Science, Maths & Technology

# Make an Ice lens

Updated Tuesday, 27th February 2007
The Rough Science crew were challenged to make a lens out of ice. You can too, by following our simple instructions.

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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

focal point (place where all parallel light rays are focussed by the lens)

This diagram shows the ideal situation, for perfect lenses. The 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 closer the blur is to a point of light, the more perfect your lens will be.

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.

Imagine 2 prisms with curved sides being placed together, 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.

What shape should you make the ice lens?
Fat lenses have short focal lengths (the distance from the centre of the lens to the focal point).

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.

For making an ice lens, which is bound to have imperfect curves, it’s better to use a small fat lens, i.e. using 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’s less chance that the kindling will light.

What you need to make an ice lens

Gloves
A sharp knife or rough bits 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
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 freezes into ice with bubbles in the middle. These bubbles scatter the light passing through the ice and stop it acting as a lens. Ice with too many cracks also scatters light. Ideally, you want a good solid piece of perfectly clear ice. Getting hold of clear ice is easier said than done.

Pond ice
It’s possible in cold winters to get it from a frozen ponds or lakes, but take care on the ice and you may have to dig down to about 12cm below the surface before you get to the clear ice.

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 100oC, 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, it cracks 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, and place the bowl into another one, placing both in the freezer. The outer bowl helps to insulate the inner one, so it cools more slowly.

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, which makes the shaping of the lens easier later.

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.

Hanging the balloon means that it freezes much much more slowly, as 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, the Rough Science lenses, whether hung up or lying flat in the freezer, all still cracked like crazy.

Challenge
The Rough Science team gave up on trying to make clear ice. Even once you’ve made an ice lens it’s hard to hold the thing with bare hands, it melts all over your kindling, and you need really bright sunlight to have a hope.

Can you do better than us and find a method of making clear ice that can be used for an ice lens that works repeatedly and, that other people can easily use? It needs to be more than 4cm!

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 take great care 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.

Check to see how well your lens works by holding it up perpendicular to the sun’s light, and move a piece of paper backwards and forwards behind the lens. You should be able to figure out where your focal point is.

Creating fire
Find some kindling that’s completely dry and ideally, in thin strips or pieces, slivers of wood, straw or paper. Place your kindling where you want to create the fire, ensuring 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 the drips fall onto the kindling. The kindling will first smoke and, hopefully, then 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.