Light Mystical, magical

What is light?  Light is a curious thing. It is...  the only thing you will ever be able to see  a wave with electric and magnetic properties  made up of particles called “photons”  Light is NOT warm. It’s energy is transformed into thermal energy when it is absorbed; until then, it’s light energy. Not thermal energy

A brief history of conflicting theories  It seems obvious to us now that light comes to you from all directions  But, it once seemed obvious to some people that light radiated from one’s eyes out to the world  Until Newton’s time, most people agreed that light was made of particles, but then someone proved it travels as a wave. Einstein showed that both ideas are right.

The speed of light  3.0 x10 8 m/s (300 million m/s) MEMORIZE THIS VALUE!  This is the universal speed limit – nothing can go faster than this  Light can travel slower than this, and does slow down in any medium that is more dense than empty space

A light year  It’s a measure of DISTANCE! It’s the distance light travels in a year.  Alpha Centauri, our nearest star neighbor, is 4 light years away  The Milky Way is 100,000 light years across  We see light from extinct stars – if the star was very far away, its light is still travelling through space towards us, even tho the star itself is now dark.

What creates light?  Light is a wave.  What creates waves? –Vibrations, of course  What creates a light wave? –a vibrating electron in the star  Electromagnetic wave – it has both electrical and magnetic properties

Frequency and Amplitude  Bigger amplitudes of light waves are BRIGHTER.  Changes in frequency/wavelength are associated with different COLORS  Blue light has a wavelength of 390 nm and a frequency of 790 THz  Red light has a wavelength of 700 nm and a frequency of 430 THz

The electromagnetic spectrum  Radio Waves  Microwaves  Infrared (IR)  Visible light (ROYGBV)  Ultraviolet (UV)  X-rays  Gamma Rays

Homework!  Task: find an image of the Electromagnetic Spectrum that goes from Power or Radio Waves to Gamma or Cosmic Rays.  Memorize the order of the wave types by name  Know which end has the longest wavelength  Know which end has the highest frequency

Special Notes: Visibility  “Visible” light: –Humans see from red to violet –Bees see from yellow to Ultraviolet –Snakes see from Infrared to green  UV light – it’s out there, but we can’t see it. It can damage our eye and skin cells, though.  We cannot see IR light, but we can sense it – it feels warm to us.

Special Note: IR Light  We are glowing with IR  Warm things = vibrating atoms  Vibrating atoms = vibrating charges  Vibrating charges = light  We cannot see, but we can sense IR  pests  application

Opaque and Transparent Materials  Transparent – light can pass through  Opaque – light cannot pass through, the material casts a shadow.

Resonance, Transparency, and opacity  If the frequency of light matches the natural frequency of a molecule, the light will cause the molecule to resonate.  Light energy will be turned to kinetic energy  The molecule is, therefore, OPAQUE to the light  If the frequencies don’t match, the material doesn’t resonate, and the material is TRANSPARENT.

Opaque Materials – Glass & UV  Glass molecules have a natural frequency of 10^15 Hz, the same as UV light.  When light in this “color” hits glass, the glass resonates; glass molecules begin to vibrate with very big amplitude, and they start bashing into one another.  The light energy is turned into heat energy. No light energy survives as light; it is turned into thermal energy. The glass gets warm.

Transparent Materials  Visible light has a smaller frequency than UV, so when it hits atoms in glass, the electrons don’t resonate nearly as much.  A glass atom will absorb the light photon. (Gulp!) The electrons will be a little more energetic for a brief time.  Then, the atom reemits a photon that is identical to the one it absorbed. (Burp!)  This is why it takes longer for light to travel through glass than through a vacuum.

Think checkpoint:  Glass is transparent to all frequencies of visible light.  Does all the sunlight incident on a pane of glass get transmitted, or not?  How can you tell?  Can you get a sunburn by sunning yourself by a window inside your house?

Think Checkpoint 2:  What frequencies of light is our atmosphere transparent to?  Opaque to?

The Greenhouse Effect  Atmosphere lets most light in  Light hits the ground and ground gets warm  Warm ground glows in IR  Air is mostly opaque to IR – most of that energy is trapped and held for a long time