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## Cosmology and astronomy

### Course: Cosmology and astronomy>Unit 1

Lesson 3: Light and fundamental forces

# Introduction to light

Light and the electromagnetic radiation spectrum. Wave and particle-like behavior, and how to calculate the wavelength or frequency of a light wave.  Created by Sal Khan.

## Want to join the conversation?

• Sometimes i see gasoline on the pavement, and when i look closely i see a rainbow. How does that rainbow form?
• Gasoline (like soap bubbles) reflects multiple colors due to an effect called "thin-film interference." Basically, gasoline forms a thick layer on top of water, so light is reflected once when it passes from air to gasoline, and another time when it passes from gasoline to water. When those two waves interfere constructively, you get different wavelengths/colors, and because the gasoline has different thickness across, you get all sorts of colors.
• What is the nature of color?
• In a nutshell: The color we percieve an object to have, is the light that was reflected by that object. For example, a (yellow) banana is reflecting light of approximately 580 nm wavelength. It absorbs the rest (or at least the rest of the visible wavelengths).

So the color we see is the light that is reflected. If the object doesn't reflect any of the visible light, it would appear black. A surface reflecting most of the visible light would be white.

• Why is a rainbow a specific shape? Or why does it go in a certain direction?
Since the rain droplets fill the entire space where it's raining, shouldn't that entire space be a rainbow? Sorry if this is a stupid question.
• Light is refracted at a very specific angle. I won't go into the mechanics here, but in a raindrop, light in fact undergoes so much refraction it bounces back in the direction the original ray came from, which is why rainbows always appear on the opposite side of the sky as the sun. This means that if you stand with the sun directly behind you, the rays of light form a triangle between you, the observer, the raindrop the light is refracted from and a point on the ground directly ahead of you.

Since raindrops will be scattered all across the sky, and you can only observe light refracted at the specific angle, the line along which the light rays travels describes an arc across the sky, and this arc is the rainbow you see.

Incidentally, the angle of refraction is also the reason why you can never catch up with a rainbow. The angle, and hence the bow's apparent distance must remain constant with respect to your position.
• A few questions......

1. how did we even get close to measuring the speed of light

2. what are Quantum Mechanics

• 1. The speed of light was originally estimated by astronomers hundreds of years ago through observation of astrological events, and today through instrumentation. What specific instruments i don't know. Just remember from chemistry reading about an experiment between two mountain tops in California and a laser, and I would imagine a similar arrangement in a vacuum.
2. "Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of the dual particle-like and wave-like behavior and interaction of matter and energy." Wikipedia.org
• Why is it that when I look at pictures of space, space is black? What makes it like that?
• Earths Hubble Sphere has a verry big diameter ( 14 billion light years ) so logical thing to say is that we should see a lot of light because the sphere has a verry big volume.

BUT, there is a thing called Doppler effect ( you can google Redshift ) which changes the frequency of wave, and makes it redder and redder until it turns infrared and we can not see it anymore.
So with our eyes we can see only the part of space in wich the Doppler effect didn't turn the light infrared. We can "see" the infrared and cosmic radiation only by using gear, but we can't see it with naked eye.
• 4.) does light has a limit? for example a candle with light can be seen 1 kilometer...what if the sun? how far it will take to be unseen in the vastness of the universe?
• why do we see sky blue in color ?why not yellow or any other color?
• We see the sky blue in colour as shorter wavelengths of light are scattered more by the atmosphere. Blue has one of the shortest wavelengths in the visible spectrum.
Violet and Indigo have a shorter wavelength than blue and so are scattered more but our eyes are more sensitive to Blue light as to Violet and Indigo light.
• It is claimed that light as a wave does not require a medium in which to propagate. How do we know that? Could it be that the propagation medium is not yet understood?
• Why can't anything travel faster than light? Is this just a simple fact in the world of astronomy?