Chapter 11 Electromagnetic Waves

What is the order of the electromagnetic spectrum from longest wavelength to shortest wavelength?

RADIO WAVES → MICROWAVES → INFRARED RADIATION → VISIBLE LIGHT → ULTRAVIOLET RADIATION → X-RAYS →GAMMA RAYS

What are characteristics of electromagnetic waves

All electromagnetic waves can travel without a medium, with the same

speed of 3 × 10

8 m/s in a vacuum.

4. The wave speed equation, v = fλ, applies to all electromagnetic waves.

5. When an electromagnetic wave travels from a vacuum to other media,

the speed and wavelength decrease. However, the frequency stays the

same as at the source when the wave goes into a different medium. 6. Electromagnetic waves undergo reflections and refractions.

7. Electromagnetic waves transfer energy.

8. Ionising waves have higher frequencies and hence higher energies. They

can eject electrons from atoms and molecules to create ions. The ions can

cause harm by disrupting cell functions and even killing them.

9. All material bodies emit a range of electromagnetic waves. In general,

higher temperature bodies tend to emit more and higher frequency

electromagnetic waves.

What are the Harmful Effects of Over-exposure to Electromagnetic Radiation –Effects of Ionising Radiation

•Ionising radiation can be high frequency electromagnetic waves or high energy

particles.

•They come from outer space, natural radioactive materials in our environment

or man-made sources.

•High energy electromagnetic waves can penetrate our body more easily than

high energy particles.

What can gamma rays be used for?

1. Disinfection and Sterilisation

•Just like UV and X-rays, gamma rays can be used for disinfection and

sterilisation.

2. Quality Control in Manufacturing

•Gamma rays are used to scan manufactured goods for hidden defects and

cracks.

3. Radiotherapy

•Gamma rays can be used to kill cancer cells (radiotherapy).

•In a procedure known as the Gamma Knife radio surgery, gamma rays are

directed at brain tumours to kill the cancer cells

What can X-rays be used for?

1. X-Ray Imaging

•The amount of X-rays passing through different types of

tissues varies.

2. Computed Tomography (CT scan)

•A more advanced medical imaging technique

is called Computed Tomography.

•A rotating X-ray scanner collects data over a

thin slice of a patient’s body before moving to

the next adjacent slice.

3. Security Scanning

•At airports, X-rays are used to scan luggages for hazardous materials.

4. Quality Control in Manufacturing

•X-rays are used to scan manufactured goods for hidden defects and

cracks.

What are the use of ultraviolet waves?

1. Disinfection and Sterilisation

•In Singapore’s water treatment plants, UV light plays an important role in

disinfection and sterilisation by killing bacteria and viruses.

2. Vitamin D Production

•Exposure to a suitable dose of UV light from the sun enables our body to

produce vitamin D which is good for our bones, muscles and immune

system.

•In places where there is little sunlight, people also use sun-tanning beds to

get a more tanned skin tone.

3. Anti-Forgery Features

•Pigments that glow under UV light are commonly used in anti-forgery

features of printed notes

What are the use of visible light?

1. Photography

•A digital camera uses a sensor to record an image.

2. Telecommunication and Medicine

•Light can be reflected repeatedly within a thin optical glass fibre

3. Artificial Light for Plant Growth

•Scientific research is ongoing to find out the role of

differently coloured light in leaf growth, flowering and root

formation

4. Hardening or Curing Materials

• The soft material used by a dentist to fill up a treated tooth cavity needs to be

cured with light so that it can be hardened.

• Over the years, different light sources were used to trigger the polymerisation

process. Currently, blue light from a LED (light emitting diode) is used for the

job.

5. Induced Chemical Reaction

• Light can cause a chemical change in photographic films.

• The films are coated with selected chemicals that react to visible light of

different frequencies, allowing them to record an image formed by a lens.

• These films respond to ultraviolet, X-rays and gamma rays too.

what are the use of microwaves?

Cooking

•The frequency of the waves in microwave ovens is specially chosen to

enhance the transfer of energy to the water molecules as water is a major

component in food.

•The energy transfer leads to an increase in the internal energy and a

corresponding higher temperature.

2. Communication

•Besides radio waves, microwaves can also be used for satellite

communication and mobile phones.

•A mobile phone converts sound waves to microwave signals. The microwaves

are sent from the mobile phone to the cell tower. The receiver's mobile

phone receives the microwave signals and converts them to sound.

11.2 What Are the Uses and Dangers of

Electromagnetic Waves?

Uses of Electromagnetic Waves – Microwaves

3. Radar (Radio Detection and Ranging)

•Using the time taken for emitted microwaves

to be reflected by an object, the distance to

the object can be calculated.

4. Cancer Treatment

•Microwaves can be used to kill cancer cells.

5. Global Positioning System (GPS)

•Microwave signals from satellites are used in

GPS systems for working out the location of a

receiver device

What kind of electromagnetic waves does a electric heater emit?

Infrared waves