physics - waves

• wavelength - distance from a point on one wave to the equivalent point on the adjacent wave
• waves in a ripple tanks

  1. Count the number of waves passing a point in 10 s to get 10 x frequency. Repeat this twice more, discard anomalies and calculate a mean value for 10 x frequency.
  2. Divide by 10 to get the frequency
  3. Measure across 10 wavelengths using a ruler, then divide by 10 to get wavelength. Taking a photo will further improve accuracy.
  4. Calculate wave speed by multiplying frequency. by wavelength .
• Measuring speed of sound in air

  The transducer produces pulses of sound (or ultrasound) waves and measures the time taken for the echo to return.

  The distance travelled by the sound wave can be found by measuring  the distance from the transducer to the reflective board with a ruler  and doubling it.

  The speed of sound in air can be determined by dividing distance travelled by time taken.
• Method 2: Waves in a solid (on a string)

  Measure the wavelength using a ruler by measuring the distance between adjacent wave crests .

  Read the frequency off the supply.

  Calculate wave speed by multiplying frequency by wavelength .

  Increase the frequency of the supply collect more data to see whether wave speed remains constant for different frequencies of waves on the string.
• Experiment 1: Determining which surfaces are the best emitters of infrared

  The cube is filled with very hot water .

  The matt black surface emits the highest intensity infrared radiation. The shiny sliver surface emits the lowest intensity radiation. This shows that matt black surfaces are the best emitters of infrared and shiny sliver surfaces are the worst emitters of infrared.

  An important control variable in this experiment is the distance from the detector to the surface of the cube. This should be measured with a ruler .
• Experiment 2: Determining which surfaces are the best absorbers of infrared

  The temperature rise of the water in the boiling tube with a  matt black surface is greatest. This shows that matt black surfaces are also the best absorbers of infrared and that shiny silver surfaces are the best reflectors of infrared.

  Important control variables in the experiment are: distance between boiling tube and heater , heating time , volume of water in each boiling tube, surface area of boiling tubes.
• All electromagnetic waves have oscillating/ vibrating electric and magnetic fields, are transverse , can travel through a vacuum , and travel at the speed of light. (3 x 108 m/s) through a vacuum.
• The order of the electromagnetic spectrum from longest wavelength to shortest wavelength (or lowest frequency to highest frequency) is: radio waves, …microwaves, …infrared., visible light, ultra violet , x-rays and gamma rays . The electromagnetic spectrum is known as a …continuous spectrum, because wavelength and frequency can have any numerical value. However, it is convenient to put electromagnetic waves into categories depending on their properties.
• Radio waves are produced by oscillations in electrical circuits (alternating current), when they are absorbed they induce alternating current in the receiver aerial at the same frequency. as the radio wave. Radio waves are used for radio communication and …terrestrial… television broadcasting.
• Microwaves can pass through a layer in the earth’s atmosphere known as the ionosphere and therefore can be transmitted  into space . Microwaves are used for satellite television broadcasting and mobile phone communication. Water molecules absorb microwaves causing the temperature of the water to increase . This principle is used in a microwave oven to cook food. Microwaves may be hazardous to us as they cause heating with our bodies.
• Infrared radiation: all objects emit and absorb infrared radiation. The hotter an object the …more infrared radiation it radiates in a given time. An object at constant temperature absorbs radiation at the same rate as it emits it. If an object absorbs radiation at a faster rate than it emits it, its temperature will increase . If an object emits radiation at a faster rate than it absorbs it, its temperature will decrease .
• We use infrared radiation for heating, thermal imaging., remote controls and fibre optic communication.
• Visible light is electromagnetic radiation that we can detect with our eyes . Amongst many other things, we use visible light for photography and fibre optic communication.
• Ultraviolet, x-rays and gamma rays have higher frequency and, therefore, higher energy . This makes them hazardous.
• Ultraviolet damages out eyes and skin, and can cause skin cancer. Some objects fluoresce (absorb UV and emit visible light) under ultraviolet light. This principle is used in security markers, washing powders and energy efficient lighting. .
• X-rays and gamma rays are both ionising radiation. This means these radiations turn atoms into ions. by removing electrons . If a human has been irradiated (absorbed x-rays or gamma rays) it is possible that atoms in their cells/ DNA may be ionised, which can either cause cell death or mutations such as causing the cell to become cancerous. The dose of ionising radiation is measured in Sieverts .
• X-rays darken photographic film, are transmitted. through soft tissue and are absorbed by bone. This is the principle behind x-ray imaging.
• Both x-rays and gamma rays can be used to kill cancer cells. This process is known as radiotherapy. It is important that the beam is narrow to minimise the damage to healthy cells. Gamma rays are also used to sterilise…. food and …medical…. equipment.
• Amplitude is the maximum displacement of the wave from the …undisturbed position.
• When light travels from a less dense medium to a more dense medium its speed decreases, its wavelength …decreases., and it changes direction towards the normal line (unless it enters the new medium along the …normal line, in which case its direction does not change and therefore the wave is not refracted).
• When light travels from a more dense medium to a less dense medium its speed …increase., its wavelength increases., and it changes direction …away from the normal line (unless it enters the new medium along the normal line, in which case its direction does not change and therefore the wave is not refracted).
• Refraction takes place at the boundary (or interface) between different media

  The normal line makes an angle of …90.0 with the interface.

  The angle of incidence is the angle between the incident ray and the normal line .

  The angle of refraction is the angle between the refracted ray and the normal line.

  Wavefronts mark positions on a wave that are separated by one wavelength. They are perpendicular to the direction of the ray
• Water waves slow down as they enter shallower water and speed up as they enter deeper water.
• Explaining why the waves are not refracted when they travel along the normal line

  On entering the shallower water (water wave) or glass (light wave) there is no change in direction because every point on the wavefront crosses the boundary at the same time and so every point on the wavefront slows down at the same time. Therefore, waves are not refracted when they travel at 90 degrees to a boundary
• Explaining why the waves are refracted when they travel at an angle to the normal line

  On entering the shallower water (water wave) or glass (light wave) the edge of the wavefront that crosses the boundary first is slowed whilst the edge that has not crossed the boundary continues at the same speed. This causes a change in direction. Hence, the wave is refracted when it crosses a boundary at some angle to the normal line.
• transverse waves - oscillations are at 90 degrees or perpecndiulcar to directions of energy transfer e.g electromagnetic waves
• longtiduadianl waves - oscillations are parrell to doirection of energy transfer. e.g sound waves