Astronomy and Spectroscopy: Telescopes, Light, and Atomic Physics

Reason for larger telescopes

To collect more light from distant objects.

Refraction of light

Change in direction of a light ray as it passes at an angle from one transparent material to another that has a different optical density.

Reflection of light

When a ray of light strikes a smooth mirror surface at an angle to the perpendicular, the ray is reflected on the 'other' side of the perpendicular but at the same angle as the incoming ray.

Newtonian telescope

Uses one curved mirror and one flat mirror at a 45° angle to the first mirror axis.

Eyepiece light direction in Newtonian telescope

Light leaves the eyepiece at right angles compared to the direction of the incoming light.

Construction of a Newtonian reflecting telescope

Constructed using a concave primary mirror and a flat, diagonal secondary mirror.

Best shape for astronomical mirror

A parabolic shape produces the sharpest images of very distant objects at its prime focus.

Prime focus of a reflecting telescope

Reached by light after one reflection.

Cassegrain reflecting telescope

Constructed using a concave primary mirror followed by a smaller convex secondary mirror that reflects light back through a hole in the primary mirror.

Light-gathering power of a telescope

Depends directly on the area of its primary mirror or lens.

Magnification of a Newtonian telescope

80 times with a primary mirror of diameter 0.25 m and focal length of 2 m when used with an eyepiece of focal length 25 mm.

Light-gathering power comparison

The Keck telescope is 2500 times greater in light-gathering power than a 20-cm telescope.

Palomar telescope comparison

The light-gathering power of a Keck telescope is 4 times greater than that of the Palomar telescope.

Light collection comparison

The 5-m telescope at Mount Palomar can collect 1,000,000 times more light than the unaided human eye.

Light-gathering power factor

The light-gathering power of the 10-m diameter Keck telescope is 4 × 10^6 times greater than an average unaided human eye.

Increasing magnification in refracting telescope

Increase the focal length of the eyepiece.

Bending of light ray in dense material

The light ray bends toward the perpendicular when it crosses the plane smooth surface of a dense transparent material at an angle.

Light ray behavior when leaving glass

The light ray bends away from the perpendicular to the surface as it leaves a glass surface traveling from glass back into air.

Speed of light in glass vs vacuum

The speed of light in glass is slower than the speed of light in a vacuum.

Effect of light passing into glass

When light passes from air into a dense but transparent material such as glass, it slows down.

Important property of glass lens

Curvature and shape of its surfaces are the most important in bending light rays to form a focused image.

Telescope using lens

A refracting telescope uses a lens as the main optical element.

Refracting telescope

An optical telescope that uses a lens to gather and focus light.

Primary image distance

The distance from the objective lens in a refracting telescope where the primary image is formed, equal to its focal length.

Aberration

A defect in design that blurs or distorts the image in a telescope.

Spherical aberration

Occurs when different parts of a lens focus light at different distances from the lens.

Correction of spherical aberration

Usually achieved by making the objective lens very thin, with a long focal length.

Chromatic aberration

The failure of a telescope objective to bring all colors of light to the same focus, appearing only in refracting telescopes.

Causes of chromatic aberration

Different colors are refracted through different angles at each surface of the lens.

Focus of different colors

Light of different colors comes to a focus at different points behind the objective lens inside the telescope.

Correction of chromatic aberration

Most often corrected in modern refracting telescopes by using a combination of two lenses made from different types of glass.

Building a telescope with minimal chromatic aberration

Should use mirrors instead of lenses.

Parabolic mirror

Shaped and polished to avoid spherical aberration by bringing parallel rays to a single focus.

Glass corrector plate

Used in Schmidt telescopes to correct for spherical aberration in the primary mirror.

Schmidt telescopes

Used mostly for photographing wide areas of the night sky.

Air turbulence

The major cause of blurred and unsharp images of objects observed through very large telescopes at the extreme limit of magnification.

Seeing

The twinkling and blurring of the image due to turbulence in the Earth's atmosphere.

Best sites for telescopes

On the tops of high mountains, above a large fraction of the disturbing atmosphere.

5-m Hale telescope issue

Light pollution from nearby cities has seriously reduced its effectiveness.

Adaptive optics

Developed to compensate for image distortion caused by the Earth's atmosphere.

Technique for detail in telescopes

Adaptive optics, where the tilt and shape of mirrors are changed many times per second to compensate for atmospheric turbulence.

First radio energy detection

Came from the center of the Milky Way Galaxy, in the direction of Sagittarius.

Combining radio telescopes

Spreading many radio telescopes across a large area and combining the signals at a central station produces much sharper images of radio sources.

Interferometer use

Using several radio telescopes together to obtain much better angular resolution or sharpness in the images.

Infrared radiation absorption

Water vapor (H2O) is the main absorber in the atmosphere that impedes observations of astronomical infrared objects.

Infrared astronomy in aircraft

Conducted to avoid the absorption of IR radiation by water vapor.

Telescopes in space

Telescopes are placed in space to view distant galaxies primarily to avoid the absorption and distortion of the light or other radiations within the atmosphere of the Earth.

Astronomy from space vehicles

Astronomy from space vehicles is particularly useful because the telescope is above the Earth's absorbing and distorting atmosphere and can measure radiation over a very wide wavelength range.

X-ray telescope technique

The X rays must be reflected at grazing incidence to the mirror surface; otherwise they will simply pass straight through the mirror.

Gamma rays absorption

Gamma rays are generally absorbed in the Earth's atmosphere.

Cherenkov light

The blue light produced when the particles gamma rays collide with move faster than the speed of light in air.

Color change of heated metal

When a rod of metal is heated intensely, its predominant color will change from red through orange to white and then to blue.

Spectrum change with temperature

The intensity of radiation would increase greatly, and the color would change from red through white to blue when heated slowly in an intense flame from 500 K to 1500 K.

Kelvin scale

The temperature scale most often used by scientists.

Kelvin scale measurement

The Kelvin scale measures temperature in Celsius-sized degrees above absolute zero.

Typical star temperature

A typical but very cool star might have a temperature of 3100° Celsius, which is about 3373 K on the Kelvin scale.

Erroneous temperature measurement

The result is erroneous because absolute temperature cannot be negative.

Melting point of water ice

The normal temperature of the melting point of water ice is 273 K.

Freezing water temperature

On the absolute Kelvin temperature scale, the temperature of freezing water is about 273 K.

Boiling water temperature

The temperature of boiling water at ordinary air pressure on the absolute scale—Kelvin—is 373 K.

Temperature range in Kelvin

The range of temperatures in the Kelvin (absolute) scale between the freezing point and boiling point of water is 100°.

Temperature change measurement

A scientist measures the temperature change between freezing water and boiling water with a thermometer calibrated in the Kelvin or absolute scale and will measure 100 degrees Kelvin (K).

Jupiter cloud temperature

The temperature at the top of the clouds on Jupiter is about 165 K, which is -108°C in degrees Celsius.

Mars surface temperature

The minimum temperature reached on the surface of Mars, -140°C, is represented on the absolute (Kelvin) temperature scale as 133 K.

Hottest temperature

100°C is hotter than the other temperatures listed.

Blackbody definition

A blackbody is an idealized object in physics and astronomy that reflects no light and emits light in a manner determined by its temperature.

Device sensitivity for night detection

To detect animals at night by the radiation they give off, the device should be sensitive to infrared wavelengths.

Sun's energy emission

The hot, dense gas existing on the Sun emits energy at all wavelengths, with a peak at one particular wavelength (color).

Vega's surface temperature

The star Vega has a higher surface temperature than the Sun and emits more IR and more UV flux than the Sun.

Object at 0 K

An example of an object that emits no radiation at all is any object at a temperature of 0 K.

Star evolution spectrum change

As a new star evolves from cool dust and gas to a hot star, the peak wavelength of its spectrum of emitted electromagnetic radiation will change from the infrared to the visible wavelengths.

Sun's red appearance

The Sun looks red when it is setting because Earth's atmosphere scatters shorter wavelength light more easily than longer, so more red light is left to reach our eyes.

Sky color

The sky appears blue due to the scattering of shorter wavelength light.

Scattering of light

The air molecules scatter blue light better than red light, so more blue light reaches our eyes.

Emission spectrum

The light from a small amount of a particular chemical element, when heated in a flame, consists of a pattern of narrow, bright emissions at wavelengths that are specific to the element and different for each element.

Spectrograph

A spectrograph is a scientific instrument that spreads out light from a source into its component colors, or spectrum.

Prism effect

When light passes through a prism of glass, the different colors or wavelengths of light are separated in angle by the prism.

Solar spectrum appearance

The spectrum of sunlight, when spread out by a spectrograph, has a continuous band of color, crossed by innumerable dark absorption lines.

Dark absorption lines

The dark absorption lines in the solar spectrum are caused by a cooler layer of gas overlying the hot solar surface, which contains many elements, including hydrogen, helium, magnesium, calcium, and iron.

Spectral lines in astronomy

Spectral lines are of particular importance in astronomy because each different element has a characteristic line spectrum.

Atoms in hot gas

Atoms in a thin, hot gas (such as a neon advertising sign), according to Kirchhoff's laws, emit light at specific wavelengths or colors in a pattern that depends on the element.

Atom structure

An atom consists of negatively charged electrons moving around a very small but massive, positively charged nucleus.

Electromagnetic attraction

The physical force that holds the components of an atom together is the electromagnetic attraction between the positive nucleus and the negative electrons.

Periodic table position

The position of an element in the periodic table is directly related to the number of protons in the atomic nucleus and hence to its positive charge.

Atomic number

The position of an element in the periodic table, its atomic number, is equal to the number of protons in the nucleus of the atom.

Chemical behavior of atoms

The property of a neutral atom that defines its chemical behavior and fixes its position in the periodic table is the number of protons in the nucleus.

Isotopes

Isotopes of a particular element in the periodic table have the same number of protons in the nucleus but different numbers of neutrons.

Age of the solar system

The age of the solar system has been dated rather precisely to 4.56 billion years by determining the age of meteorites by radioactive dating.

Ionization of an atom

Ionization of an atom occurs when an electron is removed from the atom.

Mass of ordinary matter

The majority of the mass of ordinary matter resides in the nuclei of atoms.

Neutron

A neutron is an uncharged particle similar in mass to a proton.

Bohr's model of hydrogen atom

In Bohr's model of the hydrogen atom, light is emitted whenever an electron jumps from an upper to a lower energy level or orbit.

Electron energy levels

Electrons in atoms occupy levels whose energies are fixed.

Spectrum of hydrogen gas

A low-density, high-temperature hydrogen gas gives off a series of emission lines, spaced in a mathematical sequence.

Balmer series

When electrons jump from higher levels in hydrogen atoms to the level n = 2, the resulting spectrum will consist of a series of spectral lines, some of which are in the visible range, known as the Balmer series.

Light emission origin

According to modern atomic theory, light emitted by atoms originates from transitions of electrons between electron levels of the same atom.

Doppler effect

The Doppler effect is the change in the wavelength of light caused by the source moving with respect to the observer.

Observed wavelength change

The observed change in wavelength of light due to the Doppler effect occurs only when the light source has a radial velocity (i.e., motion toward or away from the observer).

Redshift

The Doppler effect is the increase in the observed wavelength of light if the source of light is moving away from you.

Doppler shift in light

When electromagnetic radiation (e.g., light) is Doppler-shifted by motion of the source away from the detector, the measured wavelength is longer than the emitted wavelength.