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What is total internal reflection?
When light is completely reflected back at a boundary between two mediums, no wave energy is lost as a refracted ray. It occurs when light meets a less dense medium at an angle of incidence larger than the critical angle
What is the law of reflection?
angle of incidence = angle of reflection
What is the critical angle?
The angle of incidence when the angle of refraction is 90 degrees
What is specular reflection?
Reflection from a smooth surface in a single direction.
What kind of reflection do you get from specular reflection?
A clear reflection e.g. when light is reflected by a mirror
What is diffuse reflection?
When a wave is reflected by a rough surface & the reflected rays are scattered in lots of different directions.
Why does diffuse reflection happen?
The normal is different for each incoming ray which means the angle of incidence is different for each ray
What kind of reflection do you get from a diffuse reflection?
When light is reflected by something rough, the surface looks matt and you don't get a clear surface.
What determines the colour of visible light waves?
The wavelength and frequency of the light waves.
What colour of visible light has the highest frequency?
Blue
What colour of visible light has the largest wavelength?
Red
What are opaque objects?
Objects that do not transmit light. When visible light waves hit them, they absorb some wavelengths of light and reflect others
What does the colour of an opaque object depend on?
It depends on which wavelengths of light are reflected e.g. a red apple appears to be red because of the wavelengths corresponding to the red part of the visible spectrum are reflected.
What are the primary colours of light?
Red, green and blue (RGB)
Do white objects reflect or absorb all wavelengths?
White objects reflect all wavelengths of visible light equally
Do black objects reflect or absorb all wavelengths?
Black objects absorb all wavelengths of visible light equally
What are transparent and translucent objects?
Objects which transmit light
Why do translucent objects appear translucent?
Some wavelengths of light are absorbed or reflected by translucent objects. These objects will appear to be the colour of light that corresponds to the wavelengths most strongly transmitted by the object.
What are colour filters?
They're used to filter out different wavelengths of light so that only certain colours are transmitted - the rest are absorbed.
What are primary colour filters?
Colour filters that only allow their specific primary colour to be transmitted e.g. if white light is shone at a blue colour filter, only blue light will be let through and the rest are absorbed.
What will an object look like if you wore a colour filter that was different to the colour of the object?
The object would appear black e.g. if you wore red colour filters, blue objects would appear black as the blue wavelength is being absorbed.
What are the two types of lenses that refract light?
Convex lenses and Concave lenses
What is a convex lens?
A converging lens that is curved on both sides to make it thicker in the middle. It brings parallel light waves together so that they converge at the principal focus.
What is a concave lens?
A diverging lens which caves inwards. It causes parallel light waves to diverge and spread out.
What is the principal focus of the convex lens?
Where rays hitting the lens parallel to the axis all meet
What is the principal focus of the concave lens?
The point where rays hitting the lens parallel to the axis appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens
What is the focal length?
The distance from the centre of the lens to the principal focus
What are images formed by lenses?
Images are formed at points where all the light rays from a certain point on an object appear to come together.
What are the two types of images formed by lenses?
Real images and virtual images
What is a real image?
Where the light from an object converges at a point to form an image. The image can be viewed if captured on a screen because the light rays actually meet at the place where the images seem to be.
Give an example of a real image
The image formed on the eye's retina.
What is a virtual image?
When the rays are diverging, so the light from the object appears to be coming from a completely different place. The light rays don't actually come together at the point where the images seems to be, so it cannot be captured on a screen
Give an example of a virtual image.
Magnifying glass.
How would describe an image formed by a convex lens?
An image formed by a convex lens is a real image, it is inverted and smaller than the original object.
How would you describe an image formed by a concave lens?
An image formed by a concave lens is a virtual image, it is upright and smaller than the original object.
What factors affect the power of a lens?
Focal length, material of the lens and its curvature
How does the focal length affect the power of a lens?
The shorter the focal length, the more strongly it converges the rays of light, so more powerful the lens.
How does the material of the lens affect the power of a lens?
Materials that are better at focusing light can be made thinner for the lens as the lens does not need to be curved as much to achieve the same focal length.
How does the curvature of a lens affect the power of a lens?
Stronger curved surfaces from materials e.g. glass produce more powerful lens.
How does the distance between an object and a convex lens affect the size and position of the image?
An object 2F (two focal lengths from the lens produces a real, inverted image the same size as the object and at 2F on the other side of the lens. An object between F and 2F will make a real, inverted image bigger than the object and beyond 2F.
What type of waves are electromagnetic waves?
Transverse
What do EM waves transfer?
Energy
List the order of the electromagnetic spectrum in order of decreasing wavelength
Radio waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-Rays, Gamma rays.
What is the EM wave with the largest wavelength and lowest frequency?
Radio waves
What is the EM wave with the shortest wavelength and highest frequency?
Gamma rays
What properties are shared by all EM waves?
They are all transverse waves and they all travel at the same speed in a vacuum
What is the speed for EM waves in a vacuum?
3x10^8 m/s
What links the amount of energy a EM waves transfers and the frequency of the EM wave?
A high EM frequency means it can transfer high amounts of energy (and so the more dangerous it is for humans).
How are EM waves produced?
Alternating currents are made up of oscillating charges - as the charges oscillate, they produce oscillating electric & magnetic fields i.e. EM waves
What EM waves can our eyes detect?
Visible light
What are the properties of radio waves, microwaves, infrared and visible light?
Radio waves are transmitted through the body without being absorbed. Some wavelengths of microwaves can be absorbed, which can lead to the heating of cells. Infrared and visible light are mostly reflected or absorbed by the skin, which can cause some heating too.
What are the properties of ultraviolet waves?
UV is also absorbed by the skin, but it has a higher frequency so it is potentially more dangerous. It is a type of ionising radiation and when absorbed it can cause damage to cells on the surface of our skin, which could lead to skin cancer. It can also damage our eyes and cause a variety of eye conditions or even blindness.
What are the properties of X-rays and Gamma rays?
They are also ionising, so they can cause mutations and damage cells too which can cause cancer. They have higher frequencies, so transfer more energy and can cause more damage. They can also pass through the skin and be absorbed by deeper tissue.
What do all bodies emit and absorb?
EM radiation, specifically infrared.
What do the distribution and intensity of emitted and absorbed EM wavelengths depend on?
The temperature of the object.
What happens to the quantity of infrared radiation emitted by an object as temperature increases?
The hotter the object, the more infrared radiation it will emit.
What happens to the type of radiation emitted by an object as temperature increases?
The hotter the body, the shorter the wavelength of radiation released.
What is meant by intensity?
The power transferred per unit area; it is a measure of the energy transferred by a wave.
How is the intensity of an EM wavelength affected by its length?
Intensity increases more rapidly for shorter wavelengths. This causes the peak wavelength (the wavelength with the higher intensity) to decrease.
What is required for a body to be at a constant temperature?
The body would need to emit radiation at the same rate it absorbs it - it needs to radiate at the same average power that it absorbs.
What will happen if the average power that an object radiates is less than it absorbs?
The temperature of the object will increase
What two factors affect the temperature of the Earth?
The Earth's rate of absorption and emission of radiation and the amount of reflection of radiation into space.
What happens to the local temperature during the day?
Lots of radiation is transferred to the Earth from the Sun. Some of this is reflected, but most is absorbed by the Earth's atmosphere, clouds and surface. This causes an increase in local temperature.
What happens to the local temperature at night?
Radiation is emitted by the atmosphere, clouds and surface, which causes a decrease in the local temperature.
How does the Earth's atmosphere affect radiation?
The atmosphere largely absorbs or reflects radiation from the sun, preventing it from reaching Earth. Some radiation, however, is allowed to pass through and warms the earth.
What happens to the radiation emitted from the Earth?
It is absorbed and re-emitted in all directions by greenhouse gases, resulting in the greenhouse effect which warms the Earth.
Explain the practical that investigates how well different surfaces emit radiation
Wrap four identitical test tubes with material e.g. paper. The material covering each test tube should be the same but each one should have a different surface or be a different colour e.g. black and white paper, glossy and matte paper. Boil water in a kettle and fill each test tube with the same volume of water. Use a thermometer to measure the temperature of the water in the test tubes every minute. Seal the test tubes with bungs between measurements. The temperature of the water will decrease quicker for the test tubes surrounded by surfaces that are good emitters of radiation. You should find that matte surfaces are better emitters than shiny ones and black surfaces emit radiation better than white ones.
What type of waves can be produced by oscillations in an electrical circuit?
Radio waves
How can radio waves create an alternating current in a circuit?
When radio waves are absorbed, they can induce oscillations in a circuit with the same frequency as the waves themselves
Describe the setup to produce radio waves from alternating current in a circuit.
A transmitter oscillates electrons to produce radio waves. The transmitter is connected to a oscilloscope which displays the waves. The radio waves reach a receiver, where they are absorbed. The receiver is also connected to an oscilloscope. Both oscilloscopes on the transmitter and receiver will display identical images of the radio waves.
Describe what happens when a radio waves reaches a receiver from a transmitter in terms of energy and electrons
The energy carried by the waves is transferred to the electrons in the material of the receiver. The energy causes the electrons to oscillate, and, if the receiver is part of a complete electrical circuit, it generates an alternating current. The current has the same frequency as the radio waves that generated it.
What are long-wave radio waves?
Wavelengths of 1 - 10km. They can be received from halfway round the world from where they started, because long wavelengths bend around the curved surface of the earth. This makes it possible for radio waves to be received even if the receiver isn't in the line of sight of the transmitter.
What are short-wave radio signals?
Wavelengths of about 10-100m. They can be received at long distances from the transmitter because they are reflected from the Earth's atmosphere.
What devices use short-wave radio signals?
Bluetooth uses short-wave signals to send data over short distances between devices without wires. TV and FM radios also use these signals so to get reception, you must be in direct sight of the transmitter - the signal doesn't bend or travel far through buildings
How are radio waves and microwaves used in communication?
Communication to and from satellites use EM waves which can pass very easily through the Earth's watery atmosphere. These waves are usually microwaves but can sometimes be relatively high radio waves.
How are EM waves used in satellites?
For satellite TV, the signal from a transmitter is transmitted into space and picked up by the satellite receiver dish orbiting thousands of kilometres above the Earth. The satellite transmits the signal back to Earth in a different direction, where it's received by a satellite dish.
How are microwaves used in microwave ovens?
Microwaves are absorbed by water molecules in the food. The microwaves penetrate up to a few centimetres into the food before being absorbed and transferring the energy they are carrying to the water molecules in the food, causing the water to heat up. The water molecules then transfer this energy to the rest of the molecules in the food by heating - which quickly cooks the food.
How is infrared radiation used in monitoring temperature?
Infrared cameras can be used to detect infrared radiation and monitor temperature. The camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture. This is thermal imaging.
When is thermal imaging used?
This is used by police to see suspects that are trying to escape or hide in the dark.
How are infrared sensors used in security systems?
If a change in infrared radiation is detected, an alarm sounds or a security light turns on.
How is infrared radiation used in cooking/increasing temperature.
Absorbing IR radiation causes objects to get hotter. Food can be cooked using IR radiation - the temperature of the food increases when it absorbs IR radiation e.g. from a toaster's heating element.
How is infrared radiation used in electric heaters?
They contain a long piece of wire that heats up when a current flows through it. This wire then emits lots of IR radiation. The emitted IR radiation is absorbed by objects and the air in the room - energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperature to increase.
How can infrared radiation transfer information?
The distance between the transmitter (e.g. a TV remote) and the receiver (e.g. a TV) must be short and in line for the information to be sent across. Optical fibres are thin glass or plastic fibres that can carry data over long distances as pulses of infrared radiation. They usually use a single wavelength to prevent dispersion, which can otherwise cause some information to be lost.
How is visible light used in photography?
Photographic film reacts to light to form an image. Digital cameras contain image sensors that detect visible light and generate an electrical signal. The signal is then converted into an image that can be stored digitally or printed.
How is ultraviolet used in fluorescent lamps?
Gas inside lamps produce UV when electric current passes through. A coating on the inside of the the glass absorbs the UV and emits visible light. They're energy-efficient, so good to use when light is needed for long periods.
What are the uses of X-rays?
To look inside objects, including human bodies - X-rays are transmitted by flesh, but absorbed by denser material like bones or metal; In airport security scanners, to see what people have in their luggage
What are the uses of gamma rays?
Sterilising food, medical treatments such as radiotherapy, and medical equipment
