P13 Flashcards
Electromagnetic waves
Give the EM spectrum from longest wavelength to shortest wavelength
- radiowaves
- microwaves
- infrared
- visible light
- ultraviolet light (UV)
- x-rays
- gamma rays
Why are the boundaries between EM wave types only approximated and not definite ?
the spectrum is continuous, so it is hard to definitively categorise the waves
Give the visible light spectrum from longest wavelength to shortest wavelength
- red
- orange
- yellow
- green
- blue
- violet
Give the range of wavelengths for the visible light spectrum
just below 400 nm to just over 700 nm
What is the speed of all EM waves through a vacuum ?
3 x 10^8 m/s
- (300 million m/s)
Give the equation for wave speed with units
wave speed (m/s) = frequency (Hz) x wavelength (m)
What is the relationship between frequency and wavelength for EM waves ?
shorter wavelength = higher frequency
- as wavelength decreases, frequency increases
What is white light mainly emitted by ?
regular lamps and the sun
Give some uses of infrared radiation
- optical fibres IN COMMUNICATIONS SYSTEMS
[] absorbed less than visible light by the glass fibres - remote control handsets
- infrared scanners in hospital to detect unhealthy tissue with an excess of heat
- cameras to see warm-blooded beings in the dark
- electric heaters and stoves
Give some uses of microwave radiation
- satellite communications for TV and phone signals
[] better than radio waves for TV broadcasting as can travel between space and ground to satellites and spread out less than radio waves so signal isn’t weakened as much - microwave ovens that heat food by being absorbed by the water molecules and making them vibrate, heating it up
Give some uses of radio waves
- carry radio, TV and phone signals
- bluetooth
How can radiowaves, microwaves and infrared harm people ?
- infrared radiation may cause surface damage to SKIN via heating of cells
- microwaves and radio waves can cause damage to cells and heat internally
Describe the method for an investigation into the efficacy of absorption and emission of infrared by different surfaces (one with Leslie’s cube and one with cans)
LESLIE’S CUBE
- gather equipment
[] leslie’s cube (one matte white side, one matte black, one shiny metallic and one shiny black side)
[] kettle w/water
[] infrared detector
[] metre rule
[] heat-proof mat
- place the cube on top of the heat proof mat to avoid loss if heat from the cube via conduction to the working surface
- fill the cube with just-boiled water
- close the lid to prevent heat loss
- place the metre rule on the working surface with the short side flat against the base of the cube (so it is at a 90 degree angle) and approximately in the middle of the face
- using the detector at 10cm, take a reading of the infrared emission from that side and record it (noting which colour/shine of surface it was from)
- repeat at the same distance and angle etc. for the other 3 sides
- should find that the matte black side had the highest reading of infrared, and thus is better at emission of infrared than shiny or light surfaces
CANS
- gather equipment
[] 4 identical cans with surface painted/coated in different colours (matte white, matte black, shiny black, metallic shiny)
[] 4 thermometers
[] room temperature water
[] windowsill
- prepare the cans by pouring 150 cm^3 of water in each, then placing one thermometer in each, recording the start temperature of the water in each can
- place each can in front of a sunny window, all at the same distance from the window
- check and record temperature of each can every 2 minutes over 20 minutes
- record the end temperatures for each can and plot a heating curve of each
- should find that matte black can absorbed the most infrared, because the temperature increased the most and the fastest
What are the waves used to carry ANY type of signal called ?
carrier waves
- radio waves, microwaves and infrared are good examples of this
Why are waves with shorter wavelengths (like radio waves) better as carrier waves ?
- carry more information
- spread out less
- good for short range travel if wavelength is small
[] due to increasing absorption by the atmosphere
Why should children spend less time on screens according to some scientists in relevance to EM waves ?
children have thinner skulls than adults and so some scientists believe that the radiation may damage their brains if exposed too long
Describe how a signal is transmitted and detected
- oscillator supplies carrier waves to transmitter in the form of an alternating current (AC)
- signal supplied to transmitter to modulate/vary the amplitude/frequency of the carrier waves
- modulated carrier waves from transmitter supplied to transmitter aerial
[] varying AC supplied to the aerial causes it to emit [insert wave type] waves that carry the signal - waves absorbed by a receiver aerial
[] induces AC in the receiver aerial
[] this AC causes oscillations in the receiver - the oscillations in the receiver are of the same frequency as the [insert wave type] waves
- receiver circuit separates signal from carrier waves
- signal supplied to device
What are optical fibres ?
thin glass fibres that transmit signals carried by visible light or infrared by bouncing the waves inside of the fibres (they cannot escape)
What are the benefits of optical fibres ?
- more secure since the carrier waves and signal are kept within the fibres
- carry more information as visible light has a shorter wavelength than radio waves
Give some uses of UV
- can use chemicals which absorb UV and then emit as visible light for glow in the dark posters/ink, or detecting blood/fingerprints at a crime scene
- security pens to mark valuable objects
- tanning beds
What are the dangers of UV ?
- ionising radiation
[] prolonged exposure could lead to cancer - prolonged exposure can cause sun/skin burns
- can cause blindness due to carrying more energy than visible light
Why do x-ray and gamma rays have similar properties ?
both have very short wavelengths and thus carry much more energy per second
Why do x-rays and gamma rays differ ?
- x-rays
[] produced when electrons/high-speed particles are stopped - gamma rays
[] produced by radioactive substances when unstable nuclei release energy
[] shorter wavelengths than x-rays so penetrate more and are more harmful + carry mroe energy
Give some uses of gamma rays
- sterilisation of food products and medical equipment
- radiotherapy (killing cancer cells)
Describe the dangers of ionising radiation and how they can be reduced
- when UV, x-rays and gamma rays pass through substances, they knock out electrons from atoms, making them ions
[] this is called ionisation - high doses of ionising radiation kills cells
- low doses cause GENE MUTATIONS and thus sometimes cancerous growth
to prevent:
- UV blocking sunscreens are available
- those who work with x-rays or gamma rays must wear a film badge to signal over-exposure to the radiation so they can be disallowed from the work for a period of time
Give the key uses of x-rays
- imaging
[] bones and organs
[] cracks in metal objects in industry - x-ray therapy
[] killing cancer cells
Describe how an x-ray is taken in humans
- x-rays exit tube
- pass through soft tissue but are absorbed by thicker materials like foreign objects in the body, bones and teeth
- those that pass through are absorbed by film or a flat-panel detector (containing a CCD/charge coupled device) at the person’s back which appears darker on screen than the surrounding material that has absorbed them
- lead plates between the body and the tube prevent x-rays travelling to other areas of the body (unless is a full-body x-ray) to minimise dosage and thus damage via ionising radiation
- to get an x-ray of a soft tissue organ, filled with a contrast medium (something good at absorbing x-rays)
[] eg. for stomach x-rays, patients are given a barium meal
Explain how a CCD works
- charge-coupled detector
- detects x-rays, then converts them to light rays
- these light rays are used as carrier waves of electronic signals that are received by a monitor to form a digital image of the x-ray
How are x-rays used to detect cracks or internal imperfections in metal ?
- x-rays from x-ray tube fired at the metal
- flat-panel detector at back
- x-rays that pass through the thinner metal DUE TO IMPERFECTIONS/CRACKS appear dark on the final image, whilst metal with no flaws or cracks appears white due to absorbing the x-rays
How is x-ray therapy done ?
- THICK lead plates cover areas of the body that don’t have cancerous tumours so that isn’t damaged by the high dose of radiation
- very short-wavelength x-rays are fired through the gaps in the plates to the tumours and destroy them
- these x-rays are shorter in wavelength and carry more energy than x-rays used for imaging
