Waves

Waves

1. Waves basics

   • Waves transfer energy in the direction they are travelling in. When waves travel through a medium (e.g. air) the particles of the medium oscillate and transfer energy between each other but overall stay in the same place
   • Amplitude is the maximum displacement of a point on a wave from its undisturbed point. Wavelength is the distance between the same point on two waves next to each other. Frequency is the number of complete waves passing a point per second and is measured in hertz.
   • Time (s) = 1 / Frequency (Hz)

2. Concave lenses and magnification

   • Magnification = Image height / Object height
   • Unlike a convex lens, a concave lens always produces a virtual image. The image is upright, smaller than the object on the same side of the lens as the object no matter where the object is.

3. Reflection

   • When a wave arrives at a boundary between two different materials it is either: absorbed (transfers energy into the material it was trying to cross into), transmitted (carries on travelling - often leads to refraction) or it is reflected
   • Specular reflection happens when a wave is reflected in a single direction by a smooth surface. Diffuse reflection happens when the reflected rays are scattered in lots of different direction by a rough surface because the normal is different for each incoming ray so the angle of incidence is different.
   • Angle of incidence = angle of reflection

4. Electromagnetic waves

   • Radio waves, micro waves, infrared, visible light, ultra violet, x-rays and gamma rays are electromagnetic waves. This means they are transeverse waves that transfer energy from a source to an absorber. They all travel at the same speed through air or a vacuum.

5. Uses of electromagnetic waves

   • Radio waves are used to communicate because long wavelengths can diffract around the curved surface of the Earth and short wavelengths reflected from the ionosphere. They are also used in Bluetooth, TVs and radios. Micro waves are used in satellites because they can pass easily through the Earth's watery atmosphere and in microwave ovens because they penetrate a few cm into the food before being absorbed and transferring their energy to the water molecules in the food heating it up. The water molecules then transfer this energy to the rest of the food by heating which quickly cooks the food.
   • Infrared radiation is given out by all objects depending on the temperature of the object. Infrared cameras detect this infrared radiation and use it to monitor temperature. Infrared is also used to cook food and in electric heaters. Fibre optics are are thin glass or plastic fibres that carry data over long distances as pulses of visible light. The light rays are reflected back and forth until they reach the end of the fibre.
   • Ultraviolet radiation is produced by the sun and gives people a sun tan and is also generated by fluorescent lights and then absorbed and re-emitted as visible light by a layer of phosphorus on the inside of the bulb. X-rays and gamma rays are used in medicine to take x-ray photographs, treat cancer and used as a medical tracer.

6. Transverse and longitudinal waves

   • Transverse waves oscillate perpendicular to the direction of energy transfer. Some examples of transverse waves are all electromagnetic waves and ripples in water
   • Longitudinal waves oscillate parallel to the direction of energy transfer. Some examples are sound waves or seismic waves.
   • Wavespeed (m/s) = Wavelength (m) x Frequency (Hz)

7. Lenses and ray diagrams

   • There are two main types of lens: concave (caves inwards causing rays of light parallel to the axis to diverge) and convex (bulges outwards causing rays of light parallel to the axis to converge at the principal focus.
   • Three rules for refraction in a convex lens are: an incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side, an incident ray passing through the principal focus refracts through the lens and travels parallel to the axis and an incident ray passing through the centre of the lens carries on in the same direction. Three rules for refraction in a concave lens are: an incident ray parallel to the axis refracts through the lens and travels in line with the principal focus, an incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis and an incident ray passing through the centre of the lens carries on in the same direction.
   • A real image is where the light comes together to form an image on a 'screen' and a virtual image is when the rays are diverging so the light from the objects appears to be coming from a completely different place e.g. looking in a mirror. To describe an image you need to say how big it is compared to the real object, weather it's upright or inverted and weather it is real or virtual.

8. Refraction

   • When a wave crosses between materials at and angle it changes direction depending on how much the wave slows down or speed up.
   • If the wave goes to a optically denser material it slows down and bends towards the normal. However, if it goes to a less optically dense material it speeds up and bends away from the normal. If it hits the boundary face on it won't refract.

9. Dangers of electromagnetic waves

   • UV radiation damages surfce cells which can lead to sunburn, blindness and incresed risk of skin cancer. X-rays and gamma rays are types of ionising radiation which can cause gene mutation and cancer. Radiation dose is measured in sieverts and is a measure of the risk harm from the body being exposed to radiation.

10. Visible light and filters

    • Opaque objects do not transmit light they only absorb some colours and reflect others. The colour of the object depends on which wavelengths of light are most strongly reflected. Black objects absorb all colours, whereas, white objects reflect all colours. Also, transparent and translucent objects transmit light.
    • A primary colour filter transmits that colour. For example, if you look at a blue object through a blue filter it will still look blue but if you looked at a red object through the same filter it will look black as the blue light is absorbed.

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