P13 - EM Waves + Black body radiation

Question 1

What is true about objects emitting and absorbing radiation

Answer 1

• All objects, no matter what temperature, emit and absorb infrared radiation
• However, a hotter object will emit more infrared radiation in a given time than a cooler object

Question 2

How does the temperature affect the emission of infrared radiation

Answer 2

• Both the wavelength and the intensity of radiation depend on the temperature of the object
• Hotter objects emit shorter wavelength radiation
• The intensity of the radiation emitted increases with temperature

Question 3

What is a perfect black body

Answer 3

• An object that absorbs all of the radiation incident on it so no radiation is reflected or transmitted
• It is also the best possible emitter of radiation as good absorbers are good emitters

Question 4

What is true regarding temperature and the rate of absorbtion or emissions of infrared radiation

Answer 4

• If an object is warmer than the surroundings, its temperature will decreases as it emits radiation at a faster rate than it absorbs it
• If an object is cooler than its suroundings, its temperature will increase as it absorbs radiation at a faster rate than it emits it
• If an object is at a constant temperature, it is absorbing radiation at the same rate it is emitting it

Question 5

How does infrared radation affect the temperature of the earth

Answer 5

• Through the greenhouse effect
• Sun emits short wave infrared radiation
• This is then absorbed by the earth which increases the temperature of the earth
• The eath then emits this radiation and some passes back into space but some is trapped by greenhouse gases and emitted back down, increasing the temperature of the lower atmosphere

Question 6

What are EM waves

Answer 6

Transverse waves which transfer energy from the source of the waves to an absorber
They therefore don’t need a medium to travel in
They all travel at the same speed in a vacuum which is 3x10⁸m/s
They form a continuous spectrum - there is no clear cutoff from one type of Em wave to another

Question 7

Electromagentic spectrum from low to high frequency

Answer 7

• Radiowaves
• Microwaves
• Infrared radiation
• Visible light
• Ultraviolet
• X-rays
• GAmma rays

Question 8

Describe what happens when a light wave travels through air and then into a glass object and then back into air

Answer 8

• The velocity of the light slows down so it changes direction
• When waves slow down, they bend towards the normal making the angle of refraction smaller than the angle of incidence
• The light waves then travel trhough the block
• When they pass from the glass to the air, their velocity increases and they waves bend away from the normal meaning the angle of refraction is larger than the angle of incidence
• This makes the image of the object behind the glass appear to have shifted position
• This is known as refraction

Question 9

What happens to a wave if it enters a medium at right angles to the surface

Answer 9

• It doesn’t change direction as it is along the normal

Question 10

What is a wavefront and what does it allow

Answer 10

• An imaginary point that connects all the same points in a set of waves
• This line could connect all the peaks of the set of waves for example
• Allows for better visualisation of lots of waves moving together

Question 11

Why do waves change direction when changing medium in terms of wavefronts e.g. liht from air to glass

Answer 11

• When the first wave fronts start to mov into the glass, these parts of the wavefront slow down
• This causes those parts of the wavefront to get closer together so their wavelength gets smaller
• This causes the waves to change direction towards the normal

Question 12

What happens when EM waves are generated or absorbed

Answer 12

• These events cause changes to take place in atoms or the nuclei of atoms or changes taking places cause the waves to be generated

Question 13

Hazards of Ultraviolet waves

Answer 13

• Increase the risk of skin cancer
• Cause the skin to age prematurely

Question 14

Hazards of X-rays and gamma rays

Answer 14

• They are ionising radiation
• This measn they can knock electrons off atoms when they are absorbed
• This can cause mutations in genes which increases the risk of cancer

Question 15

How can radiowaves be produced and what happens when they are absorbed

Answer 15

• Theey can be produced when electrons oscillate in electrical circuits due to an alternating current
• When these radiowaves are absorbed, they cause electrons in the circuit to oscillate
• This can create an alternating frequency with the same frequency as the radiowaves

Question 16

Uses of radiowaves and why

Answer 16

• Uses to transmit radio and TV signals recieved using an aerial
• They are used because they can travel long distances before being absorbed (long range in air)
• They can also spread out lots due to their long wavelength
• They can also reflect of a layer of charged particles allowing for very long distance sending of information

Question 17

Uses of microwaves and why

Answer 17

• Uses for heating food
• This is because most food contains lots of water moelcules
• The water water molecules thenabsorb the energy of microwaves and this increase in energy increases their temperature
• They’re also used to communicate with satellites in space
• This is because they can pass thrrough the Earth’s atmosphere without being reflected or refracted unlike radiowaves

Question 18

Uses of infrared radiation and why

Answer 18

• Emitted by electrical heaters and also used to cook food
• This is because the energy carried by infrared waves is easily absorbed by the surfaces of objects increasing their temperature
• Also used in infrared cameras which allows to see heat loses

Question 19

USe of visible light and why

Answer 19

• Communication using fibre optics
• This is when pulses of light are sent down strands of glass to carry information for telephone or TV signals
• This is because visible light has a short wavelength so can carry lots of information and it stays inside the cable so is more secure

Question 20

Uses of ultraviolet and why

Answer 20

• Energy efficient lamps
• This is because UV has a short wavelength so it carries more energy that visible light
• The energy from the Uv is absorbed by the internal surfaces and converted to visible light which requires less energy than a nromal light bulb
• Another use is in sun-tanning as it replicates the UV rays sent out by the sun
• However, this has risks of skin cancer and premature skin ageing

Question 21

Uses of X-rays and gamma rays and why

Answer 21

• Used for medical imaging
• X-rays are used to detect broken bones and gamma rays are used to detect cancers
• They are used for this as they have a high penetrating power so can pass through body tissues easily
• They can both be used in medical treatments e.g. for cancer
• This is because they are ionising radiation and can carry lots of energy due to their short wavelegth so can be used to damage and kill tumour cells

Question 22

Hazards of infrared radiation

Answer 22

• Can cause burns from high intensity sources