Martin Crow Crayford Manor House Astronomical Society

BEGINNERS ASTRONOMY

Martin Crow Crayford Manor House Astronomical Society

LIGHT


This week:

The history of light.

The physics of light and the electromagnetic spectrum.

Spectroscopy.

Getting the information that light contains .

Observing at different wavelengths


Light is fundamental to our understanding of the world around us and the universe as a whole.

But light is much more than it seems. All light contains information about the processes which created it.


So what is light?

The Greeks believed that the human eye was made out of the four elements and that there was a fire in the eye which shone out making sight possible.

Euclid wrote Optica around 300BC. He questioned that sight is the result of a beam from the eye, for he asked how one could see the stars immediately if one closes one's eyes then opens them at night.

In ancient India around the 6th-5th century BC light was thought to be one of the five fundamental "subtle" elements out of which emerge the gross elements. Light was taken to be continuous in nature.

Although optics, including reflection, was studied the beginnings of our modern understanding of the nature of light had to wait until the 17th century.

The Greeks understood that light travelled in straight lines and thought of themas rays.


In 1665 Sir Isaac Newton conducted experiments on Sunlight when investigatingthe nature of colour fringing found when using telescopes of the time.


At the time people thought that colour was a mixture of light and darkness and that prisms coloured the light.


Newton’s experiment


These experiment demonstrated that white light is a combination of colours.

Also, Newton concluded that light must be corpuscular in nature because of theway the light rays could bend and reflect.


In 1676 Ole Rømer while working on a method for measuring longitude noticed (along with Cassini and Picard) that the transit timings for Jupiter’s moon Io increased and decreased suggesting that the speed of light was finite. It was generally thought that light travelled instantaneously before this.


Huygens deduced that light travelled 16 2⁄3 Earth diameters per second. (200,000 km/s)

The finite nature of the speed of light wasn’tfully accepted until Bradley discovered the aberration of light in 1727.

In 1809, with the benefit of the measurements for the A.U. yielded by the transits of Venus, Jean Baptiste Joseph Delambre calculated a speed a little over 300,000 km/s - very close to its modernvalue.

Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862.

The modern accepted value for the speed of light in vacuum is 299,792.458 km/s


The light that we see is just the tip of the iceberg.

In 1800 William Herschel was experimenting with different glass combinationsfor looking at the Sun (very dangerous). He had noticed that with certain groups he felt more heat even though he could see little light.

Herschel set up an experiment to measure the temperatures for different colours.

Herschel made repeat measurements with the thermometer bulb in the violet, green and red regions of the spectrum. In each, he observed a temperature rise, which he recorded after 8 minutes.

Average rise in red: 6.9°F, in green: 3.2°F , and in violet: 2°F.

When the central thermometer was just beyond the red end of the spectrum, Herschel noticed that the temperature was even higher than before. He realised thatthere was light that we could not see with own eyes – this was called Infrared.



In 1801 following on from Herschel’s discovery of infrared light the German physicist Johann Wilhelm Ritter looked at the other end of the spectrum thinking to find a ‘cooling’ light.

He made the observation that invisible rays just beyond the violet end of the visible spectrum were especially effective at lightening silver chloride-soaked paper.

The term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century. Eventually the term was dropped in favour of Ultraviolet light.


Around 1862 James Clark Maxwell concluded from his studies on electricity andmagnetism that “light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.”

In other words light was the propagation of electric and magnetic fields throughspace and so acted like a wave.

So light can be described by its wavelength or frequency λυ = c

Where: λ = wavelength, υ = frequency and c = speed of light

So the only thing that distinguishes radio waves or x-rays from visible light is thewavelength or frequency.


However, at the beginning of the 20th century although light could be described in terms of energy it was found that that this energy was quantised. Therefore light could also be thought of in terms of particles or photons.

E (photon) = hc/λ (electron volts)

Where h = Planck’s constant

So , how we think of light depends on what we are doing with it.

This is known as the duality of light.


The Electromagnetic (EM) Spectrum


In the early 1800s the French philosopher Auguste Comte famously said that“Some things are inherently unknowable such that we can never know what the stars are made of.”

In 1814 Joseph Fraunhofer invented the spectroscope to enable him to measure the refraction of different colours of light.

In the course of his experiments he discovered that a bright fixed line appears in the orange part of the spectrum when using the light of a candle.

Experiments to ascertain whether the solar spectrum contained the same bright line in the orange as that produced by the light of fire led him to the discovery of the 574 dark fixed lines in the solar spectrum.

These are now called the Fraunhofer lines.


About 45 years later Kirchhoff and Bunsen noticed that several Fraunhofer lines coincided with characteristic emission lines identified in the spectra of heated elements.

So there was now a way to discover what the stars were made of.

An unknown yellow spectral line signature in sunlight was first observed from asolar eclipse in 1868. This element was unknown at the time and was named Helium.


Designation Element Wavelength (nm)

Designation Element Wavelength (nm)

y O2 898.765 c Fe 495.761

Z O2 822.696 F Hβ 486.134

A O2 759.370 d Fe 466.814

B O2 686.719 e Fe 438.355

C Hα 656.281 G' Hγ 434.047

a O2 627.661 G Fe 430.790

D1 Na 589.592 G Ca 430.774

D2 Na 588.995 h Hδ 410.175

D3 or d He 587.5618 H Ca+ 396.847

e Hg 546.073 K Ca+ 393.368

E2 Fe 527.039 L Fe 382.044

b1 Mg 518.362 N Fe 358.121

b2 Mg 517.270 P Ti+ 336.112

b3 Fe 516.891 T Fe 302.108

b4 Fe 516.891 t Ni 299.444

b4 Mg 516.733


So what causes these lines?



How this effects what we see.


Another way to produce a spectrum is to use a transmission or reflection grating.


Simon Dawes

Stellar spectra using a transmission grating


Spectra of M42 using a transmission grating


Colour

Looking at the colour of a star can tell us what temperature it is.

Everything radiates in an characteristic way. It is called Blackbody radiation.


The Sun’s blackbody curve.


Astronomers measure a stars colour by measuring its brightness throughdifferent coloured filters. Then by subtracting the results a value for its rednessor blueness is obtained. This is called a Colour Index.

To do this standard filters are used so that all results produced by astronomerare the same.


Doppler shift

When sound travel travels towards you and them away from you the pitch changes.


The same effect works with light and is called red shift demonstrating the wave-like nature of light .


Red and Blue shift – Red is receding, Blue is approaching


When an object such as a star is rotating you get line broadening.This allows rotational velocities to be measured.


The intensity of light with distance follows the inverse square law.

If you double the distance the intensity will be one over the square of twowhich is a quarter.


Observing a different wavelengths is not straight forward.

The Earth’s atmosphere blocks most wavelengths.

Some wavelengths can only be observed above the atmosphere.


To catch x-rays and form an image is tricky.

The x-rays glance off the mirrors and are so focused onto the detector.


Being able to see the universe at different wavelengths has totally changed our understanding of the Cosmos and is continuing to do so.

Radio

M87


Microwave

The echo of the ‘Big Bang’.


Infrared – Herschel space telescope

Image of IC5146 showing filaments


Ultraviolet – Galex space telescope

M81 - new stars

M81 – detail of galactic core


X-rays – Chandra space telescope


Gamma ray

Showing the even distribution of these events


M31




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Martin Crow Crayford Manor House Astronomical Society