Name: 【TED-Ed】Is light a particle or a wave? - Colm Kelleher
Uploaded: 2014-01-16T08:48:21.000Z
Duration: 4 min 24 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

You look down and see a yellow pencil lying on your desk.

Your eyes, and then your brain, are collecting

all sorts of information about the pencil:

about what light is and how vision works.

thought that light originated in our eyes

and that vision happened when little, invisible probes

were sent to gather information about far-away objects.

figured out that the old, Greek theory of light couldn't be right.

In Alhazen's picture, your eyes don't send out

invisible, intelligence-gathering probes,

they simply collect the light that falls into them.

that the Greek's couldn't easily explain:

The idea is that very few objects actually emit their own light.

are simply reflecting light from a source

the light that hits your eye actually originated at the sun

and has traveled millions of miles across empty space

before bouncing off the pencil and into your eye,

which is pretty cool when you think about it.

But, what exactly is the stuff that is emitted from the sun

or is it a wave, like ripples on the surface of a pond?

Scientists in the modern era would spend a couple of hundred years

figuring out the answer to this question.

of tiny, atom-like particles, which he called corpuscles.

Using this assumption, he was able to explain some properties of light.

which is how a beam of light appears to bend

But, in science, even geniuses sometimes get things wrong.

In the 19th century, long after Newton died,

that clearly showed that light can't be made up

For one thing, two beams of light that cross paths

then you would expect that some of the particles from Beam A

would crash into some of the particles from Beam B.

If that happened, the two particles involved in the collision

The beams of light pass right through each other

with two laser pointers and some chalk dust.

For another thing, light makes interference patterns.

Interference patterns are the complicated undulations that happen

when two wave patterns occupy the same space.

and also when two point-like sources of light

And, as a bonus, understanding that light acts like a wave

leads naturally to an explanation of what color is

So, it's settled then, light is a wave, right?

In the 20th century, scientists did experiments

that appear to show light acting like a particle.

For instance, when you shine light on a metal,

the light transfers its energy to the atoms in the metal

But, we can't just forget about properties like interference, either.

So these quanta of light aren't at all like

This result, that light sometimes behaves like a particle

led to a revolutionary new physics theory called

So, after all that, let's go back to the question,

we're used to dealing with in our everyday lives.

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【TED-Ed】Is light a particle or a wave? - Colm Kelleher

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