Explaining the speed of light to school children

WORDS CANNOT express the feeling you get when you explain something new to a child and he or she looks back at you with eyes full amazement at the discovery of the new knowledge.

That’s exactly what I felt early this week when I gave a talk to Standard Eight pupils at the Nairobi Primary School.

The topic was “The Speed of Light”. It sounds like a dead topic! After all, what more can one say about the speed of something other than simply state its value? Well, there is more than meets the eye …

We started off by agreeing that speed is the distance travelled divided by the time taken. Surprisingly, this simple idea confuses a lot of people. Many are the times that I have had to explain that a car doing 60km/h travels at the same speed as a bus also doing 60km/h!

Question

Any way, I started my talk by asking a question: if a car took two minutes to travel one kilometre, what would its speed be? The answer is simple: one kilometre divided by two minutes, equals 0.5km/min.

That answer left some of the pupils a little confused because they are not accustomed to such units. So, we extended the calculation by asking this: how many kilometres would this car cover in one hour if it maintained that speed?

There are 60 minutes in one hour and the car covers 0.5km per minute; so, in one hour it will travel 60 x 0.5 = 30km. In other words, its speed can also be stated as 30km/h.

Next, we asked: what if the car was doing 60km/h, how long would it take to travel one kilometre? Of course, now the speed has doubled so the time taken halves; that is, it will take one minute.

What if the speed doubled again to 120km/h, how long would it take to travel 1km? The answer is half a minute, or 30 seconds. We continued increasing the speed in steps up to 1,200km/h – of course no car can move that fast. At 1,200km/h, an aeroplane would take just three seconds to cover one kilometre!

Distance

By now, the logic was clear: the faster you travel the shorter the time you take to cover a given distance. Then I posed an interesting question: is it possible to travel so fast that the time taken is zero?

You should have seen their faces! Obviously, the answer is no. No matter how fast you travel, you will take some time to cover the distance.

At that point, I then explained the stroke of genius that was suggested by Albert Einstein.

That is, there is a minimum time period that must elapse when travelling from one point to another, no matter how fast you travel. Put in another way, there is a maximum speed that cannot be exceeded no matter how hard you try. This maximum speed is the speed of light which is 1,079,252,848.8km/h.