The retina, at the back of the eye, is a complex structure which has a deep layer of light-sensitive cells called rods and cones, a middle layer of bipolar neurones and a surface layer of ganglion (nerve) cells. The neurones connect the rods and cones with the ganglion cells, fibres from which join to form the optic nerve.
This means that the front surface of the retina, which is about the size of a postage stamp, is not the light-sensitive part, because it's covered with blood vessels and nerve cells. But the brain ignores these obstructions; we do not see them as part of our image of the world. Instead, the back is the light-sensitive part, and the surface acts as a projection screen. The rods and cones capture the light, which is transmitted to the brain via the optic nerve, from which we gain our picture of the world.
Q Some microwave food containers seem to have metallic films on top of them. Isn't that dangerous, because metal things in microwaves cause sparks?
Metal containers can produce dangerous arcing in a microwave oven. However, many food packages actually contain thin films of metal that speed the cooking process. For example, new packaging techniques use polyethylene terephthalate (PET) film laminated to paperboard as a "heat susceptor" - a metallised film that absorbs microwaves, and becomes a miniature frying pan to brown or fry the foods in the package.
So yes, metal in sufficient thicknesses can cause sparking. But thin films can improve the cooking ability of the microwave.
Q If you've got ice cubes in a glass of water, what happens to the level of water when they melt?
The water level stays the same. The amount of space the ice cube takes up in the water is exactly the same size as the water it makes when it melts, even though some of the ice cube sticks up out of the water. This is because water shrinks when it melts and expands when it freezes, so the amount that's sticking above the water indicates the extent that the frozen water has expanded.
This, incidentally, is why the threat of sea-level rise from global warming is not due to melting icebergs. It's due to the melting of ice which presently lies on land.
Q What is the current number of species in the world?
The guesstimates for the number of species present in the world at the moment are between 10 and 30 million. Of these, more than 90 per cent are invertebrates. The World Conservation Monitoring Centre's records for 1996 are: 4,237 mammals; 22,000-plus fish; 9,672 birds; 4,000-plus reptiles; 6,500-plus amphibians; 460,000-plus plants; and more than 1 million invertebrates (though there are likely to be millions more).
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