Unit 5 – Light and Atoms

ASTR 101Prof. Dave Hanes

The Pessimist[writing in 1835]

On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are…necessarily denied to us… We shall never be able by any means to study their chemical composition.

August Comte (1798-1857)

What Newton Missed

Just as some pianos have broken keys andunplayable notes, so too:

The spectrum of the sun reveals ‘gaps’ (regions with no light). This is how we know the composition of the sun (and stars)!!

The Spectrum of the Sun[a historical drawing: Fraunhofer, 1814]

The Spectrum of a Star[note the missing colours – the ‘broken piano keys’]

Why Did Newton Miss This?

Lesson: Use a Narrow Slit

Kirchhoff’s First Lawfor hot dense bodies

Emission Lamplow density gas

Kirchhoff’s Second Lawfor hot low-density gases

Unique Patterns (‘Fingerprints’)

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Na-Flash.mp4

On Broadway

Kirchhoff’s Third Law

Kirchhoff’s Three Laws

Example: A Stellar Spectrum

The Sun’s Spectrum in Detail

Kirchhoff’s Third Law!But Where Does It Happen?

In the gases of the Earth’s atmosphere, just as the starlight reaches us?

In diffuse, spread-out gas between the stars, filling all space?

In a thin ‘stellar atmosphere’ around the outskirts of each star?

Many Stars

Simple in Principle: Collect and Disperse the Light

Complex Details and Interpretations

(here, light reflected from and emitted by Mars)

Light as Particles:The Photoelectric Effect

Application: exposure meters in cameras, automatic doors in elevators

Red Light

Green Light

Blue Light

Consider a Brick Wall

A Wavelength Dependence

Einstein’s Nobel Prize

Conclusion

Light consists not just of waves, but also of discrete lumps (photons) that act like little ‘bullets’ of energy.

Higher frequency (blue) = more energetic lumpsLower frequency (red) = less energetic lumps

Individually, they don’t carry much energy. A 100-Watt light bulb emits almost 300 million trillion photons per second!

Remember the Full Spectrum!

Gamma rays, the highest energy electromagnetic radiation, can disrupt DNA and cause cancerous mutations.

X-rays can be very penetrating, pass through fleshy tissue

Ultraviolet radiation can damage pigments, tan your skin

Infrared radiation can be felt as glowing warmth

Radio radiation is very low energy. We are awash in it all the time from radio stations and the like.

The Perplexing Wave-Particle Duality

[a digression for those interested]So light behaves like a wave but also as a particle.

Amazingly, at the quantum (= small!!) level, so too does all matter, including electrons (which are so easily visualised as little ‘billiard balls’).

Watch https://www.youtube.com/watch?v=DfPeprQ7oGc

Absorption Lines: Atoms Provide an

Understanding

Racetrack Orbits? No

Simple-Mindedly: Quantized Orbits

Quantized Behaviour

To Excite an Atom

1) Heat the gas! Collisions between atoms can ‘bump’ electrons up to higher levels; as they fall down, we get emission lines.

2) Run an electric current through the gas! This is what happens in neon lamps.

3) An orbiting electron can also be raised to a higher energy state by absorbing a photon of just the right energy. Below, red light has too little energy; blue light has too much; but green light is just right!

What Then?

Every Atom is Different- hence forensics!

Beyond Single AtomsMolecules (both simple and complex) consist of atoms bound together by the electric attraction of their electrons and protons. An entire molecule can rotate or vibrate at various rates.

CO (carbon monoxide) is a simple molecule, shaped like a baton. If rotating quickly, it can slow down by emitting a photon.

But only certain changes are possible, like changing gears on a car. So the emission is quantized – only photons of certain energies will be observed. Thus we learn about molecules in space.

Physics History

It is the interaction of atoms with light, and the science of spectroscopy, that allowed us to first understand the structure of the atom!

Indeed, these insights led to the modern theories of quantum mechanics.