Chapter 5 Basic Properties of Light The chapter

Chapter 5 Basic Properties of Light The chapter on Light is important for all studies in astronomy. Thus, it is important to review those properties in ASTR 1100. 1. Examine the nature of light as a wave and particle phenomenon in terms of its ability to carry packets of energy. 2. Note the various types of information ― line of sight speed, radiation temperature, chemical composition, etc. ― that can be established from the study of light from planets, stars, galaxies, gas clouds, etc.

Light is a phenomenon that is characterized by photons, which have both wave characteristics and particle characteristics. Because light has the properties of both a particle (the photoelectric effect) and a wave (narrow slit interference), it is usually pictured as a sine wave with an arrow attached, e. g. :

Light is a form of electromagnetic radiation since it carries both a varying electric field and a varying magnetic field at right angles to each other. It is characterized by its wavelength, λ.

Light’s Particle-Like Nature: … is demonstrated by: The Photoelectric Effect. Light incident on alkali metals and other solids (potassium, bismuth, calcium, antimony, etc. ) is able to release electrons from the surface, if they are energetic enough.

Wave Nature of Light: The wave nature of light is demonstrated by the interference effects in straight edge diffraction, single slit diffraction, and double slit diffraction experiments using a point source with either monochromatic or white light. Straight edge diffraction Single slit diffraction (top: monochromatic, bottom white light) Double slit diffraction (top: monochromatic, bottom white light)

Interference of light by a double slit.

Interference of water waves by a double slit.

Dispersion of light by a prism according to wavelength λ or frequency ν.

Low energy radiation (longest λ, shortest ν) is dispersed the least, high energy radiation (shortest λ, longest ν) the most.

The electromagnetic spectrum (schematic).

Kirchhoff’s Laws.

Kirchhoff’s Laws illustrated.

The spectrum of the Sun is an absorption spectrum, i. e. a hot gas viewed against a brighter continuum source.

A high resolution view. Most of the dark spectral lines are caused by atoms of gaseous iron in the Sun’s atmosphere.

An absorption-line spectrum made into a spectral intensity tracing.

How the colour of an object corresponds to its temperature. Initially hot objects glow red, then yellow-orange, and finally white, i. e. “white hot, ” as the temperature increases. The resulting radiation is referred to as black body radiation.

Properties of black body radiation 1. Energy (light) is emitted at all wavelengths, except for = 0 and = . 2. The form of the continuous energy distribution is given by the Planck function. 3. As temperature T increases, the energy output increases at all wavelengths . 4. As T increases, the energy output increases most rapidly for small . Wien’s law:

The brightness of a distant object decreases in proportion to the inverse square of its distance.

According to Einstein’s famous relationship: For light: Thus, since light carries energy, photons can be considered to have “mass” as they travel at the speed of light. At rest they are “massless. ” And, since mass is affected by gravity, light rays can be deflected when passing near massive objects: stars, massive galaxies, clusters of galaxies, etc.

A plexiglass simulation of a gravitational lens created by Charles Dyer and Robert Roeder (1981).

Double quasar, QSO 0957+561. The image of a single background quasar split into two halves by an intervening galaxy.

Huchra’s Lens, quasar Q 2237+030 lensed by galaxy ZW 2237+030.

Gravitational lensing in the galaxy cluster Abell 370.

A close-up view.

Another property of light arising from its wave properties is that it is subject to the Doppler Effect. The wavelengths of photons from distant objects moving relative to the observer appear to be either “stretched” to longer wavelengths or “compressed” to shorter wavelengths when viewed by us, if they are moving either away from us (“red shift”) or towards us (“blue shift” ), respectively. The Doppler Effect was first noted for sound waves, but applies to light waves as well by extension.

Interference of light by a straight edge.

The atomic and ionic lines seen in stellar spectra provide a measure of a star’s temperature. Helium lines denote hot stars (20, 000 K), molecular bands cool stars (3000 K)

The spectral lines in the spectra of stars are from atomic species in the gas of stellar atmospheres that produce electronic transitions in the specific temperature ranges applicable to stars.

Spectral types track the temperature differences between stars.

Hot stars put out a lot more light energy than cool stars, since radiance varies as T 4. (Stefan-Boltzmann Law) The spectral sequence for stars: O B A F G K M (R N S) is a temperature sequence. O stars are hottest, M stars coolest. Oven Baked Ants, Fried Gently, Kept Moist, Retain Natural Succulence Oh, Be A Fine Girl/Guy, Kiss Me (Right Now, Smack)

Astronomical Terminology Temperature. A measure of thermal energy of an object. Luminosity. A measure of an object’s absolute brightness in terms of its radiation output. Continuous Spectrum. A spectrum consisting of an unbroken band of colour from the ultraviolet to infrared regions. Absorption Spectrum. A continuous spectrum interlaced by dark lines and bands. Emission Spectrum. A spectrum consisting only of bright lines or bands. Electromagnetic Radiation. The generic description of light ranging from gamma-rays to radio waves. Doppler Effect. The change in frequency of light or sound caused by radial motion of the source relative to the observer.

Sample Questions 1. We know that the speed of light in a vacuum is 300, 000 km/s. Is it possible for light to travel at a lower speed? Explain your answer. Answer: Yes, light travels more slowly in different media, for example air, water or glass, according to the index of refraction for the medium.

2. An object somewhere near you is emitting a pure tone at middle C on the octave scale, i. e. at a frequency of 262 Hz. You, having perfect pitch, hear the tone as A above middle C on the octave, which is at a frequency of 440 Hz. Describe the motion of that object relative to where you are standing.

Answer. The object is moving toward you at a speed of 226 m/s. Because the note is heard at a higher frequency, the sound waves reach you at a shorter wavelength than that at which they were generated by the object. Wavelength and frequency are inversely proportional to one another. The sound waves are therefore “blue shifted” (appear at shorter wavelengths than originally), so the object must be travelling toward you. The actual speed of the object is found from arithmetic: