Notes: Light

The Theory of Electromagnetism

James Clerk Maxwell (1831-1879)

Scottish physicist.

Found that electricity and magnetism were interrelated.

Moving electric charges created magnetism, changing magnetic fields produced electric

charges.

His discoveries allowed us to discover the nature of light.

Maxwell found that electric charges often oscillated, which created magnetic

fields.

These electric and magnetic fields moved outward in all directions in waves, like the

ripples on a pond.

These electromagnetic waves would travel at the speed of light (3.0 x 108 m/s).

Upon further inspection, Maxwell determined that light must be a form of

electromagnetic wave that he called electromagnetic radiation.

These discoveries allowed us to understand the nature of light.

Astronomy is essentially just the science of analyzing the light from distant objects in

order to learn how those objects work.

What is Light? Light is a form of energy. It is

radiation.

Light is a wave:

It can carry energy (heat).

It has color.

It has different parts:

Crest: The top of the wave

Trough: The bottom of the wave

Rest position: Center line that divides the

crest and trough evenly.

Wavelength (λ): the distance between crests

or troughs in a wave.

Frequency (f): The # of waves that cross a

given point per unit of time.

Amplitude: The distance from the rest

position to the top of a crest or bottom of a

trough

It can be reflected and refracted (bent).

What is Light? Recap:

Light is energy, energy is radiation.

Light is a wave

Light is also something else….

Light is a particle: It can travel without a medium (like in space).

Waves can’t.

Light is weird:

It acts like a wave AND a particle - we call this the wave-particle duality.

This is a central concept behind quantum mechanics and applies to ALL matter.

Einstein proposed that light travels as waves with the energy enclosed in photons.

We call a packet (unit) of light a photon.

How Does Light Move? The speed of light (‘c’) is 3 x 108 m/s (300,000 km/s).

That’s fast enough to circle the Earth 7.5 times in one second.

Light from the sun is 8 minutes old.

It is unaffected by any medium.

It is referred to as the “universal speed limit” – nothing can go faster.

We use light to observe the universe and to figure out how far away it is from us:

A light year (ly) is the distance light travels in 1 year (9.5 x 1012 km/yr).

The closest star to us, Proxima Centauri, is 4.24 ly away.

How does light move through space?

Light travels away from its source in ALL directions.

As a result, it covers an ever widening area as it moves away from its source.

Thus, the apparent brightness of the source gets weaker with distance.

How is Light Created? At its base level, light is made by atoms.

Inside each atom is a nucleus containing protons and neutrons.

Around the nucleus are electrons, which are arranged in levels (orbits).

The farther away an electron is from the nucleus, the more energy they have

to have.

In order to change energy levels they must gain or lose energy.

To move up, they must gain energy.

To move down, they must lose energy.

They lose the energy in the form of a photon (light).

Categorizing Light Light is a form of energy called electromagnetic radiation.

The terms light and electromagnetic radiation are the same thing.

The amount of energy given off by electrons varies greatly.

This leads to widely different wavelengths and frequencies of light.

The more energy in the wave, the higher the frequency, higher the amplitude and shorter

the wavelength.

We categorize light based on those different wavelengths.

These categories are organized into the electromagnetic spectrum based on their

wavelength.

Wavelengths are measured in nanometers (nm) – 1 x 10-9 m.

Everything in the EM Spectrum goes the same speed!

The type of electromagnetic radiation is determined by the

temperature of its source.

Each type of light is the same thing, they just each have different amounts of

energy.

Light That We Can See Visible light is light that we can see.

λ = 700 nm (red) – 400 nm (violet).

It has an intermediate wavelength and intermediate amount of energy.

Visible light is a combination of many colors, which, when combined, create white

light.

Using a prism, we can refract white light to separate the colors.

This is called dispersion.

When we do this, we create a spectrum known as ROY G BIV

Red, orange, yellow, green, blue, indigo, violet.

Each color has a different wavelength and a different amount of energy.

Red light has the longest wavelength, but the lowest energy.

Violet light has the shortest wavelength, but the most energy.

We can learn about stars by looking at the spectral lines from the stars.

Each element has a different number of electrons, which produces different spectra.

All stars/elements give off ALL of ROY G BIV, however certain wavelengths show more.

If we look at the spectra from stars, their spectral lines tell us about their chemical composition.

END

Short (Relatively) Wavelength Light Gamma Rays:

λ = <0.01 nm

They have the shortest wavelength and highest frequency.

Have the most energy.

Will cause massive amounts of damage and cell death.

Will also cause mutations in DNA.

Given off by pulsars, neutron stars, and accretion disks around

black holes.

X-Rays:

λ = 10 nm – 0.01 nm.

Small enough to penetrate materials and tissues.

Given off by neutron stars, stellar coronas.

Ultraviolet Light (UV):

λ = 400 nm – 10 nm.

Means “beyond violet”, so it has more energy and shorter

wavelengths than the violet part of visible light.

Given off by very hot stars. Bees see the world in UV light.

Not quite like this

Long (Relatively) Wavelength Light Infrared:

λ = 1 mm – 700 nm.

Means “below the red”, so it has a longer

wavelength and less energy than red visible light.

It’s just heat radiation.

Given off by planets, comets, cool stars…

Radio Waves and Microwaves:

λ = 100s of meters – 1 mm.

Have the longest wavelength and frequency.

And the least energy.

Microwaves (shorter): Used to cook food (heats

the water in the food) and by cell phones.

Radio Waves (longer): Used for radios.

Given off by the Big Bang (CMBR), dying stars

(especially those being eaten by black holes).

Properties of Light The path light travels can be affected by the materials

it encounters.

Reflection: Light bouncing back off a smooth

surface.

Refraction: Light’s path being bent as it goes from

one type of transparent medium to another like when

it goes from air to water.

Different wavelengths of light are bent different amounts.

This results in dispersion.

A device called a prism can disperse light. Just ask

rainbows.

We can study the dispersion of light (called a spectrum)

using a spectrometer.

Types of Spectra In 1671, Newton first observed how spectra were made, however he

only saw a continuous band of colors (a rainbow).

This band is now called a continuous spectrum.

These spectra are now used to serve as a backdrop on which to compare gases.

In 1802, William Wollaston created a better spectrometer to focus the

spectrum of the sun.

He noticed dark lines in the sun’s spectrum.

The dark lines were where some light was being absorbed.

This band is now referred to as an absorption line spectrum.

The dark lines are superimposed on a continuous spectrum.

Later studies showed that when specific gases were heated to a certain

point, there was no continuous spectrum, but rather a pattern of bright

lines.

These bright lines appeared in the same location as the dark lines.

This became known as an emission line spectrum.

It was later found that each element

produces it’s own, unique spectrum.

Astronomers use this information to

determine the chemical composition of

stars.

Example: The element helium was

discovered in the sun before it was

discovered on Earth.

It’s name is derived from the Greek helios

which means “sun”.

How Spectra are Created

In 1895, JJ Thomson discovered the electron.

In 1911, Ernest Rutherford discovered protons and the nucleus.

The problem with Rutherford’s atomic model is that if electrons were

orbiting the nucleus, then according to Maxwell, they’d be emitting a near

constant stream of energy.

They would then lose that energy and crash into the nucleus.

Niels Bohr solved this problem.

He said that only orbits of certain sizes are possible.

If the electron stays in its orbit, no energy is released.

To move to a higher orbit, the electron has to gain energy.

To move to a lower orbit, the electron has to lose energy.

This energy is given off as photons LIGHT

Bohr took this idea further:

Each orbit required a specific amount of energy (or wavelength of light) to

get to a certain orbit.

Even further still, every element required different energies than other elements for

their orbits.

So, when light enters a gas, only the energy required for an electron

to get to a certain orbit is absorbed.

All other energies/wavelengths pass right through.

It creates an absorption line spectrum – the wavelengths that were absorbed appear as

dark lines.

When the energy is absorbed, the electron jumps up to the energy level

(orbit).

When the electron falls back to its original orbit, the energy is lost as light.

That light is given off at a wavelength that is equal to the energy difference between

the new and old energy levels.

Each atom has a unique structure, therefore so are their spectra.

Doppler Shift Light wavelength is changed by motion

of the light source – just like sound

waves are.

This means the entire spectrum of the

light source is shifted.

If the light source moves away from

you, the light looks more red to you .

This is called Doppler redshift.

If the light source moves toward you,

the light looks more blue to you .

This is called Doppler blueshift.

The motion of the source toward or

away from the observer is called its

radial velocity.

You can calculate this motion