Newton also felt that light rays were made up of particles,

1. In 1665, Isaac Newton was a young scientist studying at Cambridge University in England. One sunny day, Newton darkened his room and made a hole in his window shutter, allowing just one beam of sunlight to enter the room. He then took a glass prism and placed it in the sunbeam. The result was a spectacular multicolored band of light just like a rainbow. The multicolored band of light is called a color spectrum.

2. Newton believed that all the colors he saw were in the sunlight shining into his room. To test this, he placed another prism upside-down in front of the first prism. He was right. The band of colors combined again into white sunlight. He was the first to show that white light is made up of the colors that we see. Newton also felt that light rays were made up of particles, but nearly 150 years later Fresnel showed that light waves demonstrated diffraction, a property of waves.

3. A. J. Fresnel demonstrated mathematically that the pattern of alternating light and dark lines (diffraction pattern) produced when light travels through an aperture, or around an obstruction, would only occur if light moved as waves. Nearly 20 years later, in the early 1830’s Michael Faraday demonstrated that a magnetic field could cause an electric current to flow in a wire. This is known as magnetic induction.

4. A little more than 30 years after that James Clerk Maxwell developed a set of equations that confirmed Faraday’s idea that electricity and magnetism are simply two parts of a single phenomenon, electromagnetism. Maxwell showed that this phenomenon would produce waves which travel at the speed of light. He also suspected that there were light waves other than those that produced the light that we could see. We now refer to this collection of different waves of electromagnetic radiation (light) as the electromagnetic spectrum (EMS) Before we look at the EMS, let’s talk about waves.

5. There are two main types of waves, transverse, and longitudinal. Types of waves Light moves through space as two interacting transverse wave disturbances, one caused by the electrical field and the other by the magnetic. Parts of a wave you should be familiar with: wave Another important feature of wave motion is the inverse relationship between wavelength and frequency. That is, as one increases the other decreases. This relationship is expressed in the following equation: = c (the speed of light) λ (wavelength) (frequency) ν What about units, you ask? Well…

6. the speed of light has a constant value of 3.00 x 108 m/s Wavelength is in meters. Frequency is in: cycles/s; or 1/s; These are also known as 1 hertz (Hz) or s-1. Sample problem 1: Calculate the frequency of light with a wavelength of 5.22 x 10-10 m. c = λν Manipulating the variables to solve for ν gives, ν = c/λ / / = 3.00 x 108 m/s = 5.22 x 10-10 m / 5.75 x 1017/s = 5.75 x 1017 Hz Sample problem 2 (You try): Calculate the wavelength of a radio station signal with a frequency of 99.7 MHz. c = λν = / λ c / / ν = 3.00 x 108 m/s / 99.7 x 106/s 3.01 m =

7. OK, now let’s discuss the Electromagnetic Spectrum. microwaves Radio waves The spectrum goes from radio waves (long λ) on the right The visible part of the spectrum goes from red (7 x 10-7m) to violet (4 x 10-7m), to gamma rays (short λ) on the left. , in this sequence v increases. ROYGBIV

8. EMS Review Which would have a longer wavelength, microwaves or ultraviolet waves? Which would have a higher frequency, orange light or green light? Which would have more energy, infrared rays or gamma rays? Electromagnetic radiation song

9. At the end of the 19th century, there were two things about which science was certain: Matter was particles; everything was messed up, by a German scientist named and light was waves. Then, in 1900, Max Planck. Planck was trying to determine how the color of light radiated by a body was related to its temperature. Two separate mathematical explanations already existed. One failed to work for light at high frequencies, the other at low frequencies. Planck demonstrated that the problem could be solved by treating light as being given off in discrete units – he called them quanta - rather than being given off continuously, as previously assumed.

10. Planck found that light energy was proportional to its frequency. The relationship is given by: E = h ν E is energy. h is Planck’s constant, 6.63 x 10-34 J . s If light is only emitted or absorbed in a discrete quantum of energy, If it’s not continuous, then what is it? A particle? Evidence to support Planck’s theory came in 1905 when a Swiss patent clerk explained the photo-electric effect. When UV light is shined on a piece of metal connected to a circuit, electrons are ejected into the circuit and a current is produced. What no one had yet explained was why the intensity of light had no effect on the energy of the electrons.

11. The patent clerk proposed that if, like Planck said, light consisted of discrete quanta - photons - they would interact with the electrons like particles. He even showed the electrons had energy related to hv !! Who was this clerk? Albert Einstein Sample Problem #1 Calculate the energy of a photon of light with a frequency of 5.45 x 1014 Hz. E = hv (6.63 x 10-34 J . s) = (5.45 x 1014/s) = / / 3.61 x 10-19 J