4.1 EM Spectrum

4.1 EM Spectrum

Wavelength: distance between two successive crests or troughs.

Light: is a wave of wavelength ranging between 400 nm (violet) to

660 nm (red)

Electromagnetic Radiation

White light: consists of the rainbow colors (red – orange – yellow – green – blue – indigo – violet)

Electromagnetic Spectrum

Electromagnetic Spectrum

Why do we see the Sun looks reddish at sunrise and sunset?

• The red color has larger wavelength (longer steps).

Why do we see the sky looks blue?

• Blue light has smaller wavelength (smaller steps).

The relation between colors and temperatures

• Energy is directly proportional to frequency.

• Energy is inversely proportional to wavelength.

• Blue is more energetic than red.

• The hotter the star, the bluer the light.

• The cooler the star, the redder the light.

Atmospheric Window

• The atmosphere is transparent at visible light and the radio part.

• The atmosphere is opaque at other parts of the spectrum.

Atmospheric Window

4.2 What a telescope is?

Resolution & Angular Size

Astronomers use angles to denote the positions and apparent sizes of objects in the sky.

Angular Measure

• Basic unit of angular measure is the degree º– Full circle measures 360º– Right angle measures 90º

• Angular distance is the number of degrees across the sky between two points.

• Angular diameter or angular size is the number of degrees from one side of an object to the other side.

The angular size of the Moon is 0.5º (1/2º )

The angular size of the Sun is also 0.5º (1/2º )

Big Dipper

(Ursa Major)

Try this: What is the angular size of this sphere in degrees as you see it? (Answers

will vary from front to back of room)

Angular Measure for Small Angles

1º = 60 arcminutes = 60'

1' = 60 arcseconds = 60''

So 1º = 60x60 = 3,600 arcseconds = 3,600''

Test question: What is the angular size of the Moon ( 0.5°) expressed in arcseconds?

Answer: 1,800''

Telescopes

• Telescopes: might detect gamma rays, Radio waves, or visible light (Optical telescopes).

• Purpose of telescopes: to gather light in order to make fainter objects detectable.

• Magnification is useful for observing Sun and other planets.

• Stars are so far, they appear as points of light.

Limitations of the human eye

• Visible light only.

• Small aperture (5 to 8 mm).

• Storage & recording.

• Resolution: the ability to distinguish finer details of an image (to distinguish between adjacent objects).

Resolution

( )( sec) 1 min

500 ( ) (Human eye)m

Aarc arc

D cm

Ld m

d

L

mEye

d = linear size of objecttheta = angular size of object (in radians)L = distance to the object

Example

radarcm4109.2min1

• Dime = 2 cm = 2x10-2 m• Find L?

• Airplane d=30 m• L = ??

dL

m

Ld m

Resolution of a telescope

• Is inversely proportional to the diameter of the primary mirror or lens.

• Resolution is limited by the turbulence of Earth’s atmosphere to about ½ arc sec.

• Increasing the size of a telescope above a certain limit, no longer improves its resolution.

4.3 Lenses and Telescopes

• Refractor: is a telescope in which light is collected and focused by a lens.

• Refraction: bending of light when it moves from on substance (like air) to another (like water).

• Objective lens: bends the light at the focus (a focal length behind).

• Eyepiece: is used to examine and magnify the image focused by the objective lens.

• Field of view: an image of an area of the sky.

Chromatic Aberration: not all colors focused at the same point.

Spherical Aberration: deviation from perfect focusing

Opaqueness & Sagging

• Some limitations to the use of large refractors:– Opaqueness of the glass.– Sagging of the lens because of gravity.

4.4 Reflecting telescope

Newtonian CassegrainPrime focus

Advantages vs. disadvantages

• Blocking some of incoming light.• Need to look at the top of the telescope• Needs to be cleaned often.• Both suffer from spherical aberration.

• Can be made larger than refractors.• Can be supported. (No sagging problem)• Different colors focus at the same point.• Can be used for ultraviolet detection, since UV does

not pass through glass.

4.5 Light Gathering Power

• The amount of light collected by a telescope mirror is proportional to the area of the mirror πr2 = πd2/4

4.5a Conditions for good observation

• Seeing: The steadiness of Earth’s atmosphere.– Twinkling of stars.

• Transparency: How clear the sky is.

• Light pollution:– Why do we see more stars in the desert than

in a city.

(Mauna Kea) Hawaii4205 m above sea level

•The atmosphere above the volcano is extremely dry, which is important for submillimeter and infrared astronomy because water vapor absorbs radiation in most of this region of the electromagnetic spectrum. •The summit is above the inversion layer that separates lower maritime air from upper atmospheric air, keeping most cloud cover below the summit and ensuring the air on the summit is dry, and free of atmospheric pollution. •The summit atmosphere is exceptionally stable; the lack of turbulence creates some of the world's best astronomical seeing. •The very dark skies resulting from Mauna Kea's distance from city lights are preserved by legislation that minimizes light pollution from the surrounding area.

What is special about Mauna Kea?

4.5b Magnification

eyepieceoflengthfocal

lensobjectiveoflengthfocalionMagnificat

As the magnification increases, the field of view decreases

Example (magnification)

eyepieceoflengthfocal

lensobjectiveoflengthfocalionMagnificat

A telescope of 1-meter focal length. Calculate its magnification if the eyepiece has a focal length of 40 mm.

Example (collecting power)

Compare the collecting power of the 10-meter telescope with that of the 5-meter telescope.

4.6 Spectroscopy• Break up light into a spectrum

using:– Prism– Diffraction grating.

• Benefits:– Identify chemical constituents of star’s

atmosphere. – Determine Temperature of star’s

surface.– Find the density of the radiating

matter.

4.7 Emission Lines

• No two elements emit the same spectrum.

• Produced using a tube having low gas pressure subjected to electric discharge.

• He was discovered using this technique.

• Elements other than those present on Earth have never been detected.

Absorption Lines• The gas subtracts

energy from the continuous spectrum passing through it.

Hydrogen

Helium

Sodium

Iron

Definitions

• Spectrum: A display of electromagnetic radiation spread out by wavelength or frequency.

• Spectroscope: the device used to analyze spectrum (the spectrum making device).

• Spectrograph: if the spectrum is not seen by eye but with a photographic plate.

4.8 Doppler Effect

• Determine how fast an object is moving toward or away from us, but not the tangential speed.

c

v

0

• v = speed of the emitting body.• c = speed of light in vacuum.• Δλ = the change in wavelength.• λ0 = the original (rest) wavelength.

Approaching Object shorter

– Spectral line shifts toward the blue (Blue shifted).

veis

Receding Object Longer

– Spectral line shifts toward the red (Red shifted).

veis

Example

• If a star is moving away from us with a speed of 6000 km/s then the percentage shift in the wavelength is: 

a)2 %

b)2 %

c)5 %

d)4 %

e)4 %c

v

0

• A red light (λ=6500 A) received from a star is observed to have a wavelength that is 130 A shorter than normal.

• Determine the radial speed of the star.

• Is it approaching or receding?

Example

4.9 Observing at Short Wavelength

• Ozone Layer consists of O3 is located 20 to 40 km above Earth’s surface.

• Ozone prevents all radiation of wavelength less than 3000 A from penetrating.

• Observatories in rockets for short periods or in orbits.

• Photographic methods can be used, since ultraviolet and x-rays have enough energy to interact with silver grains on films.

• Low energy.

• Does not affect the photographic films.

• Needs special devices (electronic).

• Infrared: water vapor blocks them.– Balloons carry detectors of infrared.

• Radio Waves: Karl Jansky 1931.– Signal received 4 min earlier every day.

4.10 Observing at Long Wavelength