Journal 1/13/17 Objective Tonight’s Homework Which color gets the hottest? Which do you think gets the coldest? More importantly, why? Objective Tonight’s Homework To learn a few properties of light through experimentation Finish lab write up by next Wednesday

Learning About Light Finish up the demos today. After half of class, we’ll gather again and take some notes.

Notes on the Nature of Light What is light? Light is what we call an “electromagnetic wave”. It’s a wave made of electricity and magnetism. Light is the only wave that can move without anything to “wave through”. Like space. For reasons we’ll discuss later in the year, light is also the fastest moving thing in the universe. Nothing can go faster than the speed of light in a vacuum. This speed is 2.99 x 108 m/s. For reference, this is so fast that a beam of light could travel around the world 7 times in 1 second.

Notes on the Nature of Light Since light is a wave, it’s possible to look at both its wavelength and frequency. Remember that speed is frequency times wavelength. When we look at white light, we discover that it’s not actually one frequency, but hundreds of frequencies traveling together. Knowing this, scientists isolated single frequencies of light and found they’re the same as what we call color. Ex: wavelength frequency Red light ~700 nm 4.3x1014 Hz Purple light ~400 nm 7.5x1014 Hz

Notes on the Nature of Light Knowing the size of wave that each color of light makes is interesting. It allows us to explain why objects appear different colors. Example: A red object As white light (all colors) hits our object, some of the light waves are absorbed into the material and some are reflected. Since we see red, it means that all wavelengths of light except the red ones get absorbed, while red gets reflected. The reflected light is what reaches our eyes so that’s what we see.

Notes on the Nature of Light This leads to a few interesting facts. If something is black, it means it absorbs all frequencies of light leaving none left to reach our eyes. Since these objects absorb light and since light carries radiant energy, black objects are usually hotter, as they’ve absorbed energy. The reverse is true for white objects. They reflect all light and absorb none, so they’re usually cooler.

Notes on the Nature of Light Lastly about color: If we shine a light that ISN’T white, we can get some interesting effects. If we shine a blue light on our red block, it will appear black. Why? Because the block only wants to reflect red and there was no red in the blue light. Since none of the light is being reflected back, we’ll see it as black. Nothing to reflect back! 

Notes on the Nature of Light The last fact to discuss is also an interesting one. Since light is a wave, we say that it has orientation. This means that the wave can be twisted or turned in any direction. Normally, we don’t notice this kind of thing because our eyes aren’t set up to determine the orientation of light. However, with technology, we’ve creating polarizing filters. These filters have lines so closely together that only one orientation of light can get through.

Notes on the Nature of Light So what? Looking at things through polarized filters can produce interesting results. Polarization reveals stress lines in glass and metal. It can also absorb glare, as most reflected light is horizontally polarized. Most LCD screens emit only polarized light so looking at them through polarized lenses will render them black. 3-D movie glasses also use polarizing filters, with each lens oriented differently.

Exit Question Imagine you shine light made of all colors except green at a green object. What color will the object appear? a) green b) white c) blue d) red e) black f) yellow