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Speed of sound
Speed of sound
The speed at which sound moves is slow compared with that of light. That is why there is a time interval between seeing a flash
of lightning and hearing the crack of thunder.
Sound can only be heard through a gas (such as air), a liquid (such
as water), or a solid (such as steel). Sound does not travel through a vacuum and so does not travel through space.
In 1738 French scientists set
up a cannon on one hill and observed it from 27 kilometres away. They timed how long it
took to hear the explosion after seeing the flash of gunpowder. The distance divided by the time gave them the speed of sound.
The speed of sound in air at 0°C is 331m/s. The speed of sound varies with the medium it
is travelling through and with the temperature of the medium. At 20°C the speed of sound in air is 344m/s, and at 100°C it is 386m/s.
A sound wave requires almost three seconds to travel one kilometre. The distance of a storm centre can therefore be figured out by counting the seconds between seeing the flash of lightning and hearing the first crack of thunder. The distance in kilometres is the number of seconds multiplied by 3.
(See also: C; Refraction of sound; Sonic boom.)
Sunlight
The light that comes from the Sun is often called white light. However, it is really a yellowish light. Artists often call it a ‘warm’ light. You notice this more when the sun is low in the sky.
The yellowish colour is, in part, because the Sun does not send waves of every length in the same amounts, and in part because the
air traps some kinds of waves more than others. The more air the light has to pass through, the more some kinds of waves are trapped (those in the blue wavelengths), and the yellower or redder the light seems.
When you see sunlight from a north-facing window, it is much bluer. Artists often call it a ‘cold’ light. (See also: Afterglow.)
T
Telescope
A device that produces magnified images of distant objects, such as planets and stars.
The earliest telescopes used lenses to gather light and bring it to a focus, but gradually concave mirrors became the most common way of gathering enough light. That is because a large lens is not only difficult to make but is also extremely heavy, while a concave mirror can be made lighter and far bigger.
 Sunlight – What we call sunlight varies depending on where we see it from. Seen from space, sunlight is far more white than seen through the dust of the atmosphere. Sunlight is yellow even when the Sun is highest in the sky, but at sunset it is orange or even red. Again, it depends on the way the sunlight is affected by gases and dust in the air.
The first telescope dates back to 1608, when it was invented in the Netherlands. News of the telescope soon reached Galileo, who built his own telescope and used it to see the planets and their moons. These early telescopes had two lenses mounted at the ends of a long tube. The length of the tube was equal
to the difference between the focal lengths of the two lenses.
The convex eyepiece lens was introduced by Kepler in 1611. It made a larger field of view and allowed magnifications of over a thousand times.
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