Overtaking sound
If the plane flies faster than the speed of sound, air begins to pile up in front of it. When this air slips out of the way, it suddenly expands, making a loud noise called a sonic boom. At one time, scientists thought it would be dangerous for planes to fly 'supersonically', but now planes like Concorde break the sound barrier every day. They usually do it over the sea, so the deafening sonic boom is not heard on land.

hearing faint sounds
You may not be able to hear a pin drop, but your ear can pick up some amazingly faint sounds. The faintest sound that most people can hear is about 10 billion times quieter than the sound of a live pop concert.

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Tag :faint sounds

Sound on the move
When sound comes from aboard a slow-moving boat, the sound waves spread out evenly through the water, where they move even more quickly than in the air. When the sound comes from a plane, the pattern of sound waves is quite different. This is because the plane moves much more quickly, 'catching up' the sound waves that it makes.

Sound in the air : Sound travels through the air at abut 330 m (1082 ft) a second. It move slightly faster on a warm day than on a cold one.

Underwater sound : Sound travels nearly four times faster in water than it does in air. This means that a fish will hear the boat before a person standing nearby it on land.

Chasing sound : Most passenger jets move at about two-thirds of the speed of sound. At this speed, a plane keeps catching up the sound waves in front of it, and race away from the one it leaves behind. These trailing waves make a low-pitched rumble as the plane flies away.

Fading out : As the sound waves travel outwards, their energy is slowly soaked up by the air. Eventually, they become too weak to be heard. This is why it is hard to make yourself heard a long way off, even if you shout.

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Things that makes sound

Rocket liftoff : 150-190 dB

Jet takeoff : 120-140 dB

Motorbike : 70-90 dB

Orchestra : 50-70 dB

Whispering : 20-30 dB

Rustling leaves : 10 dB

What's in a wave?
Closely spaced waves produce a high-pitched sound, widely spaced waves a deeper one. The sound is soft if the waves are shallow, and loud if they are deep.

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THE MAKING WAVES OF SOUND

Sound is caused by waves of pressure that spread out though the air, making the air molecules squeeze together and then push apart. These waves are started by anything that is vibrating, from a car engines to the strings of violin. Unlike light, sounds always has to travel through something, so if you were floating in empty space, you would hear nothing but total silence.

How loud do sounds get?
Scientists normally measure the loudness of a sound by using units called decibels (dB). The softest sound that the human ear can hear has a decibel rating of about 4, while the loudest ones that it can cope with have a rating of over 150. The decibel scale works logarithmically. This means that if two sounds differ by 10 decibels, one of them is 10 times as loud as the other.

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Making shadows
Transparent object let light shine through them. Opaque ones, like book, block the light and cast a shadow. Because light travels in straight lines, the shadow is the same shape as the book.

Reflecting light
When light hits something shiny, like a metal can, it bounces off it. This called reflection. Shiny flat surfaces are the best at reflecting light. Thing that do not reflect light look black, because they soak up the light that falls on them.

Bending light
When light travels from on substance to another, for example from air into glass, or from glass into water, it changes direction. This is called refraction. It explains why a drinking straw in a glass looks bent.

Focusing light
Lenses are designed to bend light in a particular way. The lenses in glasses help to bend light so that it makes an image on the back of the eye. Instead of being fuzzy, the image is sharp or 'focused'.

Starlight
Light from stars can take millions of years to reach us.