Chapter 5 - EM Waves - EM Spectrum

Question 1

How do you work out optical power?

Answer 1

P = 1 / f
optical power (in dioptre - D) = 1 / focal length (in metres - m)

Question 2

What is the order of the EM spectrum (lowest frequency to highest)

Answer 2

~ radio waves
~ microwaves
~ infra-red waves
~ visible light rays
~ ultra-violet rays
~ x-rays
~ gamma rays

Question 3

What are some features of radio waves?

Answer 3

~ long wavelength
~ low energy
~ low frequency

Question 4

What are some features of gamma rays?

Answer 4

~ short wavelength
~ high energy
~ high frequency

Question 5

What is the speed of all EM waves?

Answer 5

Speed of light - 300,000,000m/s or 3 x 10^8

Question 6

What are some uses of radio waves?

Answer 6

~ communication (radio, TV)

Question 7

What are some uses of microwaves?

Answer 7

~ communication (satellite phones)

~ cooking (microwave ovens)

Question 8

What are some uses of infra-red rays?

Answer 8

~ cooking (grill or toaster)
~ remote control
~ fibre optics
~ security - sensors that can detect infra-red radiation emitted
~ night vision

Question 9

What are some uses of visible light rays?

Answer 9

~ seeing
~ photos
~ fibre optics

Question 10

What are some uses of ultra-violet rays?

Answer 10

~ security tags
~ sterilising dirty water in hot countries
~ fluorescence - absorb UV radiation and re-emit it as visible light
~ low energy light bulbs
~ detecting forged banknotes - markings do not glow
~ (tanning)

Question 11

What are some uses of x-rays?

Answer 11

~ airport security
~ imaging hard tissue - pass through muscles and fat easily but bone absorbs some x-rays
~ examine insides of metal objects

Question 12

What are some uses of gamma rays?

Answer 12

~ sterilising (food + surgical instruments)
~ radiotherapy - kills cancer cells
~ detects cancer

Question 13

How would you calculate wavelength?

Answer 13

Wavelength (m) = wavespeed (m/s) / frequency (Hz)

λ = v / f

Question 14

What can’t radio waves do?

Answer 14

Be sent into space - they get refracted (then TIR)

Question 15

What can’t infra-red do?

Answer 15

Get through glass

Question 16

How are radio waves made?

Answer 16

~ oscillations in electrical circuits can produce radio waves
~ oscillating current - electrons vibrate causing radio waves to spread out

Question 17

What are the dangers of radio waves?

Answer 17

~ very very low danger

Question 18

What are the dangers of microwaves?

Answer 18

~ (low intensity) possible risk of cancer from mobile phone

~ (high intensity) cook from inside - bodies mostly water - microwaves heat cells from inside

Question 19

What are the dangers of infra-red?

Answer 19

~ burns from an intense source

Question 20

What are the dangers of visible light?

Answer 20

~ damage eyes (retina) from intense source - blindness

Question 21

What are the dangers of ultra-violet?

Answer 21

~ sun burn

~ skin cancer

Question 22

What are the dangers of x-rays?

Answer 22

~ cancer (ionising radiation)

Question 23

What are the dangers of gamma rays?

Answer 23

~ cancer (ionising radiation)

~ radiation sickness (nuclear bombs)

Question 24

What is the most common EM injury?

Answer 24

Burns (from infra-red)