# The Nature of Light In Astronomy. Herschel’s Infrared experiment Invisible (to our eyes) light immediately beyond the color red is call infrared light.

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The Nature of Light In Astronomy

Herschel’s Infrared experiment Invisible (to our eyes) light immediately beyond the color red is call infrared light.

The Earth’s atmosphere absorbs most of EM spectrum, including all UV, X ray, gamma ray and most infrared. We have to build telescopes for these wavelengths in space. Only radio and visible light can get complete through the atmosphere.

Light is an electromagnetic wave.

Wavelength and Frequency wavelength  frequency = speed of light = constant

Particles of Light Particles of light are called photons. Each photon has a wavelength and a frequency. The energy of a photon depends on its frequency (or equivalently on its wavelength).

Wavelength, Frequency, and Energy  f = c = wavelength, f = frequency c = 3.00  10 8 m/s = speed of light E = h  f = photon energy h = 6.626  10 -34 joule  s = photon energy –The speed of light is constant. –If the frequency increases the wavelength decreases and vise versa

Thought Question The higher the photon energy, A.the longer its wavelength. B.the shorter its wavelength. C.Energy is independent of wavelength.

Thought Question The higher the photon energy, A.the higher its frequency. B.the lower its frequency. C.Energy is independent of frequency.

How do light and matter interact? Emission Absorption Transmission —Transparent objects transmit light. —Opaque objects block (absorb) light. Reflection or scattering

Interactions of Light with Matter Interactions between light and matter determine the appearance of everything around us.

Thought Question Why is a rose red? A.The rose absorbs red light. B.The rose transmits red light. C.The rose emits red light. D.The rose reflects red light.

A single Hydrogen Atom Only emits light at fixed wavelengths (i.e. fixed energy values). Each transition corresponds to a unique photon energy, frequency, and wavelength. When electrons moves from a low level to a higher level a photon must be absorbed (excited state). When an electron drops from a high to a low level a photon is emitted ( a relaxed state). Level 1 is called the ground state it is the state of lowest energy. The higher levels get closer together in energy. Energy levels of hydrogen (the energy levels of other atoms are different)

A single Hydrogen Atom Every type of atom its own unique collection energy levels (colors). We can tell what something is made from by looking at its spectrum for its chemical fingerprint. Studying the lines emitted and absorbed by objects is called spectroscopy. Spectroscopy is the how we measure the composition of astronomical objects such as stars and galaxies. There are many millions of spectral lines in our Sun’s atmosphere. Energy levels of hydrogen (the energy levels of other atoms are different)

Chemical Fingerprints Each type of atom has a unique spectral fingerprint.

Chemical Fingerprints Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

Three Types of Spectrum Continuous spectrum Emission Spectrum Absorption spectrum

Continuous Spectrum Comes from dense objects (solids or dense gases have this kind of spectrum) … in the image above we use a bulb. Atoms packed together disturb each other and cause lines to spread out and merge together. The most famous continuous spectrum is called a blackbody spectrum. A blackbody spectrum has a shape that only depends on its temperature.

Continuous Spectrum A blackbody spectrum has a shape that only depends on its temperature. If we measure the shape of a blackbody spectrum then we can determine its temperature. This is how we first measured the temperature of stars like the Sun.

Emission Line Spectrum Comes from low density hot gases (NOT from solids). Gas must be hot enough to excite electrons to upper levels. Gas must be low density enough to prevent atoms from disturbing each other’s energy levels.

Emission Line Spectrum We can use this method to determine the composition of the hot gas. Examples of astronomical objects with strong emission lines –The corona of our Sun (i.e. its hot outer atmosphere) –The hot gas of an exploding supernova.

Absorption Line Spectrum Comes from low density cool gases that are illuminated by a hot light source that has a continuous spectrum. The cool gas absorbs/removes specific lines from the continuous spectrum of the hot light source. We can use this method to tell what the cool gas is made from.

Absorption Line Spectrum We can use this method to tell what the cool gas is made from. Example –The stars behind a cool (10K) molecular cloud provide a continuous light source that allows us to measure the composition of the cloud. –Starlight passing through Jupiter’s atmosphere can be used to measure Jupiter’s composition.

Example: Solar Spectrum The photosphere is dense and provides a continuous light source. The chromosphere absorbs some lines. The extremely hot corona adds emission lines to the spectrum

ABCDE Questions ? Q1. Which letter(s) label(s) absorption lines? Q2. Which letter(s) label(s) emission lines? Q3. Which letter(s) label(s) the peak of the visible\optical light? Q4. Which letter(s) label(s) the peak of the infrared light?

Extrosolar Planet Example If we measure the spectrum of other stars that have planets. –We find that they cooler gas of the planet absorbs some of the star’s light and changes its spectrum. –We can get a direct detection of the planet and measure the composition of its atmosphere. –The problem is that the absorption lines are usually very weak.

Now work on the tutorial book. First do the EM spectrum page 45-47. Then do types of spectrum page 61-62.

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