**Speed of Light**

The universal physical constant c, or the speed of light in vacuum, is essential in many fields of physics. Its actual speed is 299792458 metres per second (about 300000 kilometres per second, or 186000 miles per second). It’s precise because a metre is defined as the length of the route taken by light in vacuum over a time interval of 1/299792458 seconds by international agreement. According to special relativity, the speed at which ordinary matter, energy, or any signal carrying information may travel through space is limited to c.

How let us calculate **The speed of light is 3.00×108 m/s. How fast is this in miles per hour (miles/h)?**

**Solution **

To convert this measurement, we need to convert meters to miles and seconds to hours. To do this, we need the following relationships:

1000 meter = 1 kilometer

60 seconds = 1 minute

60 minutes = 1 hour

We can now set up the equation using these relationships so the units cancel out leaving only the desired miles/hour.

speedMPH = 3 x 108 m/sec x (1 km/1000 m) x (0.621 mi/1 km) x (60 sec/1 min) x (60 min/1 hr)

Note all the units cancelled out, leaving only miles/hr:

speedMPH = (3 x 108 x 1/1000 x 0.621 x 60 x 60) miles/hr

speedMPH = 6.706 x 108 miles/hr

**Answer**

**The speed of light in miles per hour is 6.706 x 108 miles/hr.**

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