04+EM+SPECTRUM+AND+INVISIBLE+LIGHT

**Syllabus**

** 2.1 Demonstrate an understanding of how Herschel and Ritter contributed to the discovery of waves outside the limits of the visible spectrum. ** ** 2.2 Demonstrate an understanding that all electromagnetic waves are transverse and that they travel at the same speed in a vacuum. ** ** 2.3 Describe the continuous electromagnetic spectrum including (in order) radio waves, microwaves, infrared, visible (including colours of the visible spectrum), ultraviolet, X-rays and gamma rays. ** ** 2.4 Demonstrate an understanding that the electromagnetic spectrum is continuous from radio waves to gamma rays, but the radiations within it can be grouped in order of decreasing wavelength and increasing frequency. **

=** 'Writing' by Richard Gladdis ** = = = =** 'Boldifying' By Lotte Cordon ** = = = = = =** Part 1 ** = =** Ritter's Discovery of UV ** = = = ** In 1801, he was experimenting with silver chloride, a chemical which turned black when exposed to sunlight. He had heard that exposure to blue light caused a greater reaction in silver chloride than exposure to red light. Ritter decided to measure the rate at which silver chloride reacted when exposed to the different colors of light. To do this, he directed sunlight through a glass prism to create a spectrum. He then placed silver chloride in each color of the spectrum. Ritter noticed that the silver chloride showed little change in the red part of the spectrum, but increasingly darkened toward the violet end of the spectrum. This proved that exposure to blue light did cause silver chloride to turn black much more efficiently than exposure to red light. **

** Johann Ritter then decided to place silver chloride in the area just beyond the violet end of the spectrum, in a region where no sunlight was visible. To his amazement, he saw that the silver chloride displayed an intense reaction well beyond the violet end of the spectrum, where no visible light could be seen. This showed for the first time that an invisible form of light existed beyond the violet end of the spectrum. This new type of light, which Ritter called Chemical Rays, later became known as ultraviolet light or ultra﻿violet radiation. Ritter's experiment, along with Herschel's discovery, proved that invisible forms of light existed beyond both ends of the visible spectrum. **



=** Herschel﻿﻿﻿'s discovery of IR ** = = = ** In 1800, he directed sunlight through a glass prism to create a spectrum and then measured the temperature of each color. Herschel used three thermometers with blackened bulbs and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as control samples. As he measured the individual temperatures of the violet, blue, green, yellow, orange, and red light, he noticed that all of the colors had temperatures higher than the controls. Moreover, he found that the temperatures of the colors increased from the violet to the red part of the spectrum. After noticing this pattern Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region where no sunlight was visible. To his surprise, he found that this region had the highest temperature of all. **

= = ** Herschel performed additional experiments on what he called "calorific rays" beyond the red portion of the spectrum. He found that they were reflected, refracted, absorbed and transmitted in a manner similar to visible light. What Herschel had discovered was a form of light beyond red light, now known as infrared radiation. Herschel's experiment was important because it marked the first time that someone demonstrated that there were types of light that we cannot see with our eyes. **



= = = = = = =<span style="font-family: 'Comic Sans MS',cursive;">** Part 2 ** =

=﻿<span style="font-family: 'Comic Sans MS',cursive;">** ﻿﻿Transverse Waves ** =

<span style="font-family: 'Comic Sans MS',cursive;">** A transverse wave is a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer. If a transverse wave is moving in the positive //x//-direction, its oscillations are in up and down directions that lie in the //y–z// plane. ** <span style="font-family: 'Comic Sans MS',cursive;">**﻿** <span style="font-family: 'Comic Sans MS',cursive;">** If you anchor one end of a ribbon or string and hold the other end in your hand, you can create transverse waves by moving your hand up and down. Notice though, that you can also launch waves by moving your hand side-to-side. This is an important point. There are two independent directions in which wave motion can occur. In this case, these are the //y// and //z// directions mentioned above. Further, if you carefully move your hand in a clockwise circle, you will launch waves that describe a left-handed helix as they propagate away. Similarly, if you move your hand in a counter-clockwise circle, a right-handed helix will form. These phenomena of //simultaneous// motion in two directions go beyond the kinds of waves you can create on the surface of water; in general a wave on a string can be //two-dimensional//. Two-dimensional transverse waves exhibit a phenomenon called polarization. A wave produced by moving your hand in a line, up and down for instance, is a linearly polarized wave, a special case. A wave produced by moving your hand in a circle is a circularly polarized wave, another special case. If your motion is not strictly in a line or a circle your hand will describe an ellipse and the wave will be elliptically polarized. **

<span style="font-family: 'Comic Sans MS',cursive;"> <span style="font-family: 'Comic Sans MS',cursive;">** Part 2 **

<span style="font-family: 'Comic Sans MS',cursive;"> <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;">** EM waves are typically described by any of the following three physical properties: the frequency //f//, wavelength λ, or photon energy //E//. Frequencies range from 2.4×1023 Hz (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to the wave frequency, so gamma rays have very short wavelengths that are fractions of the size of atoms, whereas wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma rays have the highest energy (around a billion electron volts) and radio waves have very low energy (around femto electron volts). These relations are illustrated by the following equations: ** <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;"> <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;">** where: ** <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;"> <span style="color: #e86d17; font-family: 'Comic Sans MS',cursive; font-size: 120%;">**Radio Waves** <span style="color: #afe01a; font-family: "Comic Sans MS",cursive;">
 * <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;">** //c// = 299,792,458 m/s is the speed of light in vacuum. **
 * <span style="color: #afe01a; font-family: 'Comic Sans MS',cursive;">** //h// = 6.62606896(33)×10−34 J s = 4.13566733(10)×10−15 eV s is Planck's constant. **
 * <span style="color: #f15913; font-family: 'Comic Sans MS',cursive;">Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight. ****﻿﻿**