بسم الله الرحمن الرحيممحاضرة االربعاء

والخميس23-24-2011 Chapter 5

Light The Cosmic Messenger

The Two Laws of Thermal RadiationLaw 1 (Stefan-Boltzmann law): Each square

meter of a hotter object’s surface emits more light at all wavelengths.

بوتلزمان ستيفان قانون

المنبعثة الكاملة الطاقة أن على بولتزمان ستيفان قانون ينص ) تتناسب ) مساحة وحدة لكل الثانية في واط األسود الجسم من

كلفن الجسم حرارة لدرجة الرابعة القوة .مع

E(T) = σ. T4 - σ حيث بولتزمان ستيفان ثابت

σ =5.67.10-8 W/m2.K4

Law 2 (Wien’s law )Hotter objects emit photons with a higher average energy, which means a shorter average wavelength. That is why the peaks of the spectra are at shorterwavelengths for hotter objects. For example, the peak for the15,000 K star is in ultraviolet light, the peak for the 5800 K Sun isin visible light, and the peak for the 3000 K star is in the infrared.

لالنزياح فين العظمي قانون الموجة طول أن يقولاالسود الجسم إلشعاع

حرارته درجة مع عكسيا تناسبا : تتناسب

b=2.897786.10-3(K.m):

التالي الشكل انظر

Tb

max

How does light tell us the speedof a distant object?

Spectral lines shift to shorter wavelengths when an object is moving toward us, and to longer wavelengths when an object is moving away from us.

Doppler Effect then:Object moves away from us, longer wavelength (redshifted)

Object moves towards us, shorter wavelength (blueshifted)

How do telescopes help us learnabout the universe?The Two Key Properties of a Telescope

1. Light-collecting area 2. Its angular resolution.

The light-collecting area tells us how much total light the telescope

can collect at one time. Telescopes are generally round, so we usually

characterize a telescope’s “size” as the diameter of its light-

collecting area.

For example, a “10-meter telescope” has a light-collecting area that

is

10 meters in diameter. Such a telescope has a light-collecting area

more

than a million times that of the human eye. Because area is

proportional

to the square of a telescope’s diameter, a relatively small increase in

diameter can mean a big increase in light-collecting area. A 10-meter

telescope has five times the diameter of a 2-meter telescope, so its

light collecting

area is 25 times as great.

Light-collecting area

Angular resolution is the smallest angle over which we can

tell that two dots—or two stars—are distinct. For example, the human eye has an angular resolution of about arcminute , meaning that two stars can appear distinct only if they have at least this much angular separation in the sky. If the stars are separated by less than arcminute, our eyes will not be able to distinguish them individually and they will appear as a single star.

Its angular resolution

يرجي المناظير دراسة عن للمزيدمن السادس الفصل على االطالع

الحديثة الفلك علم مبادئ كتاب156صفحة

. . أحمد بري العزيز عبد د أ للمؤلف

How is technology revolutionizingastronomy?

Adaptive optics allows ground-based

telescopes to overcome atmosphericdistortion.

This chapter’s main purpose was to show how we learn about theuniverse by observing the light of distant objects. “Big picture” ideasthat will help you keep your understanding in perspective include the

following:• Light and matter interact in ways that allow matter to leave“fingerprints” on light. We can learn a great deal about the objects

we observe by carefully analyzing their light. Most of what weknow about the universe comes from information that we receive

from light.• The visible light that our eyes can see is only a small portion of the

complete electromagnetic spectrum. Different portions of the spectrummay contain different pieces of the story of a distant object, so

it is important to study all forms of light.• There is far more to light than meets the eye. By dispersing the

lightof a distant object into a spectrum, we can determine the object’s

composition, surface temperature, motion toward or away from us,and more.

• Technology drives astronomical discovery. Every time we builda bigger telescope, develop a more sensitive detector, or open up

a new wavelength region to study, we learn more about theuniverse.