Light and Waves Flashcards
What are waves?
a means of transferring energy
What doesn’t happen with waves?
There is no transfer of matter
What are transverse waves?
Waves where the direction of energy transfer is perpendicular to the direction of oscillations
What are longitudinal waves?
Waves where the direction of energy transfer is along the direction of oscillations
What is an example of a transverse wave?
light, waves travelling on the surface of water
What is an example of a longitudinal wave?
sound
What is amplitude?
the maximum movement of particles from their resting position caused by a wave
What happens when waves strike a concave barrier?
They become curved and are made to converge
What happens when waves strike a convex barrier?
They are made to diverge and spread out.
What happens when waves enter a shallower region?
their wavelength becomes shorter and because the frequency is constant the velocity also decreases
What happens when the waves enter the deeper region?
their wavelength increase and so does the velocity
What is necessary for refraction to occur?
the boundary between the shallow water and the deep water is at an angle to the direction in which the waves are moving
What happens when waves enter shallow vs deep region?
Shallow: bend towards normal and slow down
Deep: bend away form the normal and speed up
What are the common features to all electromagnetic waves?
they are all transverse
they all travel at 3x108 m/s
they can all be diffracted, refracted and reflected
they all transfer energy
What is the wave with the longest wavelength?
radio
Which wave has the most penetrating power?
gamma
Which is the most high frequency wave?
gamma
What are radio waves used for?
broadcasting and communication
reflect of ionosphere
How are radio waves produced?
They are emitted by a transmitter, cross an aerial and the information is receives as they are detected
What detects radio waves?
TV aerials and radio
What are the uses of microwaves?
cooking, radar and satellite transmissions
How do microwaves heat food?
The waves cause water to vibrate more and increase there amplitude. This increase in kinetic energy is essentially an increase in temperature and so the water molecules become very hot.
The food is cooked throughout not just on the outside
What are the dangers of microwaves?
They can heat human tissue
How are the dangers of microwaves reduced?
Microwaves have metal screens that reflect them and keep them inside the oven.
How are microwaves used for communication?
The waves pass through the Earth’s atmosphere and are used to carry signals to orbiting satellites.
They also carry messages sent from phones so therefore can pass through solids (glass/brick)
What do all objects do?
emit IR
What makes more IR be emitted?
If something is hotter
What detects IR?
skin, blackened thermometers, IR cameras
What are the uses of IR?
night vision equipment, remote controls for TV, stereo and videos
cooking in grills and toasters
optical fibres
Why are IR used for remotes?
They have a low penetrating power and so are unlikely to interact with other signals.
What are the dangers of IR?
skin burning and if cells absorb too much they are killed/damaged
What emits visible light and detects it?
luminous objects
detected by the eye, cameras, LDRs and photographic film
What are the uses of Visible light?
seeing
light from lasers is used to read compact discs and barcodes
optical fibres: communication and seeing inside the body
Which colour has the longest wavelength?
red light
What is the frequency of red light like?
low frequency
What are the dangers of UV light?
blindness - harmful to eyes
skin cancer - damage to skin
How does UV light cause cancer?
causes cancer by ionizing cells under skin surface
What is UV light used in?
UV tanning lamps
some chemicals fluoresce when exposed to UV light
What is helping to reduce UV light?
ozone absorbs it
What does fluorescence mean?
an object that absorbs UV light and emits visible light
What emits X-rays?
X-ray tubes
What detects X-rays?
photographic film
What are X-rays used for?
in radiography to observe internal objects of the body
security checks in airports
in the industry to check the internal structures of objects for cracks etc
What are the dangers of X-rays?
excessive exposure can cause cancer
How can you protect against X rays?
stand behind a lead screen
protective clothing
What are gamma rays emitted by?
radioactive material
What are gamma rays detected by?
Geiger- Muller tube
What are the uses of gamma rays?
sterilise medical equipment
kill micro-organisms so food keeps for longer
radiotherapy
What are the dangers of Gamma?
They have a high penetrating power causing mutations in genes that can lead to cancer.
A small dose can cause cells to become cancerous and a large dose can kill cancer cells.
What are the benefits of analogue signals?
when the signal is amplified so too is the noise which weakens the clarity of the signal
if signals have a similar frequency they interfere with each other and it is difficult to distinguish each signal
Quantisation (when a continuous range is rounded) can give rise to a loss of lots of INFO
wider range of frequencies are needed to broadcast an analogue signal
What are the benefits of digital signals?
regeneration of signal is clear and exact as noise is ignored easily
Many signals can be transmitted at once with one cable
Quantisation doesn’t lose any information so more info can be transmitter in smaller space without any COMPROMISE on the quality of the signal
More programmes can be broadcast over the same frequencies
Digital systems are easier to design and build
Digital systems deal with easy to process data
Can be handled by microprocessors
Wider bandwidth so the response is clearer yet the noise is lower
Carries more info than analogue because the generator can switch between two values quickly in short space of time compared to analogue where the values are so wide ranging it takes longer to generate
Why can digital signals carry more info?
They can also squeeze in more programmes, because digital signals can carry more information per second than analogue signals.
When quantisation occurs less info is lost therefore more info is carried. Also they have a large bandwidth.
What is an analogue signal?
Analogue signals can vary in frequency, amplitude or both continuously.
What are digital signals?
Digital signals are a series of pulses consisting of just two states, ON (1) or OFF (0). There are no values in between.
Explain the meaning of the critical angle
When light travels from one medium to another it is refracted; it changes angle due to change in density.
Past a certain angle the light will simply be refracted back into the medium it is in, this angle is the critical angle.
What is light?
a transverse wave
What are the characteristics of images in a plane mirror?
image is as far behind as the object is in front
virtual
same size
laterally inverted
What is refraction?
when a ray of light travels from air into glass or water it slows down as it crosses the boundary between the two media.
This change in speed may cause the ray to change direction; refract.
What is the formula for refractive index?
n = sin i / sin r
n = speed of light in a vacuum / speed of light in material
What happens when the angle of incidence is less than the Ac?
A ray of light is refracted as it passes from a more dense medium to less dense one, but a small ray of light is also reflected.
What happens when the angle of incidence is equal to the Ac?
The light is refracted at 90 degrees to the normal and there is also a small reflected ray
What happens when the angle of i is more than the Ac?
TOTAL INTERNAL REFLECTION; light reflected back into the denser medium…no refraction occurs
What is the formula for the c?
sin c = 1/n
What can ruin the clear image in a plane mirror?
Several faint images form around the main image due to partial internal reflections at the non-silvered glass surface of the mirror
How are these extra faint images removed from the mirror?
When high quality images are required prisms are used to alter the direction of light rather than mirrors.
Where are prisms used?
bicycle reflectors and binoculars
What is an optical fibre made of?
an outer cladding of less optically dense glass and an inner core of optically dense glass
How is TIR achieved?
The fibres are very narrow so light entering the inner core always strikes the boundary of the two glasses at an angle greater than the Ac.
No light escapes b/c all is Reflected.
What are large numbers of OP.FIBRES called?
bundle
What is done to bundles?
tapered to produce a magnified image
How does an endoscope work?
Light travels down one bundle and illuminates object to be viewed
Light that is reflected by the object travels up a second bundle of fibres
an image of the object is created by the eyepiece
How are optical fibres used in modern telecommunications?
Electrical signals from telephone converted to light pulses by tiny lasers
The light pulses are sent into the ends of an op.fibres
Light sensitive detector at the other end changes the pulses back to electrical signals
These then flow into the telephone receiver; earpiece
What happens when white light passes through a prism?
it emerges as a band of colours; spectrum
Why is a spectrum formed?
White light is a mixture of colours and each colour travels through at a different speed so each colour is refracted by a different angle.
What is dispersion?
When each different colour of the spectrum emerges from the prism travelling in a different direction b/c each colour is refracted by a differing amount
Why does red light disperse the least?
The smaller the wavelength of the passing light, the greater is the refractive index observed. Therefore because red has the longest wavelength is deviates the least.
The deviation depends directly on the refractive index. As white light passes through prism, violet, the minimum wavelength, observes maximum refractive index for the prism and since deviation depends directly on the ref index, the violet wavelength gets deviated to the maximum extent.
