Waves 2 Flashcards
Electromagnetic waves, Light & Lenses.
Use of radio waves
Television and radio.
They transfer energy from TV and radio transmitters to TVs and radios.
They are not transmitted by the ionosphere, instead, they are refracted and then reflected to a receiver.
Why are radio waves suitable for television and radio?
Reflected by ionosphere (the ionized part of the Earth’s atmosphere) so can broadcast over long distances.
Use of microwaves
Satellite communications, cooking food.
Why are radio waves suitable for satellite communications and cooking food?
They’re able to pass through the atmosphere to satellites.
Has a heating effect.
What are electromagnetic (EM) waves?
Transverse waves that transfer energy from the source of the waves to an absorber. (can be an observer?)
All types of electromagnetic wave travel at the same velocity through a vacuum (space) or air - 3 x 10⁸ m/s
What do electromagnetic (EM) waves form?
A continuous spectrum.
How are the waves that form the electromagnetic spectrum grouped?
In terms of their wavelength and their frequency.
List the EM waves from low to high frequency.
Radio waves
Micro waves
Infrared radiation
Visible light
Ultraviolet
X-rays
Gamma rays
EM wave with highest frequency?
Gamma rays
EM wave with lowest frequency?
Radio Waves
EM wave with longest wavelength?
Radio Waves
EM wave with shortest wavelength?
Gamma rays
What differentiates the EM waves?
Their frequencies and wavelengths which are inversely related (highest frequency = shortest wavelength, lowest frequency = longest wavelength).
Why can our eyes only detect a limited range of electromagnetic waves?
Our eyes only detect visible light.
What gives us colours?
Different wavelengths in the visible light region
(longest =red - hence lowest frequency, shortest = blue)
Which EM waves transfer energy from the Sun to the Earth?
Infrared
Visible light
UV
What is the wavelength of radio waves?
10³ m
What is the wavelength of gamma rays?
10⁻¹² m
Order the wavelengths of visible light from long to short (remember- the wavelength determines the colour)
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with what?
wavelength
Some effects, for example __________ are due to the difference in ________of the waves in different _________.
refraction
velocity
substances
Students should be able to use wave front diagrams to explain refraction in terms of the change of…
…speed that happens when a wave travels from one medium to a different medium.
How can radio waves be produced/generated?
By oscillations in electrical circuits.
They can also induce an alternating current in an electrical circuit.
What can happen when radio waves are absorbed?
When radio waves are absorbed, they may create an alternating current with the same frequency as the radio wave itself.
This means that radio waves themselves can induce oscillations in an electrical circuit.
What can result in electromagnetic waves being generated or absorbed (over a wide frequency range)?
Changes in atoms and the nuclei of atoms.
Where do gamma rays originate?
From changes in the nucleus of an atom.
Which EM waves can have hazardous effects on human body tissue?
-Ultraviolet waves
-X-rays
-Gamma rays
What does the severity of the effect of some radiation on human body tissue depend on?
-The type of radiation
-The size of the dose
What is radiation dose?
A measure of the risk of harm resulting from an exposure of the body to radiation.
What is the unit for radiation dose?
Sieverts (Sv).
1 sievert = how many millisieverts?
1000
Students should be able to draw conclusions from given data about the what?
Risks and consequences of exposure to radiation.
Risk of X-rays and gamma rays
They are are ionising radiation that can cause the mutation of genes and hence cancer. (or the mutation kills cells)
Risk of ultraviolet waves.
They can can cause skin to age prematurely and increase the risk of skin cancer.
Damage to skin cells = sunburn.
Can damage the eyes leading to eye conditions.
Use of Infrared radiation
electrical heaters, cooking food, infrared cameras.
Use of visible light
Fibre optic communications
To help us to see
Use of ultraviolet waves
Energy efficient lamps
Sun tanning
[Security markers (invisible markings on passports, bank notes, etc. - can detect forgeries).]
Use of X-rays and gamma rays
Medical images and treatments.
Why are microwaves suitable for cooking food?
Microwaves which can be absorbed by water molecules are suitable for cooking food in microwave ovens, which transfer energy to food by microwaves. These are then absorbed by water molecules in food.
This is because when they are absorbed by water molecules in food, the energy of the waves is also absorbed.
Food molecules then vibrate faster and transfer energy to neighbouring molecules.
Energy spreads through the food via convection or conduction.
Why are some microwaves suitable for satellite communications?
Microwaves which cannot be absorbed by water molecules are suitable for use in satellite communications.
This is because they can pass through the atmosphere without being absorbed or refracted.
They are transmitted by the ionosphere and then re-transmitted back to a receiver.
Why is infrared useful for electrical heaters and cooking food?
When the metals are heated to high temperatures, the metals gain lots of energy (particles vibrate faster) so emit lots of infrared radiation.
They heat food by transferring the energy
Why is infrared useful for infrared cameras?
Infrared radiation is emitted from all objects with thermal energy however…
Animals are warmer than the surrounding environment and therefore emit more infrared radiation
Infrared cameras can detect the infrared radiation emitted by animals…
This allows them to form live images of living organisms. (shows up orange/yellow, cool= blue).
Why is visible light useful for communications using fibre optic cables?
Thin glass/plastic cables (the fibre optic cables) are able to transmit pulses of visible light over long distances. This is because the light waves are reflected each time they hit the inner surface.
Why is Ultraviolet useful for energy efficient lamps?
Fluorescent lights generate ultraviolet radiation which is absorbed by a layer of phosphorus (coating the inside of the glass bulb).
The phosphorus gains lots of energy - enough to re-emit it as visible light.
Why is Ultraviolet useful for sun tanning?
UV chemically modifies melanin in the skin, causing the skin pigment to darken.
Why are X-rays useful for medical imaging and treatments?
They can be used to view the internal structure of objects- including humans:
X-rays are fired at a patient.
X-rays are absorbed by materials which are very dense, like bones, but transmitted through materials which are not very dense, like lungs and intestines.
The X-rays that pass through are detected by a detector plate to form a black area on screen.
Areas such as bones where radiation has been absorbed previously show up as white/grey for partial absorption.
Why are gamma rays useful for medical imaging and treatments?
Used in radiotherapy (steal from P4)
Why are gamma rays useful for sterilising medical equipment and food?
Gamma rays can kill microorganisms without causing damage to equipment (like boiling would= melting).
This can keep food fresh for longer and safe to eat as there are no microorganisms to break it down.
What is fluorescence?
The process by which UV light is absorbed, then converted and re-emitted as visible light.
Why is glass/plastic used to make fibre optic cables?
These materials totally reflect the visible light waves and the reflection is specular so light doesn’t scatter.
This means that the signals are less likely to be distorted during transmission.
(alt-copper wires and electricity)
Is more infrared radiation emitted by a solenoid or a normal wire?
A solenoid.
What is visible light?
The spectrum of wavelengths that our eyes can detect.
Why do we get toast and not warm bread?
Infrared radiation cannot penetrate the surface of objects.
Which EM waves are only harmful in high quantities?
-Radio
-Micro
How does a lens form an image?
By refracting light.
What can happen to EM waves?
They can be:
-reflected off a surface
-refracted when they move from one material into another
-transmitted when they pass through a material
-absorbed by different materials
The extent to which these 4 things happen depends on the material (e.g. UV can be absorbed by skin but not Earth’s atmosphere) and the wavelengths of the EM waves.
Why can X-rays and gamma rays not reach the surface of the Earth?
They are absorbed by the upper atmosphere.
Explain why it is not possible to get a suntan if you are inside a house, but it is possible to listen to the radio. (4)
The UV rays that cause suntans are not transmitted by the walls of houses- they are absorbed by them.
Radio waves are transmitted by the walls of houses so can be detected by a radio receiver inside the house.
Explain why we don’t need a communication satellite to relay some radio waves.
They are refracted by the ionosphere and reflected back to receivers on the surface of the Earth.
What does the behaviour of EM waves in different materials depend on?
Their wavelength and velocity.
EM waves have different velocities in different _________…..
materials.
This is linked to the density of the material - denser = slower.
Dangers of infrared radiation
Transfers thermal energy so too much can cause skin burns.
Dangers of microwaves
Can heat the water inside our bodies causing cell damage/killing cells.
What are the three types of radio wave?
-Long wave
-Short wave
-Very short wave
What can long waves do?
This type of radio wave can travel long distances and diffract (bend) around the curved surface of the earth.
What can short waves not do?
Diffract (bend) around the curved surface of the earth.
What are fibre optic cables used for?
To transmit data really quickly over very large distances (halfway around the earth).
Uses of gamma rays in hospitals
-To treat cancer
-To perform medical imaging
What are oscilloscopes used for?
To display the wave frequency of an alternating current.
Does background radiation do us harm?
No.
How do lenses form images?
By refracting light.
(changing the direction of the waves at the boundary).
Focal length
The distance from the lens to the principal focus.
What does a convex lens do?
Bring parallel lines to a focus at the principal focus (behind the lens).
What are ray diagrams used for?
To show the formation of images by convex and concave lenses.
Is the image produced by a convex lens real or virtual?
The image can be either real or virtual.
When would an image produced by a convex lens be virtual?
If an object is placed between the lens and the principal focus.
Is the image produced by a concave lens real or virtual?
Always virtual.
What makes a lens more powerful (refract more strongly)?
-Made of a material which refracts light less strongly
-More curved
What would make focal length shorter?
A thicker lens because light is refracted more strongly so comes to a point closer to the lens.
Where are images formed?
At points where all the light rays from a particular point on an object appear to come together.
Real image
Light rays actually come together to form an image.
Virtual image
Light rays don’t actually come together where the image appears to be.
These ‘virtual rays’ are shown using a dashed line.
How many principal focuses are there?
2, one either side of a lens at the same distance away each side. They always sit on the axis (parallel line all the way through the middle of the lens, horizontally).
Convex ‘converging’ lens
Focuses light using refraction.
‘Light rays come together to form an image. The image can be captured on a screen.’
Concave ‘diverging’ lens
Disperses light using refraction.
‘Light rays don’t come together where the image appears to be. The image cannot be captured on a screen’.
The more powerful the lens..
…the shorter the focal length as light is refracted more strongly.
Which words do we use to describe an image?
-Real/virtual
-Inverted/upright
-Smaller/bigger
Why are the images we see not inverted?
They are! Our brain just corrects this too quickly to notice!
How do you draw a ray diagram for a concave lens where the object is greater than 2F?
-Horizontal light ray from the top of the object to the lens
-Diagonal light ray from the top of the object through the centre of the lens
-Diagonal virtual (dashed lines) and real (solid lines) light ray through the principle focus and the lens
-Smaller upright image at the intersection of the real light ray and virtual light ray
How do you draw a ray diagram for a convex lens when the image is greater than 2F?
-Horizontal light ray from the top of the object to the lens
-Diagonal light ray from the top of the object through the centre of the lens
-Diagonal light ray on the right side of the lens through the principle focus
-Smaller inverted image at the intersection of the two light rays of the right of the lens
F
Principal focus
2F
Double the distance of the principal focus from the lens.
How to draw ray diagrams for a convex lens with the object at less than 1F from the lens?
-Horizontal light ray from the top of the object to the lens
-Diagonal light ray from the top of the object through the centre of the lens
-Diagonal light ray on the right side of the lens through the principle focus
-Virtual rays traced back to where the rays would meet
-Draw object here: virtual, upright, larger.
How can the magnification produced by a lens can be calculated?
using the equation:
magnification = image height ÷ object height
What units should image height and object height both be measured in?
mm or cm (same units for both!)
How are concave vs convex lenses represented in ray diagrams?
concave - a line with outgoing fins
convex - a line with inward turned fins
Each colour within the visible light spectrum has its own…
…narrow band of wavelength and frequency.
Which colour within the visible light spectrum has the longest wavelength?
Red at approximately 700nm
Which colour within the visible light spectrum has the lowest frequency?
Red
Which colour within the visible light spectrum has the highest frequency?
Violet
Specular diffusion
Reflection from a smooth surface in a single direction.
Diffuse reflection
Reflection from a rough surface which causes scattering.
What does a magnification less than one mean?
The image is smaller than the object.
How do colour filters work?
They absorb certain wavelengths (and colour) and transmit other wavelengths (and colour).
What determines the colour of an opaque object?
The colour of an opaque object is determined by which wavelengths of light are more strongly reflected.
Wavelengths that are not reflected are absorbed.
What colour will an object appear if all wavelengths of colour are reflected equally?
White
What is black?
An absence of light.
What colour will an object appear if all wavelengths of colour are absorbed?
Black
Objects that transmit light are either…
transparent or translucent.
Why does an opaque object have a particular colour?
Opaque objects do not allow for the transmission of light: wavelengths are either reflected or absorbed.
The wavelengths an object reflects determines its colour.
If all are reflected = white, and if none are reflected = black.
If red reflected = red and so on.
What is the effect on light of passing through filters?
Colour filters only allow certain wavelengths of visible light to be transmitted - the rest are absorbed.
What is the effect of viewing objects through filters?
Primary filters allow wavelengths only of red, green, or blue to be transmitted so only objects that reflect wavelengths of that certain colour will be seen. (others will appear black).
Colour filters not of primary colours allow wavelengths of the same colour as the filter and of the primary colours that can be added together to make that colour - through.
E.g. a yellow filter would let yellow, red, and green light through. A red filter would only transmit red light.
The colour of an object is related to what?
The differential absorption, transmission and reflection of different wavelengths of light by the object.
A transparent material…
Transmits nearly all of the light e.g. glass.
(Only a fraction is reflected or absorbed).
A translucent material…
Only transmits some of the light e.g. frosted glass.
The proportion of light transmitted determines how well we can see through.
All bodies (objects), no matter what ___________, emit and absorb ________ _________.
temperature
infrared radiation
What is a perfect black body?
An object that absorbs all of the radiation incident on it.
A black body does not reflect or transmit any radiation.
Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.
What does the intensity of any emission (of infrared radiation) depend on?
The temperature of the body (object).
What does the wavelength distribution of any emission (of infrared radiation) depend on?
The temperature of the body (object).
e.g. the sun (surface 5700°C) which is much hotter than a cup of tea (90°C) will emit visible light and ultraviolet which have a shorter wavelength than infrared.
Thermal equilibrium
When thermal radiation is absorbed and emitted from an object at the same rate.
A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation.
When does the temperature of a body (object) increase?
When the body absorbs radiation faster than it emits radiation.
Body =
Object.
Factors affecting the temperature of the Earth?
-The rates of absorption and emission of radiation
-Reflection of radiation into space.
When does the temperature of a body (object) decrease?
When the body emits radiation faster than it absorbs radiation.
Give an everyday example to illustrate the balance between incoming radiation absorbed and radiation emitted.
The water inside a kettle cools down because it emits infrared radiation faster than it absorbs it.
What is intensity (P6-waves)?
The energy emitted per square metre per second/ the power emitted per square metre.
As an object gets hotter, the _________ increases and the __________ ____ ___________ __ ___ _______ _________ decreases.
intensity
wavelength that corresponds to the maximum intensity (peak wavelength)
Order in terms of hotness: red, white, blue
Blue
Red
White (hence a safety flame’s colour)
On an intensity wavelength graph, what does the area under the line tell us?
How intense/total power - larger area=bigger
Violet light wavelength
410nm so refracted more than other colours
How can we split up white light?
Using a prism.
The white light ray is refracted into a spectrum of colours (dispersion).
What do primary colours (R,G,B)add to make?
White
What are the primary colours (visible light)
Red
Blue
Green
What are the secondary colours (visible light)?
Yellow
Cyan
Magenta
White
How to make yellow light?
Red and Green
How to make cyan light?
Green and Blue
How to make magenta light?
Red and Blue
How might we investigate the transmission of primary and secondary colours?
Using a light box and coloured filters.
How many wavelengths make up yellow light (secondary)
3
(yellow, green, red)
Wavelength of microwaves
10⁻²m
Wavelength of infrared
10⁻⁵m
Wavelength of UV
10⁻⁸m
Wavelength of an X-ray
10⁻¹⁰m
How does temperature affect the wavelength that is radiated most intensely (the peak wavelength)?
The peak wavelength decreases (moves to the left) as temperature increases (moves up).
Object A emits s a peak wavelength of light of 600nm, and object B emits a peak wavelength of light of 750nm.
Which of the 2 objects is hotter? Explain your answer.
Object A is hotter.
The hotter the object, the shorter the peak wavelength.
How does temperature affect the intensity of all wavelengths emitted by a black body?
The intensity of all wavelengths increases with temperature.
A hotter, bigger, or brighter object gives out more what?
Energy. (in the form of infrared radiation)
(HT only) Students should be able to use information, or draw/interpret diagrams to show how radiation affects the temperature of the Earth’s surface and atmosphere.
MHMMM?
Blue light wavelength?
≈500nm
Green light wavelength?
≈550nm
Yellow light wavelength?
≈570nm
Orange light wavelength?
≈600nm
The hotter the body, the more ________ _________ it radiates in a given ____.
infrared radiation
time
Intensity
A measure of how much energy radiation transfers to a given area in a certain amount of time.
What does a high intensity mean?
More
What makes an object cool down?
Emitting more radiation than is absorbed.
This causes the object to lose energy and cool down.
What makes an object heat up?
Absorbing more radiation than is emitted.
This causes the object to gain energy and heat up.
What happens to an object is it absorbs and emits the same amount of energy?
It will stay the same temperature because its gaining and losing the same amount of heat.
What goes on the x- axis of a graph of emitted radiation?
wavelength of the radiation emitted
What goes on the y- axis of a graph of emitted radiation?
intensity
What does a graph of emitted radiation show?
How much of each wavelength is emitted.
The higher the intensity, the hotter the object so the more of that wavelength is emitted.
On a graph of radiation emitted, what three EM waves go on? (order from left to right on the graph)
UV, visible light, infrared
On a graph of emitted radiation, how would a hotter and cooler object compare?
The curve for the hotter object would be taller and further to the left.
This shows that it emits more radiation overall, and that it emits shorter wavelengths than the cooler object.
What happens as the temperature of an object increases?
The intensity of every emitted wavelength increases (this is why the graph gets higher).
The intensity of shorter wavelengths increases more than the intensity of longer wavelengths (which is why the graph shifts to the left).
Why do Bunsen burner flames change colour?
The hotter the flame, the shorter the wavelengths of light emitted.
This is why cooler flames are orange and hotter ones are blue.
Why can’t we see the radiation from objects at room temperature?
All wavelengths emitted from the objects are in the infrared range.
The _____________________ applies to our entire planet.
Balance between absorbing and emitting radiation
Balance of absorbing and emitting radiation - THE EARTH!
The earth receives EM radiation from the sun.
It emits its own infrared radiation. (because its a big warm object)
The earth has an atmosphere which can reflect, absorb, and emit radiation.
The Earth’s temperature stays fairly constant because some parts are always in the day at the same time as others are in night.
Why does Earth’s temperature increase during the day?
More energy is absorbed by the Earth and atmosphere than is being emitted.
This increases the local temperature.
Why does Earth’s temperature decrease at night?
More energy is emitted by the Earth and atmosphere than is being absorbed.
This decreases the local temperature.
