Electromagnetic spectrum p. 52

Electromagnetic spectrum

frequency wavelength

lowest longest

highest shortest

redorangeyellowgreen

blueindigoviolet

Visible light

Infrared

Microwaves

Radio waves

Gamma rays (𝛾 − 𝑟𝑎𝑦𝑠)

X-rays

Ultraviolet

𝑣 = 𝜆𝑓𝑐 = 𝜆𝑓Speed of light

(electromagnetic waves)3 x 108 m.s-1

Wavelength in m

Frequency in Hz

Wave equation

𝑐 = 𝜆𝑓

ExampleCalculate the frequency of an electromagnetic wave with a wavelength of 3,2 nm.

m = x 10-3

𝜇 = x 10-6

n = x 10-9

p = x 10-12

3 x 108 = 3,2 x 10-9 x 𝑓

𝑓= 9,38 x 1016 Hz

Uses of electromagnetic waves,p. 54

Radio waves:

• Radio and TV-broadcasts• Detecting information around the

world• Communication

Uses of electromagnetic waves,p. 54

Microwaves:

• Microwave ovens• Satellite communication• Radar / position control of aeroplanes

Uses of electromagnetic waves,p. 54

Infrared:

• Transfer of energy and heat• Navigation • Conventional ovens• Remote controls

Uses of electromagnetic waves,p. 54

Visible light:

• Production of food in plants (photosynthesis)

• Enables us to see things around us

Uses of electromagnetic waves,p. 54

Ultraviolet light:

• Detecting false banknotes• Lighting

Uses of electromagnetic waves,p. 54

X-rays:

• Medical imaging• Security control

Uses of electromagnetic waves,p. 54

Gamma rays:

• Kills cancer cells• Sterilisation of equipment • Increases shelf life of food

Penetrating ability of the different kinds of electromagnetic radiation

Visible light

UV light

X-rays

Gamma-rays

Increase in

En

ergy

Dangers

UV-rays: skin aging , DNA-change, skin cancer (UVB is more damaging than UVA)

X-rays: extended exposure can cause cancer and cell damage

Gamma rays: can cause serious damage to DNA, different types of cancers

A photon as a parcel of energy found in light.

Particle nature of electromagnetic radiation

Relationship between the energy of a photon and the frequency and wavelength of the light.

𝐸 = ℎ𝑓

Energy in joule (J)

Planck’s constant (6,63 x 10-34 J.s)

Frequency in Hz

𝑐 = 𝑓𝜆

i.e. 𝑓 = 𝑐

𝜆

Therefore 𝐸 = ℎ𝑓 = ℎ𝑐

𝜆

𝐸 = ℎ𝑓

ExampleCalculate the energy of photon witha frequency of 2,45 GHz.

k = x 103

𝑀 = x 106

G = x 109

T = x 1012

= 6,63 x 10-34 x 2,45 x 109

= 1,62 x 10-24 J

Homework

p. 59,nos. 6, 7, 11.4, 11.5, 15

6.1 Write down the wave equation for electromagnetic radiationin the approved symbols.

𝑐 = 𝑓𝜆

6.2 The frequency of an electromagnetic wave is 6 x 1016 Hz.What is the wavelength of the wave in air?

𝑐 = 𝜆𝑓

3 × 108 = 𝜆 × 6 × 1016

𝜆 = 5 × 10−9 m

6.3 The period of a electromagnetic wave is 4 x 10-14 s. Calculatethe wavelength of the wave in air.

𝑐 = λf

3 × 108 = λ x 2,5 x 1013

𝜆 = 1,2 × 10−5 m

𝑓 =1

𝑇

=1

4 × 10−14

= 2,5 × 1013 𝐻𝑧

7. Charges in the antenna of a radio transmitter oscillate with a frequency of 650 kHz, while another one has a frequency of120 MHz. Which electromagnetic wave has the highest frequency?

𝑅𝑎𝑑𝑖𝑜 1:650 𝑘𝐻𝑧

= 650 x 103 Hz= 650 000 Hz

𝑅𝑎𝑑𝑖𝑜 2:120 𝑀𝐻𝑧

= 120 x 106 Hz= 120 000 000 Hz

𝑅𝑎𝑑𝑖𝑜 2 has the highest frequency.

11.4 All electromagnetic waves move at the same speed of300 million m.s-1. Calculate the frequency of X-rays witha wavelength of 5 × 10−10 m.

𝑐 = 𝑓𝜆

3 × 108 = 𝑓 × 5 × 10−10

𝑓 = 6 × 1017 Hz

11.5 It takes about 8,5 seconds for light from the Sun to reach theEarth. Calculate the distance (in meter) between the Sun andthe Earth.

𝑠𝑝𝑒𝑒𝑑 =𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒

𝑡𝑖𝑚𝑒

3 × 108 =𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒

8,5

distance = 2,55 × 109 m

15. A certain radio programme is picked up at frequencies of 5 x 106 Hz in the short wave band and 100 MHz in the FM-band. Calculate the correspondingwavelengths.

𝑐 = 𝑓𝜆

3 × 108 = 5 × 106 × 𝜆

𝜆 = 60 𝑚

𝑐 = 𝑓𝜆

3 × 108 = 100 × 106 × 𝜆

𝜆 = 3 𝑚

Homework

p. 62,nos. 16, 17, 28, 29, 30, 31

16. Visible light ranges from red (λR = 670 nm) to violet (λV = 400 nm).Calculate the frequency of:

16.1 red light

16. Visible light ranges from red (λR = 670 nm) to violet (λV = 400 nm).Calculate the frequency of:

16.2 violet light

17.1 Calculate the frequency of a microwave with a wavelength of 5 cm.

17.2 An X-ray has a wavelength of 5 nm. What is the frequency of the X-ray?

28.1 Calculate the energy of a photon with a frequency of 3 x 108 Hz.

28.2 Calculate the energy of a photon with a frequency of 12 THz.

29.1 Calculate the energy of a photon with a wavelength of 660 nm.

29.2 Calculate the energy of a photon with a wavelength of 532 nm.

30.1 Calculate the wavelength of a photon with energy of 3,3 x 10-19 J.

30.2 Calculate the wavelength of a photon with energy of 5,4 x 10-19 J.

31.1 Calculate the frequency of a photon with energy of 1,33 x 10-17 J.

31.2 Calculate the frequency of a photon with energy of 9,93 x 10-10 J.