Option G: Electromagnetic waves

Question 1

Outline the nature of electromagnetic (EM) waves. (6)

Answer 1

• Involves the oscillation of electric and magnetic fields
• An oscillating electric charge produces varying electric and magnetic fields
• Changing magnetic and electric fields propagate through space
• Transverse waves
• All have the same speed in a vacuum
• The speed of an electromagnetic wave is independent of the velocity of the source

Question 2

Health hazards associated with transmission lines (3)

Answer 2

• electrical power lines on pylons are not insulated along their length and are thus extremely dangerous if they become unattached from the pylon
• some statistical evidence exists that suggests that there are regions near some power lines where more children are diagnosed with leukaemia than would be expected if the causes were random
• since electrical power lines carry alternating currents that produce changing extra-low-frequency electromagnetic fields, they are able to induce currents within any conductor, including nearby humans

Question 3

What are risks associated with transmission lines dependent on? (3)

Answer 3

• current (density)
• A.C. frequency
• length of exposure

Question 4

Name the different regions of the electromagnetic spectrum. (7)

Answer 4

• Gamma (γ) rays
• X-rays
• Ultraviolet
• Visible light
• Infrared
• Microwaves
• Radio waves

Question 5

Wavelength of gamma waves

Answer 5

10^-10 m and smaller

Question 6

Wavelength of X-rays

Answer 6

10^-10 m – 10^-9 m

Question 7

Wavelength of ultraviolet waves

Answer 7

10^-9m – 10^-7 m

Question 8

Wavelength of visible light

Answer 8

10^-7 m – 10^-6 m

Question 9

Wavelength of infrared waves

Answer 9

10^-6 m – 10^-3 m

Question 10

Wavelength of microwaves

Answer 10

10^-3 m – 10^-1 m

Question 11

Wavlength of radio waves

Answer 11

10^{<span>-1</span>} m – 10^{<span>5</span>} m

Question 12

Frequency of gamma waves

Answer 12

3 x 10¹⁹ and higher

Question 13

Frequency of x-rays

Answer 13

3 x 10¹⁷ - 3 x 10¹⁹

Question 14

Frequency of ultraviolet waves

Answer 14

7.5 x 10¹⁴ - 3 x 10¹⁷

Question 15

Frequency of visible light

Answer 15

4.3 x 10¹⁴ - 7.5 x 10¹⁴

Question 16

Frequency of infrared waves

Answer 16

3 x 10¹² - 4.3 x 10¹⁴

Question 17

Frequency of microwaves

Answer 17

3 x 10⁹ - 3 x 10¹²

Question 18

Frequency of radio waves

Answer 18

3 x 10⁹ and lower

Question 19

Source of gamma waves

Answer 19

Radium

Question 20

Source of x-rays

Answer 20

X-ray machine

Question 21

Sources of ultraviolet waves

Answer 21

Sun

Question 22

Sources of visible light

Answer 22

Light bulb

Question 23

Source of infrared waves

Answer 23

Electric heater

Question 24

Source of microwaves

Answer 24

Microwave ovens

Question 25

Source of radio waves

Answer 25

TV/radio aerials

Question 26

Describe what is meant by the dispersion of EM waves.

Answer 26

EM waves refract in transparent media at different angles depending on their wavelengths which separates them. eg dispersion of light in a rainbow.

Question 27

Describe the dispersion of EM waves in terms of the dependence of refractive index on wavelength.

Answer 27

Dispersion occurs because the refractive indexes are slightly different for each of the different frequencies.

Question 28

Define: transmission

Answer 28

Energy that travels through a medium. The direction of energy transfer may be affected by refraction taking place when the energy entered the medium, but energy will be transmitted in straight lines.

Question 29

Define: absorption

Answer 29

Medium may absorb some of the energy, causing its temperature to increase and decreasing the energy transmitted. The medium may reemit some of this absorbed energy.

Question 30

Define: scattering

Answer 30

Radiation can be scattered by the medium.

Question 31

Example of scattering using the Earth’s atmosphere

Answer 31

Blue light is scattered in all directions as a result of interaction with small dust particles in the atmosphere. This is the reason that the sky appears blue

Question 32

Example of transmission using the Earth’s atmosphere

Answer 32

The transmitted light will not contain the same amount of blue. The grazing incidence to the atmosphere at sunset and sunrise means that light from the Sun travels a greater length through the atmosphere and sunsets or sunrises appear to be red.

Question 33

Example of absorption using the Earth’s atmosphere (2)

Answer 33

• Harmful UV radiation is absorbed by the ozone layer in the atmosphere, which would otherwise be harmful to creatures living on the surface of the Earth, including humans
• The atmosphere absorbs strongly in the infrared radiation region. This process is involved in the greenhouse effect. Increasing the carbon dioxide content of the atmosphere will increase the absorption and result in global warming

Question 34

Define: monochromatic

Answer 34

Laser light is monochromatic. It contains only a very very narrow band of frequencies.

Question 35

Define: coherent

Answer 35

Laser light is coherent. The oscillations that make up the waves are linked together. Light is always emitted in photons. Each photon in laser light is in phase with all the other photons that are emitted.

Question 36

Outline the mechanism for the production of laser light. (4)

Answer 36

1. Light photons are produced when an atomic electron falls from a higher energy level down to a lower energy level
2. Normally electrons will always occupy the lowest available energy levels in an atom
3. The production of laser light involves a process that promotes a large number of electrons to a higher energy level - this is known as population inversion
4. These electrons are stimulated to fall down and emit light of a particular frequency

Question 37

Outline applications of the uses of lasers. (6)

Answer 37

• medical applications - destroying tissue in small areas, attaching the retina, corneal correction
• communications
• technology - bar code scanners, laser disks
• industry - surveying, welding and machining metals, drilling tiny holes in metals
• production of CDs
• reading and writing CDs, DVDs, etc.

Question 38

Define: principal axis

Answer 38

The line going directly through the middle of the lens

Question 39

Define: focal point

Answer 39

The point on the principal axis to which rays that were parallel to the principal axis are brought to focus after passing through the lens. A lens will thus have a focal point on each side.

Question 40

Define: linear magnification

Answer 40

The ratio between the size of the image and the size of the object.

Question 41

Defining equation for the power of a convex lens

Answer 41

P=1/f

Where:

P is the power of the lens in dioptres

f is the focal length in m

Question 42

Defining equation for linear magnification

Answer 42

m=h_i/h_o

Where:

m is linear magnification (no unit)

h_i is image size in m

h_o is object size in m

Question 43

Define: real image

Answer 43

The image formed where the light rays are focussed.

Question 44

Define: virtual image

Answer 44

One from which the light rays appear to come but don’t actually come from that image like in a mirror.

Question 45

Thin lens formula

Answer 45

1/u + 1/v = 1/f

Where:

u is the distance between the object and the lens

v is the distance between the image and the lens

f is the focal length

Question 46

Define: far point

Answer 46

The distance between the eye and the furthest object that can be brought into focus (taken to be infinity)

Question 47

Define: near point

Answer 47

The distance between the eye and the nearest object that can be brought into clear focus (taken to be 25 cm)

Question 48

Define: angular magnification

Answer 48

The ratio between the angle that an object subtends normally and the angle that its image subtends as a result of the optical instrument.

Question 49

Define: objective lens

Answer 49

First lens. Forms a real magnified image of the object being viewed.

Question 50

Define: eyepiece lens

Answer 50

Second lens. Acts as a magnifying lens. The rays from this real image travel into the eyepiece lens and they form a virtual magnified image.

Question 51

Define: spherical aberration

Answer 51

Term used to describe the fact that rays striking the outer regions of a spherical lens will be brought to a slightly different focus point from those striking the inner regions of the same lens. In general, a point object will focus into a small circle of light, rather than a perfect point.

Question 52

Define: chromatic aberration

Answer 52

Term used to describe the fact that rays of different colours will be brought to a slightly different focus point by the same lens. The refractive index of the material used to make the lens is different for different frequencies of light.

Question 53

Describe how spherical aberration in a lens may be reduced. (2)

Answer 53

• The shape of the lens could be altered in such a way as to correct for the effect. The lens would no longer be spherical. A particular shape only works for objects at a particular distance away
• Decreasing the aperture. The technical term for this is stopping down the aperture. The disadvantage is that the total amount of light is reduced and the effects of diffraction would be made worse

Question 54

Describe how chromatic aberration in a lens may be reduced.

Answer 54

The effect can be eliminated for two given colours and reduced for all by using two different materials to make up a compound lens. This compound lens is called an achromatic doublet. The two types of glass produce equal but opposite dispersion.

Question 55

When are two sources coherent? (2)

Answer 55

• they have the same frequency
• there is a constant phase relationship between the two sources

Question 56

Describe the effect on the double-slit intensity distribution of increasing the number of slits. (3)

Answer 56

• the principal maxima maintain the same separation
• the principal maxima become much sharper
• the overall amount of light being let through is increased, so the pattern increases in intensity

Question 57

Outline the use of a diffraction grating to measure wavelengths.

Answer 57

The accurate experimental measurement of the different wavelengths of light contained in a given spectrum. If white light is incident on a diffraction grating, the angle at which constructive interference takes place depends on the wavelength. Different wavelengths can thus be observed at different angles. The accurate measurement of the angle provides the experimenter with an accurate measurement of the exact wavelength (and thus frequency) of the colour of light that is being considered. THe apparatus that is used to achieve this accurate measurement is called a spectrometer.