Part 3: Light
John P. Pratt
Light: Wave or Particle?
- Newton proposed that light is a particle, like little bullets.
- Huygens argued that it is a wave, like sound.
- From 1650-1900 the wave theory was accepted.
- Now it is believed that light is "both" an electromagnetic wave and a particle called a photon.
- Waves have a wavelength, which is the distance between peaks in the waves.
- Waves have a frequency, which is the number of cycles per second passing a point.
- The frequency times the wavelength equals the speed of the wave: fl = s.
- That means that the higher the frequency, the shorter the wavelength.
The Visible Spectrum of Light
- Newton showed that white light is made up of all the colors of the rainbow.
- The colors of the spectrum are red, orange, yellow, green, blue and violet
- The wavelength of light goes from 400 (violet) to 700 (red) nm ( 1 nm = nanometer = 10-9 meter).
The Full Spectrum of Light
- Visible light is only a tiny portion of the entire electromagnetic spectrum.
- Gamma Rays are super high energy, very short wavelength, being about 0.01 nm.
- X-ray are very high energy so they penetrate flesh but not bones, being about 0.1 to 1.0 nm.
- Ultraviolet waves are too short to see, being in the 10-100 nm range.
- Visible light is 400-700 nm, going from violet to red.
- Infrared waves are too long to see, being in the 1 - 100 µ range (i.e. 1,000 nm to 0.1 mm).
- Microwaves are in the .1 mm to 1 mm range.
- Radio waves extend from 1 cm to many km in wavelength.
The relation of wavelength and frequency for light
- The speed of light is constant, and is called simply "c". It equals 3.0 x 108m/sec.
- So frequency x wavelength = speed, or fl = c.
- Example: What is the wavelength of FM 100 waves? (100 means 100 MHz, or 100 million cycles per sec.)
- l = c/f = 3 x 108m/sec / 100 x 106/sec
- l = 3 m. So the wavelength of music on FM radio is about 3 meters. (10 feet).
- AM radio (at 1000 KHz) is about 300 meters.
Wave Properties of light
- Refraction: light slows down in a transparent medium, causing it to bend.
- Dispersion: Refraction depends on wavelength, causing blue to bend more than red, giving
- Diffraction: Light bends around corners.
- Doppler Effect: If a light source is moving away from us, it's spectrum is shifted towards the red.
Particle Properties of light
- Light comes in discrete units, like little bullets, called photons
- The photoelectric effect:
- A tiny blue light can knock some electrons off a metal plate.
- A bright red beacon cannot eject any electrons from the same plate.
- Hence, light cannot be a wave, or the huge red light would eject at least a few electrons.
- It must be that blue light is like high power bullets, and red light like BB's.
- The energy of light is proportional to its frequency: E = hf, where h is a constant.
- Temperature is a measure of the average speed of a body's molecular motion.
- Astronomers use absolute temperature measured in degrees Kelvin, or simply Kelvins.
- One kelvin is the same as 1° C.
- Absolute zero = 0 K = -273° C is where essentially all motion stops.
- All objects radiate (give off light) according to their temperature.
- Wien's Law: The hotter an object, the bluer the radiation it emits.
- (The hotter an object the more light it emits at all wavelengths, but the peak shifts towards blue.)
- Wavelength varies inversely with temperature: 10 times the temperature = 1/10 the wavelength.
- A hot gas does not emit a smooth spectrum as do hot solid or liquid bodies.
- Instead, the light it emits is restricted to a few very precise wavelengths called emission lines.
- The theory is that electrons are dropping down fixed energy levels in the atomic electron shells.
- Electrons in the lowest energy level are in the "ground state." Others are in "excited states."
- The most important visible line is the red line called H alpha in hydrogen gas.
- When light passes through a cool gas, it absorbs out the same lines that it would emit if hot.
- Thus the light spectrum has black lines in it which identify the element and its temperature.
- Molecules give off groups of narrowly spaced lines called bands.
- They are much lower energy than atoms and tend to be in the infrared wavelengths.
- There are both emission and absorption lines for molecules, as for atoms.
- Water vapor absorbs much infrared light, so observatories are on mountains to get above it.
Telescopes come in two varieties
Telescopes have three important features:
- Refractors use a lens for the principal objective.
- Reflectors use a mirror for the objective.
- Magnification is the increase in apparent angular diameter of an object.
- Resolution measures the smallest angular separation between two objects that can seen.
- Light-gathering power measures how much more light it sees compared to the unaided eye.
What to look for in a telescope
- Magnification comes from a long focal length, combined with eyepieces of short focal length.
- Resolution comes from a large diameter objective (and high quality optics).
- Light-gathering power comes from a large diameter objective.
- Therefore, the most important feature of a telescope is the size of the objective.
All big telescopes are reflectors
- Lenses have chromatic aberration, causing red and blue fringes on images. Mirrors don't do that.
- Mirrors can be supported all across the back so the reflecting shape does not sag.
- Reflectors are shorter, so they don't suffer from long flexing tubes.
- Mirrors are cheaper than lenses because the light does not have pass through the glass.
Why a pair of good 10 x 50 binoculars for $50 are better than a 100x 60 mm telescope for $100.
- They have just as much light gathering power. 10 x 50 means 10 power, 50 mm diameter.
- Most objects in the sky are dim, not small, so it is only light gathering power that is needed.
- The Andromeda galaxy covers about 3° (6 moon diameters) of sky. It is just too dim to see more than the tiny center with the unaided eye.
- The Andromeda galaxy looks about the same in either binoculars or a small telescope.
- It is 10 times larger in a telescope, but 10 times dimmer because the light is spread out.
- So you can only see about a tenth as much, ten times bigger, so it looks the same.
- You can take binoculars to football games and operas, or go birdwatching.
Astronomers usually replace visual observations with instruments.
- Photographic images can be digitized and made into false color images.
- Photometers measure the brightness of objects.
- Spectrophotometers measure the brightness in different colors.
Space Telescopes have great advantages
- Eliminates all distortion from the air.
- Allows ultraviolet, x-ray and gamma-ray astronomy.
- The background is a million times darker for infra-red astronomy because the earth radiates.
- The long wavelengths require huge antennas, so they are ground based.
- The long wavelengths yield very poor resolution.
- Interferometry used many widely separated telescope to improve resolution by blending signals.