3.1 - RADIATION

 When you admire the colors of a rainbow, you are seeing light behave as a wave.  When you use a digital camera to take a picture of the same rainbow, the light hitting the camera’s detector acts as a particle.

 The distance between successive peaks of a wave is known as wavelength, usually represented by the Greek letter lambda, λ.  Wavelength is related to frequency, the number of waves passing a stationary point in 1 second. Wavelength Formula λ = v/f λ = wavelength; in meters (m) v= wavelength; in m/s  speed of EM radiation ~ 3.0 x 10 8 m/s f = frequency, in Hertz (Hz)

 According to the International System (SI) of Units, the unit used for frequency is Hertz (Hz), named after the German physicist, Heinrich Hertz.  Previous name for Hertz  cycles per second.  The period (T) is the length of time taken by one cycle.  Inversely proportional to the frequency; T = 1/f Frequency Formula f = 1/T f= frequency; in Hertz (Hz) T = period; in seconds (s) Heinrich Hertz

 When you tune into your favorite FM radio station such as 96.1, which means 96.1 MHz (million cycles per second), you are adjusting your radio to detect radio waves with what wavelength? Answer in meters.  FM radio stations transmit frequencies in megahertz (MHz).  AM radio stations transmit frequencies in kilohertz (KHz).

 Find the wavelength of the radio station “The X” on your radio dial.

Notice, the higher the frequency, the shorter the wavelength. The amplitude of a wave is the maximum disturbance from an undisturbed position; it describes the intensity of the wave.

 Different waves have different wavelengths along with different speeds. Wave Velocity v = λf v = λ/T v = velocity of the wave; in m/s λ = wavelength; in meters (m) f = frequency; in Hertz (Hz) T = period; in seconds (s) Example 1: A wave has a wavelength of 5.2 meters and a frequency of 2.5 Hz. What is its speed, or velocity? Example 2: A wave has a speed of 25 m/s. If its frequency is 80. Hz, what is its wavelength? Example 3: A 450. nm light ray has what frequency? (Hint: 1 nm = 1 x m)

 Sound is another example of a wave, but is created through vibration  mechanical disturbance.  It requires a medium, which is a substance or material that carries a wave.  Therefore, it cannot travel through a vacuum  a region free of matter, as light can. If the tines on a tuning fork vibrate at 256 Hertz (vibrations per second) = middle C

 There is no sound on the moon (no medium – air), but there is plenty of reflected sunlight.  A common misconception is that radio waves are related to sound.  In fact, they are a type of light, therefore radio communications work just fine on the moon.  Remember, light does NOT require a medium.

 Light is a form of electromagnetic radiation and carries energy through space as electric and magnetic waves.  All electromagnetic waves travel at the speed of light, c  300,000 km/s  We use the word light to refer to this, but in fact visible light only makes up a small part of the electromagnetic spectrum, as you will soon discover.

 Although electromagnetic radiation can behave as a wave, it can also behave as a flood of particles.  A particle of electromagnetic radiation is called a photon.  The amount of energy a photon carries depends inversely on its wavelength.  Shorter λ photons carry more energy, longer λ photons carry less. Planck’s Equation E = hc/λ E = hf E = energy, in joules (J) h = Planck’s constant; x J·s c = speed of light; 3.0 x 10 8 m/s λ = wavelength; in meters (m) f = frequency, in Hertz (Hz) Max Planck

 What is the energy, in both Joules and electron volts (eV), of a 450-nm blue photon?  An electron volt is the amount of energy gained by an electron when it accelerates through an electric potential of 1 volt  convenient unit of energy. Hint: 1 eV = x J

 A spectrum is an array of electromagnetic radiation displayed in order of wavelength.  Wavelengths of light are measured in units of nanometers (nm) or Ångströms (Å): 1 nm = m 1 Å = m = 0.1 nm  Visible light has wavelengths between 4000 Å and 7000 Å ( nm.) Anders Jonas Ångström

 Just as you sense the wavelength of sound as pitch, you sense the wavelength of light as color. 400 nm.700 nm.

 Gamma rays, X-rays, and some radio waves are absorbed high in Earth’s atmosphere, and a layer of ozone (O 3 ) at altitudes of about km. absorbs ultraviolet radiation.  Water vapor in the lower atmosphere absorbs the longer wavelength infrared radiation.  Only visible light, some shorter- wavelength infrared, and some radio waves reach Earth’s surface through two wavelength regions called atmospheric windows.