Download
1 / 63
630 likes | 745 Views
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.
E N D
Wavelength: distance between two successive crests or troughs. Light: is a wave of wavelength ranging between 400 nm (violet) to 660 nm (red)
White light: consists of the rainbow colors (red – orange – yellow – green – blue – indigo – violet)
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.
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).
d L d = linear size of object theta = angular size of object (in radians) L = distance to the object Eye
d L Example • Dime = 2 cm = 2x10-2 m • Find L? • Airplane d=30 m • L = ??
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.
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.
Opaqueness & Sagging • Some limitations to the use of large refractors: • Opaqueness of the glass. • Sagging of the lens because of gravity.
Prime focus Cassegrain Newtonian
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.
What is special about Mauna Kea? • 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.
4.5b Magnification As the magnification increases, the field of view decreases
Example (magnification) 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.
