9.2 Penetrating ability  (Page 3/3)

Cellphone users are recommended to minimise their exposure to the radiation, by for example:

1. Use hands-free to decrease the radiation to the head.
2. Keep the mobile phone away from the body.
3. Do not telephone in a car without an external antenna.

Exercise - penetrating ability of em radiation

1. Indicate the penetrating ability of the different kinds of EM radiation and relate it to energy of the radiation.
   
2. Describe the dangers of gamma rays, X-rays and the damaging effect of ultra-violet radiation on skin.
   

Summary

1. Electromagnetic radiation has both a wave and a particle nature.
2. Electromagnetic waves travel at a speed of $3\times {10}^{8}m·s^{-1}$ in a vacuum.
3. The Electromagnetic spectrum consists of the follwing types of radiation: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma-rays.
4. Gamma-rays have the most energy and are the most penetrating, while radio waves have the lowest energy and are the least penetrating.

Indigenous knowledge systems: animal behaviour

People have believed that animals can predict earthquakes and other natural disasters for centuries. As early as 373 B.C., historians recorded a massive exodus of animals, including rats, snakes and weasels, from the Greek city of Helice days before a quake struck causing massive devastation.

This topic is much debated and there are people that claim different behaviours for different kinds of animals, for example:

• Dogs and cats : are believed by some to howl or bite their owners before natural disasters, they cite factors like a much stronger sense of smell.
• Sharks : have been reported to move to deeper water before hurricanes, possibly because a sensitivity to changes in the air pressure preceding the hurricane.
• Elephants : will allegedly trumpet and flee to higher ground before a tsunami arrived.

Others argue that many animals detect certain natural signals, such as the early tremblings of an earthquake, long before humans. This means they have opportunity to react before we can but argue that they exhibit no special understanding, they just flee as would any person hearing a shout of fire.

Another problem cited with these seemingly clairvoyant animals is that their psychic powers often are based on behaviors that people only recall after the event. Some animal behaviors happen frequently, but are not remembered unless an earthquake, tsunami, or mud slide follows. For example, if you see a dog cross a road, you just remember you saw a dog cross the road. But if an earthquake shook your neighborhood five minutes later, would you say the dog was fleeing?

Project: indigenous knowledge: animals and natural disasters.

Carry out research on the behavior of animals before natural disasters.

Pick one type of natural disaster (earthquake, flood, tsunami, etc.) and see what you can find about animals reacting to that type of disaster. Ask people you know about what they have heard to get a sense of folklore.

Then research the topic to find more information and remember to critically assess all information. Things to consider:

• What scientific research has been conducted?
• Which countries does that type of disaster usually occur in?
• Do any of the native people of that country have legends/ideas about animals reacting to the disaster?
• What do people believe leads to this behavior? i.e. do the animals have some mystic ability or are they more sensitive to anything then we are (such as low frequency radiation)

Present your findings to your class. Critically analyze all the information you collect and decide what you believe.

End of chapter exercise

1. What is the energy of a photon of EM radiation with a frequency of $3\times {10}^8$  Hz?
   
2. What is the energy of a photon of light with a wavelength of 660 nm?
   
3. List the main types of electromagnetic radiation in order of increasing wavelength.
   
4. List the main uses of:
   1. radio waves
   2. infrared
   3. gamma rays
   4. X-rays
5. Explain why we need to protect ourselves from ultraviolet radiation from the Sun.
   
6. List some advantages and disadvantages of using X-rays.
   
7. What precautions should we take when using cell phones?
   
8. Write a short essay on a type of electromagnetic waves. You should look at uses, advantages and disadvantages of your chosen radiation.
   
9. Explain why some types of electromagnetic radiation are more penetrating than others.