Light

Astronomy 315Professor Lee

CarknerLecture 4

Scale Exercise What is scale for solar system (0.0016 ly)?

measure ball diameter = real/model = scale scale = 0.0016/2 =

What is the model value for the distance to Sirius (9 ly)? real/scale = model 9 /0.0008= 11250 cm = Distance to other end of Science Building

Change scale so that ball equals 9 ly new scale = 9/2 =

Find size of galaxy in model (100000ly) 100000/4.5 = 22222 cm = Distance to Old Main

Disturbing the Universe

Can’t visit directly or send probes Would take ~100000 years to get to nearest

star

Can do some simulations in the lab But how do we know if they are right?

Light

What is light?

How do these properties give us information about the object that emitted the light?

What is Light?

EM radiation can be thought of in two different ways:

As a stream of photons (particle) Light is both a particle and a wave

We use what ever formulation is most useful

Properties of Light When we examine a light emitting

object, what do we want to know? Energy

Photon Flux

How much total energy is emitted by an

object depends on how much energy each photon has and how many of them are emitted

Wavelength Each photon has a wavelength

Energy is inversely related to the wavelength () Long wavelength = Short wavelength =

We will often measure wavelength in meters or nanometers (1 billionth of a meter, or 1X10-9 m)

Waves

Speed and Frequency

c = 3 X 108 m/s = 186,000 miles/s We can use this speed to write the

frequency:c = f

Frequency is directly related to energy High frequency = high energy Low frequency = low energy

Color

This is called visible light

Short wavelength, high energy = blue Long wavelength, low energy = red

A Spectrum

Star Colors Stars come in 4 basic colors

How is Light Produced?

Every object in the universe emits blackbody radiation that depends on its temperature

Given in degrees Kelvin Room temp = 300 K

Higher T means more radiation

Spectrum

The radiation is a continuum of wavelengths called a spectrum

We can describe the spectrum as a curve on the intensity versus wavelength diagram

Peak Wavelength and Temperature

A higher temperature produces a spectrum that peaks at shorter wavelengths

Wien’s Law: max = 3,000,000/T Where T is in Kelvin and is in

nanometers

Intensity and Temperature A higher temperature means more total

energy emitted Stefan-Boltzmann law: P = AT4

is the Boltzmann constant (5.67 X 10-8 W/m2 K4)

A is the surface area of the object (in m2) T is the temperature in Kelvin

Using Radiation Laws Wien’s Law

If you can find the peak wavelength you can find the temperature

Stefan-Boltzmann law Hot objects emit more energy then cool objects The intrinsic brightness of a star depends on both

its temperature and size

Alberio This is the double

star Alberio Two stars orbiting

around each other

Both are the same distance from Earth

Size of star image proportional to brightness

What is the relative temperature and size of the stars?

The Electromagnetic Spectrum Light can have a wide range of wavelengths

This corresponds to a wide range in energies

Today we call the range of wavelengths the electromagnetic spectrum

The EM Spectrum

The EM Spectrum and You

You see in visible light, feel infrared as heat and get a sunburn from ultraviolet

Microwave and radio have long wavelengths and low energy

Next Time

Read Chapter 5.1-5.8