Transcript Unit 1
Units to cover 24, 25, 29 30 Energy Carried by Photons • A photon carries energy with it that is related to its wavelength or frequency E hc h • From this we see that long wavelength (low frequency) photons carry less energy than short wavelength (high frequency) ones. This is why UV waves give us a sunburn, and X-rays let us look through skin and muscles! Seeing Spectra • Seeing the Sun’s spectrum requires a few special tools, but it is not difficult – A narrow slit only lets a little light into the experiment – Either a grating or a prism splits the light into its component colors – If we look closely at the spectrum, we can see lines, corresponding to wavelengths of light that were absorbed. Emission Spectra • Imagine that we have a hot hydrogen gas. – – – – Collisions among the hydrogen atoms cause electrons to jump up to higher orbitals, or energy levels Collisions can also cause the electrons to jump back to lower levels, and emit a photon of energy hc/ If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength of 656 nm If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength of 486 nm • We can monitor the gas, and count how many photons of each wavelength we see. If we graph this data, we’ll see an emission spectrum! Emission spectrum of hydrogen • This spectrum is unique to hydrogen – Like a barcode! • If we were looking at a hot cloud of interstellar gas in space, and saw these lines, we would know the cloud was made of hydrogen! Different atom, different spectrum! • Every element has its own spectrum. Note the differences between hydrogen and helium spectra below. Absorption Spectra • What if, instead of hot hydrogen gas, we had a cloud of cool hydrogen gas between us and a star? – Photons of an energy that corresponds to the electronics transitions in hydrogen will be absorbed by electrons in the gas – The light from those photons is effectively removed from the spectrum – The spectrum will have dark lines where the missing light would be – This is an absorption spectrum! – Also like a barcode! Types of Spectra • Kirchoff’s Laws: – If the source emits light that is continuous, and all colors are present, we say that this is a continuous spectrum. – If the molecules in the gas are wellseparated and moving rapidly (have a high temperature), the atoms will emit characteristic frequencies of light. This is an emission-line spectrum. – If the molecules of gas are wellseparated, but cool, they will absorb light of a characteristic frequency as it passes through. This is an absorption line spectrum. Spectra of Astronomical Objects If absorption lines are seen in the spectrum of an object, what are we seeing? • • • • a. A gas in front of a source of continuum radiation. b. We are seeing the glow from a gas by itself. c. A source of continuum radiation is in front of a gas. d. We are seeing the glow from a continuum source by itself. Doppler Shift in Light • If an object is emitting light and is moving directly toward you, the light you see will be shifted to slightly shorter wavelengths – toward the blue end of the spectrum, or blue-shifted • Likewise, if the object is moving away from you, the light will be red-shifted. • If we detect a wavelength shift of away from the expected wavelength , the radial (line-ofsight) velocity of the object is: VR = Dl l ´c Telescopes • Telescopes have been used for hundreds of years to collect light from the sky and focus it into an eyepiece. An astronomer would then look through this eyepiece at planets, nebulae, etc. • The human eye is not very sensitive to dim light, and was replaced in astronomy by the film camera. • Film is sensitive to only around 10% of the impinging light, and is usually replaced by a… The Charge-Coupled Device (CCD) • • The CCD, similar to those found in commercial digital cameras and phones, utilizes the photoelectric effect to collect around 75% of the visible light that is focused on it! It has revolutionized astronomy – images can be recorded and downloaded to a computer anywhere in the world for analysis • The science of developing new methods for sensing, focusing and imaging light in astronomy is called instrumentation Outside the visible spectrum • • • • Many objects of astronomical interest are visible only in wavelengths other than the visible! Much can be learned from studying a star, planet or nebula in multiple wavelengths. Radio telescopes can be used from the ground to image pulsars and other bodies • Observations in other wavelengths require instrumentation to be lifted above the Earth’s atmosphere. X-ray, Gamma ray and infrared wavelength telescopes are currently in orbit! Radio Telescopes • Radio telescopes, like the one in Arecibo, Puerto Rico, collect radio waves from astronomical objects and events Size Matters! • • • Aperture size is very important when collecting light! A large collecting area allows astronomers to image dim and distant objects. For a telescope with an aperture a distance D in diameter, Collecting Area = p 4 ´ D2 Refraction • • • Light moves at a fixed speed, c. The value of c changes depending on what substance, or medium, it moves through. The speed of light in vacuum is around 300,000 km/s. Its speed through glass or water, however, is slightly slower • • If a beam of light (or light ray) enters a new medium at an angle, light on one side of the ray enters first, and slows. This slowing of one part of the ray causes the ray to change direction, similar to driving a car from asphalt onto sand can make a car swerve. This bending of a light ray’s path is called refraction Refraction in Water Dispersion • • • • The amount a light is diffracted depends on its wavelength A prism spreads the light out, using this effect This dispersion of light is a problem in refracting telescopes, as the focal planet will be at a slightly different location for each wavelength of light This leads to chromatic aberration, a blurring effect. Lenses • A lens is a specially shaped piece of glass that bends light rays passing through it so that they focus a particular distance away (the focal length) at a particular location (the focal plane). • A sensor such as a human eye, a camera or CCD, if placed in the focal plane can image the light Refracting Telescopes • Telescopes that use lenses to focus light are called refracting telescopes, or refractors. • Large refractors are difficult to build! – Glass is heavy, and glass lenses must be supported only by their rims, a difficult engineering problem – Glass sags under its own weight, defocusing the light! – Refractors suffer from chromatic aberration, a blurring effect due to changes in the focal plane of the lens for different wavelengths of light Reflecting Telescopes • Reflecting telescopes, or reflectors, use a curved mirror to focus light • Mirrors can be supported from behind, and so can be much larger than refractors • Larger sizes mean that more light can be collected and focused, allowing astronomers to image dimmer or more distant objects • Most modern telescopes are reflectors. Different styles of reflectors X-Ray reflectors • • X-rays only reflect at glancing angles, otherwise they are absorbed or pass through the mirror! X-Ray mirrors are designed to gently reflect the incoming photons, focusing them at the end of a long tube-shaped array of mirrors