Light

Mystical, magical

What is light?

Light is a curious thing. It is . . . the only thing you will ever be able to seea wave with electric and magnetic

propertiesmade 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 x108 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 acrossWe 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 courseWhat creates a light wave?

– a vibrating electron in the starElectromagnetic 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 WavesMicrowavesInfrared (IR)Visible light (ROYGBV)Ultraviolet (UV)X-raysGamma 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 atomsVibrating atoms = vibrating chargesVibrating charges = lightWe cannot see, but we can sense IRpests application

Opaque and Transparent Materials

Transparent – light can pass throughOpaque – 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 inLight hits the ground and ground gets

warmWarm ground glows in IRAir is mostly opaque to IR – most of that

energy is trapped and held for a long time