The Electromagnetic Spectrum

Transcription

1 The Electromagnetic Spectrum

2 Learning Objectives! What is Electromagnetic Radiation?! What are spectra? How could we measure a spectrum?! How do wavelengths correspond to colors for optical light? Does red or blue have the longest wavelength?! What is the speed of light? Is it infinite? How was it first measured? Can anything travel faster than light?! What is the Doppler Shift? A redshift? A blueshift? How can we use the Doppler Shift to measure the speed at which very distant objects are moving?! What are the types of Electromagnetic Radiation? Can they all penetrate the Earth s atmosphere? Can you order them by wavelength? By the energy they carry?

3 Light & Astronomy! Astronomers usually cannot control experiments! We get information from the objects we study (stars, planets, galaxies) by observing the light the objects emit! We use telescopes to collect the light that comes to the Earth from distant objects! To be able to extract the maximum information from the light we collect, we must understand light

4 Colors of the Rainbow! Newton used a prism to separate white light into its component colors! Called a spectrum! He then used a second prism to recombine the spectrum back into white light! This demonstrated that white light is a mixture of a spectrum of many colors

5 Particle or Wave?! How does light behave?! Huygens: light travels in the form of waves of energy! Newton: light is composed of a large number of particles

6 Light is a Wave!! In 1803, Thomas Young demonstrated that light travels as a wave! When light is passed through a double slit, a pattern of light and dark bands is produced! This can only be explained by wave-like behavior

7 How Do Colors Arise?! The color of light is determined by its wavelength! Visible light has extremely small wavelengths! Usually measured in nanometers (billionths of a meter)! Wavelengths range from 400 nm (violet light) to 700 nm (red light)! Colors, from longest wavelength to shortest: red, orange, yellow, green, blue, violet

8 The Speed of Light! The ancient Greeks thought the speed of light was infinite! In the 1600s Galileo realized very fast is not infinite! Galileo was the first to suggest an experiment to measure the speed of light, in which two people with lanterns signal each other! This isn t possible over distances of a few miles because of the extreme speed at which light travels Speed = 2 separation / time ~ 2 miles: 10 microseconds

9 The Speed of Light! In 1676, Ole Rømer saw that Io passed behind Jupiter sooner in time as the Earth got closer to Jupiter! He concluded that light from the event took longer to reach us when the Earth was farther from Jupiter! He measured that light travelled 8¼ minutes per AU! Today, we know the speed of light to be c = m/s (186,000 miles per second, or 8.3 minutes per AU) June December ~16.5mins delay, extra 2AU distance

10 Velocity and the Doppler Effect! The Doppler Effect arises from the relative motion between an observer and a source of light (or sound)! Waves are squashed in the direction of motion! For light waves, this is called a blueshift! Waves are stretched in the direction opposite the direction of motion! For light waves, this is called a redshift

11 Electromagnetic Radiation! In the 1860s, James Clerk Maxwell created new theories of electricity and magnetism! These theories suggested that light is traveling electric and magnetic waves! Which is why we refer to light as electromagnetic radiation Magnetic Field Electric Field Electromagnetic wave direction

12 Invisible Forms of Electromagnetic Radiation! Visible light has a range of wavelengths from nm (nanometers)! But what about light with longer or shorter wavelengths?! Maxwell s theories placed no restrictions on the possible wavelengths of light! There are types of light outside of optical or visual wavelengths (the range of wavelengths that we can see with our eyes)

13 Light is also a Particle!! In 1905, Einstein showed that light also behaves as a particle! Light particles are called photons! Particles, like little bullets, have energy! The energy of a photon decreases as the wavelength of the light increases! If you half the wavelength of a photon of light, you double its energy

14 The energy of E-M radiation that is emitted by a process depends on the atoms and molecules responsible for that process. In general, violent processes are energetic and emit short wavelengths (UV, x-rays). Gentle processes are less energetic and emit long wavelengths (IR, radio)

15 Opaque Atmosphere! The atmosphere blocks some wavelengths! This means that IR, UV, X-rays, gamma-rays are best observed from space or, at least, from high in our atmosphere

16 Important bad drawing

17 Next Time How Hot is a Star?