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It's a kind of energy called "electromagnetic (EM) radiation" (but this kind of radiation is not harmful, except for an occasional sunburn). There are other kinds of EM radiation too (radio waves, microwaves, x-rays, etc.), but light is the part WE can see, the part that makes the rainbow.
How FAST?
About 186,000 miles per second [300,000 kilometers per second], so light from the sun takes about 8 minutes to go 93 million miles [149 million kilometers] to earth. Does this seem SLOW? Well, if you could DRIVE to the sun at 60 mph [100 kph], it would take you 177 years to get there! In one second, light can go around the earth 7 times!
How STRAIGHT?
Perfectly straight, until something bends it. The straight paths of light are called LIGHT RAYS.
... with an object or person.
This makes a shadow.
(change its path with a mirror) This is called a REFLECTION.
See this
Change its direction by making it pass into another transparent material of different density, like glass or water. This is called REFRACTION, and it's how lenses work. There are actually other ways to bend or deflect light, including diffraction gratings and holographic lenses. Scientists have also found that gravity can bend light!
Put your cursor over a picture to see what it is