6.2 Electromagnetic waves Flashcards
Properties of EM waves
Transverse waves that transfer energy from the source of the waves to an absorber.
All electromagnetic waves share the following properties:
They are all transverse.
They can all travel through a vacuum.
They all travel at the same speed in a vacuum.
There are 7 types of electromagnetic waves, which all together form a continuous spectrum.
EM spectrum
The electromagnetic spectrum is arranged in a specific order based on the wavelengths or frequencies.
The higher the frequency, the higher the energy of the radiation.
Radiation with higher energy is:
Highly ionising.
Harmful to cells and tissues causing cancer (e.g. UV, X-rays, Gamma rays).
Radiation with lower energy is:
Useful for communications.
Less harmful to humans.
Visible light
Visible light is defined as the range of wavelengths which are visible to humans.
Visible light is the only part of the spectrum detectable by the human eye.
However, it only takes up 0.0035% of the whole electromagnetic spectrum
In the natural world, many animals, such as birds, bees and certain fish, are able to perceive beyond visible light and can see infra-red and UV wavelengths of light.
The different colours of waves correspond to different wavelengths:
Red has the longest wavelength (and the lowest frequency and energy).
Violet has the shortest wavelength (and the highest frequency and energy).
Wavelength and frequency are inversely proportional.
Transfer of energy by EM waves
Electromagnetic (EM) waves carry energy and so can transfer energy from one point to another point.
EM radiation is an energy transfer pathway.
EM waves with a shorter wavelength carry a higher energy.
This includes UV, X-rays and gamma rays.
The higher the energy of the EM wave, the more dangerous it is.
Energy transfer by microwave
Water molecules absorb certain wavelengths of microwave radiation.
Therefore, microwave ovens transfer energy by radiation to the thermal store of the food placed inside it.
Energy transfer by infrared
All hot objects emit infrared radiation.
The emitted waves can then be absorbed by other objects, warming them up.
Therefore energy is transferred by radiation to the thermal store of the object (and the surroundings).
Energy transfer from the sun
The Sun emits several types of EM radiation, including:
Visible light waves allow living creatures to see.
Infrared waves heat up the Earth.
Ultraviolet waves cause suntans and sunburn.
Energy from the Sun is transferred by radiation.
Refraction of EM waves
Some effects, for example, refraction, are due to the difference in velocity of the waves in different substances.
Although all electromagnetic waves travel at the same speed in a vacuum, when they encounter certain materials (water, glass, oil) they will slow down.
How much they slow down depends on the material.
This slowing of electromagnetic waves causes them to refract.
Practical 9 (reflection of light on a smooth mirror.)
Aim: To investigate specular reflection off a smooth surface.
Procedure - Set up the apparatus as shown in the diagram
In the middle of the paper use a ruler to mark a straight line of about 10 cm long.
Use a protractor to draw a 90° line that bisects (cuts in half) the 10 cm line
Place the mirror on the first line as shown in the diagram above.
Switch on the ray box and aim a beam of light at the point where the two drawn lines cross at an angle.
Use the pencil to mark two positions of the light beam:
A point just after leaving the ray box.
The point on the reflected beam about 10 cm away from the mirror
Remove the ray box and mirror.
Use a ruler to join the two marked positions to the point where the originally drawn lines crossed.
Use the protractor to measure the two angles from the 90° line. The angle for the ray towards the mirror is the angle of incidence, and the other the angle of reflection.
Repeat the experiment three times with the beam of light aimed at different angles.
Practical 9 (refraction of light by a perplex block)
Aim: To investigate the refraction of light by a perspex block.
Procedure - Place the glass block on a sheet of paper, and carefully draw around the block using a pencil.
Switch on the ray box and direct a beam of light at the side face of the block
Mark on the paper:
A point on the ray close to the ray box.
The point where the ray enters the block.
The point where the ray exits the block.
A point on the exit light ray which is a distance of about 5 cm away from the block.
Draw a dashed line normal (at right angles) to the outline of the block where the points are.
Remove the block and join the points marked with three straight lines.
Replace the block within its outline and repeat the above process for a ray striking the block at a different angle.
EM waves and atoms
Atoms can interact with electromagnetic (EM) waves in one of two ways, they can be:
Absorbed
Emitted
When an EM wave hits an atom, it can be absorbed by one of the electrons giving it energy.
This causes the electron to move up to a higher energy level
If an electron moves down to a lower energy level it will emit an EM wave in the process.
In this way, atoms can absorb and emit electromagnetic waves over a wide range of frequencies.
The energies associated with electron transitions tend to be in the visible and ultraviolet range (and sometimes X-rays).
Higher energies (ie. gamma rays) can only be achieved when EM waves interact with the nucleus.
The nucleus of an atom can absorb and emit EM waves in a similar way to electrons.
Radio waves
Radio waves can be produced by connecting an antenna to a high frequency alternating current (a.c.) power source.
The oscillation of charge in the a.c. circuit produces radio waves with the same frequency of oscillation.
In the transmitting antenna:
The charge from the alternating current oscillates up and down the antenna
This produces radio waves that can be absorbed by a similar aerial some distance away.
In a receiving aerial:
The metal aerial absorbs the radio waves
This creates an alternating current with the same frequency as the transmitted wave.
Hazardous effects of high energy EM waves
As the frequency of electromagnetic (EM) waves increases, so does the energy.
Beyond the visible part of the spectrum, the energy becomes large enough to ionise atoms.
As a result of this, the danger associated with EM waves increases along with the frequency.
The shorter the wavelength, the more ionising the radiation.
Although the intensity of a wave also plays a very important role.
Because of ionisation, ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue.
The effects depend on the type of radiation and the size of the dose.
They can damage cells and cause mutations, making them cancerous.
Practical 10 (investigating infrared radiation)
Aim: investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.
Procedure - Set up the four identical flasks painted black, grey, white and silver.
Fill the flasks with hot water, ensuring the measurements start from the same initial temperature.
Note the starting temperature, then measure the temperatures at regular intervals .
Radiation dose
A measure of the risk of harm resulting from an exposure of the body to ionising radiation.
Radiation dose is measured in sieverts (Sv).
1 Sv is a very large amount of radiation, so it is more commonly measured in millisieverts (mSv) where 1 Sv = 1000 mSv.
Typically, background radiation is about 1.5 to 3.5 mSv per year.
Whereas, 8 Sv is enough to cause death, even with treatment.
Dangers of Microwaves
Certain frequencies of microwaves are absorbed by water molecules.
Since humans contain a lot of water, there is a risk of internal heating from microwaves.
This might worry some people, but microwaves used in everyday circumstances are proven to be safe.
Microwaves used for communications (including mobile phones) emit very small amounts of energy which are not known to cause any harm.
Microwave ovens, on the other hand, emit very large amounts of energy, however, that energy is prevented from escaping the oven by the metal walls and metal grid in the glass door.
Dangers of ultraviolet
Ultraviolet is similar to visible light, except it is invisible to the human eye and carries a much higher energy.
If eyes are exposed to high levels of UV it can cause severe eye damage
Good quality sunglasses will absorb ultraviolet, preventing it from entering the eyes.
Ultraviolet is ionising meaning it can kill cells or cause them to malfunction, resulting in premature ageing, and diseases such as skin cancer.
Sunscreen absorbs ultraviolet light, preventing it from damaging the skin.
Dangers of x-rays and gamma rays
X-rays and gamma rays are the most ionising types of EM waves.
They are able to penetrate the body and cause internal damage.
They can cause the mutation of genes and cause cancer.
Fortunately, the level of X-rays used in medicine is kept to minimum levels at which the risk is very low.
Doctors, however, will leave the room when taking X-rays in order to avoid unnecessary exposure to them.
People working with gamma rays have to take several precautions to minimise their exposure and are routinely tested to check their radiation dose levels.
For example, radiation badges are worn by medical professionals such as radiographers to measure the amount of radiation exposure in their body.
Application of EM waves
Radio waves - communication (TV and radio)
Microwave - heating food, Communication (WiFi, mobile phones, satellites)
Infrared - Remote controls, Fibre optic communication, Thermal imaging (medicine and industry), Night vision, Heating or cooking things, Motion sensors (for security alarms), Electrical heaters, Infrared cameras
Visible light - Seeing and taking photographs/ videos, Fibre optic communications
Ultraviolet - Security marking (fluorescence), Fluorescent bulbs (energy efficient lamps), Getting a suntan
X-ray - X-Ray images (medicine, airport security and industry)
Gamma rays - Sterilising medical instruments, Treating cancer
Convex lens
In a convex lens, parallel rays of light are brought to a focus.
This point is called the principal focus.
This lens is sometimes referred to as a converging lens.
The distance from the lens to the principal focus is called the focal length.
This depends on how curved the lens is.
The more curved the lens, the shorter the focal length.
A convex lens is drawn by a straight with arrows facing outwards.
Concave lens
In a concave lens, parallel rays of light are made to diverge (spread out) from a point.
This lens is sometimes referred to as a diverging lens.
The principal focus is now the point from which the rays appear to diverge from.
A concave lens is drawn by a straight with arrows facing inwards.
Real images
An image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen.
A real image is one produced by the convergence of light towards a focus.
Real images are always inverted.
Real images can be projected onto pieces of paper or screens.
Virtual images
An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen.
A virtual image is formed by the divergence of light away from a point.
Virtual images are always upright.
Virtual images cannot be projected onto a piece of paper or a screen.
Magnification
The magnification depends on:
The distance of an object from the lens
The power of the lens
Magnification = image height / object height
Specular reflection
Reflection from a smooth surface in a single direction.
When light reflects off a smooth surface, such as a mirror, specular reflection occurs.
This is what gives a mirror its shiny appearance.
This is why a reflection can be seen clearly in a mirror.
Diffuse reflection
Reflection from a rough surface causes scattering.
When light reflects off a rough surface, which applies to the majority of surfaces, diffuse reflection occurs.
This is what gives objects a dull or matt appearance.
This is why a reflection cannot be seen clearly from a table surface, for example.
Even though a table’s surface may look smooth from afar, it is actually made up of many tiny ridges which the light rays are scattered off.
When light scatters, it leaves the surface in all directions.
White light
White light is a mixture of all the colours of the spectrum.
Each colour has a different wavelength (and frequency), making up a very narrow part of the electromagnetic spectrum.
White light may be separated into all its colours by passing it through a prism
This is done by refraction.
Violet light is refracted the most, whilst red light is refracted the least.
This splits up the colours to form a spectrum.
Colour filter
Colour filters work by absorbing certain wavelengths and transmitting other wavelengths.
These certain wavelengths correspond to certain colours.
When white light passes through a coloured filter, some colours are absorbed whilst others are able to pass straight through.
The colour that is transmitted is the same colour as the filter.
Colour and reflection of light
The colour of an opaque object is determined by which wavelengths of light are more strongly reflected.
Wavelengths that are not reflected are absorbed.
Hence, this is why different objects appear to be different colours.
