Teacher Background Information on the EM Spectrum
The electromagnetic spectrum is a way of organizing the different types of EM radiation we find in the universe. It is a continuous spectrum, meaning where one part finishes, another starts with no gaps. The parts of the EM spectrum all have one thing in common: they all travel at the same speed in a vacuum. All the waves travel at 3 x 108m/s or 300,000,000 m/s. The different parts all have different uses and dangers which are related to their wavelength and frequency. As the frequency increases (and the wavelength decreases) the energy of the waves increase. The frequency and wavelength of the waves are related using v=fλ, where v is the velocity measured in m/s, f is the frequency measure in Hertz, and λ is the wavelength measured in m.
Radio Waves have the longest wavelength, the lowest frequency, and the lowest energy. Radio waves are used for broadcasting and communication. There are no real dangers associated with radio waves as their energy is so low. Heinrich Hertz discovered radio waves and used them to show that James Clerk Maxwell’s theory of electromagnetism was correct.
Microwaves have the second longest wavelength. They are most commonly known for their use in microwave ovens. Microwave ovens work by making the water molecules in your food vibrate. The microwave oven was invented by Percy Spence and the first commercial microwave oven was made in 1947. The original microwave ovens cost $5,000! Microwaves are also used in mobile phones allowing us to communicate with people all around the world. Microwaves can be dangerous. If you were to put a person in a microwave oven, it would heat the water molecules inside the body and cook the internal tissues.
Infrared is what we, as humans, feel as warmth or heat. It is used in thermal imaging. The police use it at night to find people who are hiding. Even though the fugitive isn’t reflecting any visible light, they will still give off infrared. Infrared is also used in optical fiber communication. Optical fiber communication is when information is send down long optical cables using infrared by bouncing the signal of the sides of the fiber. Too much infrared can cause burns to your skin.
Visible Light is the narrow range of EM radiation that we can see. White light is made up 7 colors: red, orange, yellow, green, blue, indigo, violet. We can use a prism to split light into its constituent colors because the different wavelengths refract different amounts, splitting light like this is called dispersion. A good way to remember this is to think of the letters making a name - Roy G. Biv. For most of human history, visible light was the only type of radiation we thought existed before Herschel’s discovery of infrared. Visible light has a range of uses, including optical telescopy and optical microscopy. There are no real dangers associated with it, but too much visible light can damage the eye.
Ultraviolet was discovered by Johann Ritter a year after the discovery of infrared. He carried out an experiment where he measured how quickly silver chloride turned from white to black when exposed to different colors of light. Ritter found that the silver chloride turned black the fastest just passed the violet in the invisible part of the spectrum. We now know this as UV radiation. UV is the radiation that causes skin to tan and is used in tanning beds. It is also used to check for counterfeit bills: using a special type of ink that absorbs UV and emits visible light patterns can be printed on the bills that appear to glow when put under a UV lamp. Cashiers can check bills to see if they have these markings. UV can also be used to sterilize drinking water as it can kill harmful microorganisms by disrupting their DNA. High exposure to UV can also lead to an increased risk of skin cancer, which is why physicians recommend that we wear sunscreen on sunny days.
X-rays were discovered by Wilhelm Röntgen and were originally called X-rays because they were of an unknown origin, just as in math the unknown variable is often called ‘x’. They are commonly used for imaging in medicine and security. X-rays are very penetrating which means they are difficult to stop; this allows them to be used for creating images of bones. They are only absorbed by dense material like bone and metal and can easily travel through soft tissue. They are also used in airport security, allowing security officers to check the inside of a passenger's luggage without having to open the suitcase. X-rays are ionizing and they can cause cancer. This is why radiographers in hospitals will stand behind a screen or leave the room when they take an X-rays.
Gamma rays are the highest energy EM waves. They have the shortest wavelength and the highest frequency. They are extremely ionizing and penetrating. They can be very hazardous to human health and are linked to the development of cancer. As well as being a cause of cancer they can also be used to treat cancer. This may come across as counter-intuitive: "How can a type of radiation cause and treat cancer?" The answer to this, in short, is that gamma rays damage cells, when this happens to healthy cells this can cause them to change and it could cause cancer. When cancerous cells are exposed to gamma radiation, they are also damaged and can be killed. This can help stop the spread of cancer. Gamma rays were discovered by French scientist Paul Villard at the start of the 20th century when he was experimenting with radon gas.
Essential Questions for the Electromagnetic Spectrum
- What are the uses and dangers of the different parts of the electromagnetic spectrum?
- What are electromagnetic waves made of?
- How are frequency and wavelength related to danger?
Additional EM Spectrum Lesson Plan Ideas
- Have your students make storyboard to show what the world would be like without microwaves.
- Get your students make a safety poster for medical professionals working in a hospital. How can they keep themselves safe from some of the harmful EM waves that are used in hospitals?
- Have students create their own mnemonic devices for remembering the order to the EM spectrum (or the colors of light) and make a visualization to help them remember it.