the center of the lens to the focal point).

Sketch of short focal length

Thin lenses have longer focal lengths. This is because fat lenses have sides that are more curved than thin lenses, so they bend the light rays more. So fat lenses are described as being more powerful.

Sketch of long focal length

To make an ice lens, which is bound to have imperfect curves, it's better to use a small fat lens, i.e. use a short focal length. That's because with a longer focal length, there's more chance for the light blur to spread out. This means the light energy is more widely distributed and consequently there will be less chance for the kindling to light.

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What you need to make an ice lens

Gloves
A sharp knife or rough pieces of concrete
Some dry kindling — straw, thin bits of paper, thin pieces of dry wood
Clear ice (water that's been boiled for at least ten minutes)
A container to freeze the water in
A freezer (not set at too low a temperature)

Why does the ice need to be clear?
Water that contains dissolved gases will freeze into ice with bubbles in the middle. These bubbles scatter the light passing through the ice and keep it from acting as a lens. Ice with too many cracks also scatters light. Ideally, you want a good solid piece of perfectly clear ice. But getting that piece of clear ice is easier said than done.

Pond ice
In cold winters it's possible to get ice from a frozen ponds or lakes, but take care on the surface. You may have to dig down to about 4 inches below the surface before you get to the clear ice.

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Making clear ice
You need to boil water for at least 10 minutes to remove the gases that are dissolved in the water. Note: when you heat water, gradually getting it up to 100°C (212°F), you see some tiny bubbles forming. This isn't the water boiling yet; these are the dissolved gases bubbling out.

Now you need to slowly freeze the water. If it freezes too quickly, will crack because of the temperature differences across the ice.

Some things to try to make clear ice:
a) bowl
b) bowl in bowl
c) balloon trick

a) bowl
Put the de-gassed water into a bowl and put it in the freezer. Once it's frozen, there are likely to be bubbles and cracks, so use only the area that is clear.

b) bowl in bowl
To slow down the freezing process, try putting some de-gassed water in a bowl, place the bowl into another one, and then place both in the freezer. The outer bowl helps to insulate the inner one, so it cools more slowly.

Sketch of bowl inside bowl

c) balloon trick
Try putting the de-gassed water (after it's cooled) into a balloon, so that it freezes in a near spherical shape. That makes the shaping of the lens easier later.

Sketch of front of balloon

We also tried putting a round loop of plastic inside the balloon first, which really gave it a fantastic lens shape. You can place or hang this in a freezer.

Sketch of side view of balloon

Hanging the balloon means that it freezes much, much more slowly. Air is a bad conductor of heat, so the cold air in the freezer is much less efficient at cooling than, say, the cold, solid bottom of the freezer would be. Nevertheless, all the Rough Science lenses, whether hung up or lying flat in the freezer, still cracked like crazy.

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Shaping the ice
Once you have a piece of clear ice, you need to make it into a lens shape. You can do this by grinding the ice on cement or a stone. Use gloves to keep your hands warm. You can also shave the lens with a knife to get the rough shape of a lens, but be very careful with the knife.

Once you have an approximate lens shape, use the heat of your hand to melt the ice lens smooth (and, incidentally, make your fingers miserably cold in the process).

The edges of your lens should look something like this.

Sketch of lens shape

Check to see how well your lens works by holding it up perpendicular (at a 90° angle) to the sun's light, and moving a piece of paper backwards and forwards behind the lens. You should be able to figure out where your focal point is.

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Creating fire
Find some kindling that's completely dry - ideally thin strips or slivers of wood, straw or paper. Place your kindling wherever you want to create the fire, as long as it's in sunlight!

Hold your lens perpendicular to the sun and move it backwards and forwards to focus the brightest spot of light onto the kindling. The lens will probably melt a little, so take care not to let water drip onto the kindling. The kindling will first smoke and then, hopefully, ignite. Carefully add more kindling and very small sticks until a good fire is established. If it doesn't work, try again when the sun is at its highest in the sky.

Sketch of lens perpendicular to the sun

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Related websites

Primitive Ways
How to make an ice lens with step-by-step pictures

How stuff works
How a lens works

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Photo: Rough Scientists at work
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