Lecture 2: Light the Messenger

Readings: Sections 5-1, 5-2, 5-5, and 5-9

 

Because of the vast distances involved in astronomy, we cant rely on laboratory investigations to study the Universe. How can we learn about things that are so distant?

 

 

Answer LIGHT!

Doesnt need a medium to travel in, so it can travel through the vacuum of space, unlike sound or water waves.

 

 

 

Everything we know, we know from the light emitted by astronomical objects with the exceptions of

 

         n (neutrinos)

        

         nearby space stuff (solar wind, comets, planets)

                 

 

Key Ideas About Light:

 

       Light is Electromagnetic Radiation

 

         Light as Waves and Photons

 

         Electromagnetic Spectrum

                  sequence of photon energies

 

         Doppler Effect

                  Relative motion between source & observer

                  Way to measure speeds at a distance

 

Electromagnetic Radiation

 

       Light is a self-propagating electromagnetic disturbance that moves at the speed of light (see Section 5-2 in the book and Figure 5-6)

 

We can treat light as either

 

         Electromagnetic Waves

 

 

         Photons (particles of light)

 

 

because it has properties of both.

 

Wave Nature of Light

 

Electromagnetic Waves

         Periodic changes in the strengths of electric & magnetic f         fields

        

         Travels through a vacuum at the speed of light

 

Speed of light is a constant for all light waves

        

                  c=299,792.458 km/sec

 

Independent of wavelength or frequency     

 

We can measure the wavelength and frequency of light.

They are related to the speed of light c by the equation

 

 

 

Photons: Particles of Light

 

Can also treat light as particles or photons

 

Photon: massless particle that carries energy at the speed of light.

 

This particle has energy that depends on the frequency/wavelength of the light.

 

    

 

 

n = frequency

h = Plancks constant

c = speed of light

l = wavelength

 

The Electromagnetic Spectrum (see Figure 5-7)

 

Sequence of photon energies from low to high is called the

Electromagnetic Spectrum

 

Low energy=low frequency=long wavelength

 

 

High energy=high frequency=short wavelength

 

 

The different types or bands in the electromagnetic spectrum interact with matter in different ways. For example, our eyes are very focusing viual light, but not X-rays. One of the major ways they are different is their ability to come through the Earths atmosphere.

 

 

Visual light can penetrate our atmosphere.

         Light we can see with our eyes

                  Wavelengths: 400-700 nm

                  Frequencies: 7.5x1014-4.3x1014 waves/sec

 

Observing at different wavelengths

 

Gamma-rays – must be observed from space

 

X-rays – must be observed from space

 

Ultraviolet – must be observed from space

 

Visible – observed from the ground or space. In the daytime, its really tough.

 

Infrared – observed from the ground or space. Detectors must be kept really cold.

 

Radio – observed from the ground

 

 

How Bright is a Light Source?

 

         Luminosity: total energy (photon) output of a source per second

         Apparent Brightness: how bright it appears from a distance

 

I will use faint and bright to refer to the apparent brightness of a source. For example, the Sun is very bright, but it turns out that it is not very luminous. Rigel is much more luminous than the Sun, but it is much fainter than the Sun.

 

 

Luminosity is measured in Power Units (Energy/second). It is an intrinsic property of the source and is independent of our distance from it.

 

Apparent Brightness is measured in Flux Unites (Energy/area/second) and measures how bright an object appears to be as seen from a distance. It does depend on the distance from an object and is what we can actually measure.

 

 

Inverse Square Law of Brightness

 

 

The Doppler Effect (see section 5-9)

 

Shift in the observed wavelength when the source is moving relative to the observer.

 

Amount of the shift and its sign depend on

         Relative speed of the source & the observer

         Direction (towards or away)

 

Examples:

 

Sound

 

Light

 

Cats & Mouse

 

The Doppler effect for light

 

         Moving away from the observer, wavelength gets longer, redshift

 

         Moving towards the observer, wavelength gets shorter, blueshift

 

Note that this does not mean the light becomes red or blue!

 

We can use this to measure the speed (v) of an object by noting the lobs of the light that hits us and knowing the lem (the known emitted wavelength)

 

 

 

A Note about Equations

 

You do not need to memorize the equations for the exam, but you do need to know qualitatively how the quantities are related.

 

For example, for the equation

 

you can remember that

or the words higher frequency=shorter wavelength, lower frequency=longer wavelength

 

or have a mental image of a wave in your head

 

or whatever works for you.