light and EM spectrum

Question 1

EM waves are

Answer 1

• transverse
• travel at the same speed in a vacuum

Question 2

explain that all electromagnetic waves transfer
energy from source to observer

Answer 2

• EM waves transfer energy from a source to an absorber
• higher frequency = more energy it transfers = more dangerous for humans
• ex - when you warm yourself by an electrical heater, infrared waves transfer energy from the thermal energy store of the heater (the source) to you thermal energy store (the absorber)

Question 3

Core Practical: Investigate refraction in rectangular glass blocks
in terms of the interaction of electromagnetic waves with matter

Answer 3

• need to do in dim room
• thin ray of light to easily see middle of ray to trace and measure the angles
• use a ray box containing a light bulb , thin slit cut in one side to allow thin ray of light out of box
• light is refracted at the boundary between air and glass, use this experiment to investigate how much light is refracted when it passes through a glass block
  1. rectangular glass block on paper and trace
  2. use ray box to shine ray of light at the middle of one side of the block
  3. trace incidence and emergent rayon other side of block, remove block, using straight line join up incident and emergent ray = shows path of refracted ray through block
  4. draw normal at point where light ray entered block, use protractor to measure the angle between incident and normal (angle of incidence) and angle between refracted ray and normal (angle of refraction)
  5. do same for point where ray emerges from the block
  6. repeat 3 times, keeping angle of incidence as the ray enters the block the same
  7. calculate average for each of the angles

Question 4

extra practical details

Answer 4

• should see light bends towards normal as it enters block (angle of refraction is less than angle of incidence)
• because air has one of the lowest optical densities that there is so light ray will almost always slow down when it enters the block
• should then see ray of light bends away from normal as it leaves the block because light ray speed up as it leaves the block and travels through air
• all EM waves can be refracted but this experiment uses visible light so you can see ray being refracted as it travels through the block

Question 4

Recall the main groupings of the continuous electromagnetic
spectrum including (in order)

Answer 4

radio waves, microwaves, infra red, visible light, ultraviolet, x-rays, gamma rays

Question 5

Describe the electromagnetic spectrum

Answer 5

• continuous from radio waves to gamma rays
• radiations within it can be grouped in order of decreasing wavelength and increasing frequency

Question 6

what can our eyes detect

Answer 6

a limited range of frequencies of electromagnetic radiation

Question 7

recall what means by waves being absorbed at boundaries

Answer 7

wave is absorbed by second material, wave transfers energy to the material’s energy store
often the energy is transferred to a thermal energy store - heating

Question 8

recall what means by a wave being transmitted at a boundary

Answer 8

wave is transmitted through second material - wave carries on travelling through new material - often leads to refraction
can be used in communications as well as in lenses of glasses and cameras

Question 9

recall what means a wave is reflected at a boundary

Answer 9

the incoming ray isn’t absorbed or transmitted but is sent back away from the second material
how echoes are created

Question 10

Recall that different substances may absorb, transmit,
refract or reflect electromagnetic waves in ways that
vary with wavelength

Answer 10

(whether an EM wave is absorbed, transmitted, refracted or reflected when meeting a boundary depends on the materials at the boundary and the wavelength of the EM wave ex some materials absorb some wavelengths of light but reflect others this is what causes things to be a certain colour

• radio waves = transmitted through body without being absorbed
• some wavelength =s of microwaves can be absorbed
• IR and visible light are mostly absorbed or reflected by the skin
• UV absorbed by skin
• x-rays and gamma = transmitted through body tissues with little absorption

Question 11

Explain the effects of differences in the velocities of
electromagnetic waves in different substances

Answer 11

• EM waves slow down when encountering certain materials which causes them to refract
• even though EM waves travel at the same speed in a vacuum, how much they slow down depends on the material
• this slowing of EM waves is caused by differenced in the velocity of waves travelling through different substances

Question 12

how does the danger of EM waves depend on frequency

Answer 12

• higher frequency of the EM waves the more energy it transfers (and so more dangerous for humans)

Question 13

Describe the harmful effects on people of excessive exposure to
electromagnetic radiation, including:

Answer 13

• microwaves = internal heating of body cells
• infrared = skin burns
• UV = damage to surface cells and eyes leading to skin cancer and eye conditions
• x-rays and gamma rays = mutation or damage to cells in the body

Question 14

describe some uses of radio waves

Answer 14

• send signals as can bend around world = long wave radio, short wave radio = reflected by earth’s atmosphere
• satellite transmissions = high frequency radio waves transmitted into space picked up by satellite dish
  -(broadcasting, communications, satellite transmissions)

Question 15

describe how microwave ovens work

Answer 15

• microwave ovens = microwaves penetrate up to a few cm into food
• where they are absorbed into water molecules, then transfer their energy into them this causes water to heat up
• the heat from water is transferred to the molecules in food

Question 16

describe how microwaves are used for communications

Answer 16

• communications to and from satellites=
• satellite TV signal from transmitter is transmitted into space and picked up by satellite receiver dish
• satellite transmits signal back to earth in a different direction where its received by a satellite dish on ground

Question 17

describe how infrared is used for cooking

Answer 17

• absorbing IR radiation causes objects to get hotter, the temperature of food increases when it absorbs IR radiation

Question 18

describe how infrared is used- thermal imaging

Answer 18

• infrared cameras = detects IR and monitors temperature
• camera detects IR and turns it to electrical signals which is displayed on screen = thermal imaging
• thermal imaging is used for police to see if suspects are trying to hide in the dark

Question 19

describe how IR is used for infrared sensors

Answer 19

• infrared sensors = a change is detected and alarm sounds or security light turns on

Question 20

describe how IR is used for short range communications and optical fibres

Answer 20

• transfer information, small distances and receiver must be in line of sight of emitter for sending files and for tv remotes
• optical fibre ( thin glass or plastic fibre that carries data over long distances) = send singles wavelength to prevent dispersion
• use total internal reflection to send data over long distances

Question 21

describe how visible light is used

Answer 21

• photography = reacts to light to form and image - traditional cameras, digital cameras = image sensors which detect visible light and generate electrical signal which is converted to and image that can be stored digitally or printed
• vision = light we see
• illumination = use visible light to illuminate things so we can see them

Question 22

describe how ultraviolet is used

Answer 22

• fluorescent lamps = UV is absorbed and visible light is emitted
• security pens = under UV light ink glows but otherwise invisible - help identify stolen property
• bank notes and passports = use similar technique to detect forgeries
• sterilise water = kills bacteria

Question 23

describe how x-rays are used

Answer 23

• internal structure = affect photographic film in same way as light to take x-ray photos
• medical x-rays = radiographers take x-rays, they are transmitted by flesh but absorbed by denser material like bones or metal - x-ray is directed through body or object onto a detector plate, brighter bits = where fewer x-rays get through producing a negative image (plate starts off white)
• airport security scanners = detect hidden objects

Question 24

describe how gamma rays are used

Answer 24

• sterilise food and medical equipment = kills microbes
• detection of cancer = medical imaging such as tracers use gamma rays to detect cancer
• cancer treatments = radiation targeted at cancer cells and kills them
• doctors have to be careful to minimise danger to healthy cells when they treat cancer like this

Question 24

recall that changes in atoms and nuclei can

Answer 24

• generate radiations over a wide frequency range
• be caused by absorption of a range of radiations

Question 24

recall that radio waves can

Answer 24

produce or induce oscillations in electrical circuits

Question 25

long wavelength =

Answer 25

low frequency

Question 26

short wavelength =

Answer 26

high frequency