Why Does Thunder Follow Lightning?
Have you ever noticed how you see a lightning flash and then, some seconds later, hear the
thunder? The sound must be either produced later or take longer to reach you. But the
same happens at the cricket at which you see the batsman hit the ball and hear it seconds
later!
The light from the lightning flash reaches your eyes almost instantaneously. Light travels
at nearly 300 000 km per second. The sound takes some time to reach us as it only travels
about 330 metres per sec at sea level. That’s only 1/3 km per sec. Light travels almost a
million times as fast.
USEFULNESS OF THIS FACT: Knowing how fast light and sound travel can be quite
useful. If a storm is 3 km away, its light reaches our eyes in 3/300 000 thousandth of a
sec. It is a period of time too small for our senses to detect. However, the sound would
take about 9 seconds to do that distance. It follows that if we count the number of
seconds between the thunder and the lightning, then divide it be 3, we will know that the
distance to the storm in kms. This could be useful if we witnessed, for instance a lightning
strike resulting in a fire or if we needed to do
something before the storm arrived.
WHAT IS LIGHTNING? Lightning is the giant
electrical spark between clouds or between clouds and
the ground. Friction within the clouds causes an
electrical charge, similar to the one that builds up
between your comb and your hair as you comb it on a
dry day. A single lightning bolt is really a series of
such sparks in rapid succession.
WHAT IS THUNDER? The heat of sparks causes a rapid expansion of air surrounding the
lightning bolt. Huge sound waves result. Sound is a vibration of air particles. This is the
thunder we hear.
***TURN OVER THIS SHEET TO COMPLETE THE QUESTIONS
COMPLETE THE FOLLOWING QUESTIONS ON FILE PAPER
1.
What is the speed of (a) Light and (b) Sound in air?
2.
How much faster than sound is light as quoted in
the passage?
3.
State in one sentence why thunder follows the
lightning flash.
4.
It is 180 000 000km to the sun. How long does
it take for its light to reach us?
5.
Below are some time lapses taken between the
lightning flash and the thunder. Work out how far
away the flash was in each case.
(a) 3 sec (b) 5 sec (c) 7 sec (d) 1.5 sec (e) 2.5
sec
6.
What causes (a) Lightning and (b) Thunder?
7.
At the cricket you are sitting about 500m from the batsman. What will be the time
lapse of between your seeing and hearing a hit?
8.
When Krakatoa, an active volcano island off the coast of Indonesia exploded in
1883, the explosion was heard in Adelaide about 3500 km away!! How long would it
have take for the sound to reach there?
9.
What is sound?