CORE PRACTICAL: Investigating Refraction
A – Place a piece of paper on the desk and set up the power supply, ray box and single slit so that a single ray shines across the paper
B – Place a rectangular glass block on the paper and draw around it
C – Shine light into the block and mark where the light ray goes
D – Remove the block. Use a ruler to join the paths of the light outside of the block to show how the light travelled in the block
E – Measure the angles of incidence and refraction where the light entered and left the block
F – Repeat with the ray entering at different angles. The angle of refraction increases at the angle of incidence increases, but the angle of refraction is always less than the angle of incidence. There is no linear relationship between the angles of incidence and refraction
Light
A ray diagram models how light rays change at boundaries. During reflection, the angle of incidence is equal to the angle of reflection. A normal is drawn perpendicular to the mirror.
Light travels at different speeds in different mediums. When a ray moves from into a material where it travels at a different speed, it usually changes direction. The angle of incidence and the angle of refraction are measured from the normal.
When light passes from glass into air with a small angle of incidence, most of the light passes through the interface but a little is reflected. When the angle of incidence equals the critical angle, the refracted light passes along the interface of the glass. At angles greater than the critical angle, the light is completely reflected in a process called total internal reflection
Most objects have rough surfaces on close inspection and so reflected light is scattered in all directions, called diffuse reflection. Very smooth surfaces reflect light evenly by specular reflection
Light from the sun is called white light and contains all the colours of the visible spectrum. When white light hits a coloured surface, some of the colours that make it up are reflected and the others are absorbed. For example, when white light hits a red object, all other colours are absorbed except red, which is reflected. Filters are pieces of transparent material that absorb some of the colours. For example, a green filter allows green light through while all other colours would be absorbed
A lens is a piece of transparent material shaped to refract light in particular ways. The power of a lens describes how much it bends light that passes through it. A more powerful lens is more curved and bends light more
A converging lens is wider in the middle than at the edges. It makes rays converge at the focal point. The focal distance is the distance between the focal point and the centre of the lens. A diverging lens is thinner in the middle and the focal point is where the rays seems to be coming from after passing through the lens
A converging lens can be used to focus the rays of light onto a screen, creating a real image where light rays converge. An object closer to a converging lens than the focal point will form a virtual image which cannot be projected on a screen
Electromagnetic Waves
All electromagnetic waves are transverse and travel at the same speed in a vacuum. All EM waves transfer energy from source to observer. For example, light travels from a source and is reflected by an object, transferring energy. Our eyes can only detect a limited range of frequencies of electromagnetic radiation i.e. visible light. Infrared radiation is emitted by all objects depending on how hot the object is. The heating effect of infrared can be shown by splitting sunlight then measuring temperature at each colour and beyond the spectrum, where there will be a temperature increase
The Electromagnetic Spectrum is continuous and can be groups as shown in the table below. Different substances may absorb, transmit, refract or reflect EM waves in ways that vary with wavelength.
Radio waves are produced by oscillations in electrical circuits. A metal rod or wire can be used as an aerial to receive these waves and induce oscillations in the electrical circuit. Waves travel in a straight line unless reflected or refracted. Some frequencies of radio wave can be refracted by the ionosphere, enough to be sent back to earth at certain angles. Microwaves are not refracted in the atmosphere and so are used for satellite transmissions
The potential danger associated with an EM wave increases with increasing frequency. The table below details the dangers of each type of EM wave
EM radiation is produced by changes in the electrons or nuclei of atoms. e.g. Heating objects makes electrons rearrange, producing IR or light. Changes in nuclei produce gamma radiation. Radiation can also cause changes in atoms, such as causing atoms to lose electrons to become ions
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| Radio Waves | Microwaves | Infrared |
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Ultraviolet | X-Rays | Gamma Rays |
| Transmit broadcasts and TV programmes. Used in communications, e.g. radio comms sent via satellite | Communications and satellite transmissions incl. mobile signals. Microwave ovens transfer energy to food
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Short range communication e.g. TV remotes. Optical fibres sent information by IR. Grills heat food using IR. Thermal images show IR given off by objects. Security systems use sensors to detect IR
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Disinfect water by killing microorganisms. Fluorescent materials used in security marking, visible under UV. Lowenergy bulbs produce UV with current and coating emits light
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Pass through materials like fat and muscle but cannot pass through bone, so used to make images of inside the body. Also inspects luggage at airports
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Sterilise food and surgical instruments. Kill cancer cells in radiotherapy. Used in detection by injecting gamma-emitting chemical and monitoring movement as it is collected
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| No danger so used in transmission | Internal heating of body cells | Skin burns | Damage to surface cells and eyes, leading to skin cancer and eye problems | Mutations to DNA base sequence or damage to cells in the body | ||
Radiation and Temperature
All bodies emit radiation, and the intensity and wavelength distribution of any emission depends on their temperature. The higher the intensity, the higher the temperature and the shorter the wavelength
For a body to be at constant temperature it must radiate the same average power that it absorbs. If the average power it radiates is less than the power it absorbs then the temperature increases, but if the power radiated is higher then the temperature decreases
- Greenhouse gases in the atmosphere naturally absorb some energy, keeping the Earth at higher temperature than if there were no atmosphere. This is the greenhouse effect
- The Earth and atmosphere are at constant temperature if the power absorbed is equal to the power radiated
- Extra greenhouse gases are emitted by the Earth due to human activity, and these gases absorb energy, increasing temperature. The power absorbed is more than the power radiated and so the Earth gets warmer
- The power from the sun is absorbed by the Earth and the Earth radiates this same power, while extra greenhouse gases absorb more energy. The Earth has a new, higher constant temperature
CORE PRACTICAL: Investigating Radiation
A – Cover four boiling tubes in different materials: dull grey, shiny silver, dull black and shiny black
B – Pour in the same volume of hot water into each tube
C – Insert a bung with thermometer into each tube. Measure the temperature change over 2 minutes
D – The dull colours should emit more energy than shiny surfaced colours, while the black colours should have a greater temperature decrease