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Video transcription

Hi, I'm Steve Jones and I'm going to tell you how fast light travels. Well, light itself is a form of electro-magnetic radiation. Now, electro-magnetic radiation, it, comes in many forms. And starting at one end, we have radio and then we have we have microwaves. And next to microwaves we have infrared and then we have visible radiation which we call light. And then we have ultraviolet and onto x-rays at this end. So we have many types of electro-magnetic radiation and the similarity between them is the speed. The speed of electro-magnetic radiation is always 3 X 10 8 m/s or 186,000 miles/sec or 300,000 km/sec in vacuo; in vacuo. That means in a vacuum. Of course if I actually ask light to travel through something like glass; glass is a good example; it would go down to 200,000 km/sec because it travels slower in transparent media. So if it's in plastic, it would be about 250,00; in glass, 200,000 and so on. So the speed of light depends not just on what type of radiation it is, because all of the radiation have the same speed; all of these radiations. It depends also, on what it is traveling in. But in vacuo, the answer of those three, take your choice.