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Sound and light (light (Imaging in mirror (appears to be behind the mirror…
Sound and light
light
how does light travel
Light travels as waves. These are transverse waves, like the ripples in a tank of water. The direction of vibration in the waves is at 90° to the direction that the light travels. Light travels in straight lines, so if you have to represent a ray of light in a drawing, always use a ruler.
Unlike sound waves, light waves can travel through a vacuum (empty space). They do not need a substance to travel through, but they can travel through transparent and translucent substances.
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refraction
the light slows down going into a denser substance, and the ray bends towards the normal
the light speeds up going into a less dense substance, and the ray bends away from the normal
Focusing
convex lens
A convex lens is made from a transparent material that bulges outwards in the middle on both sides. It can focus light so that appears to meet at a single point, called the focal point. Light is refracted as it passes into, then out of, the lens.
The pinhole camera
A pinhole camera consists of a box or tube with a translucent screen at one end and a tiny hole (the pinhole) made in the other end. Light enters the box through the pinhole and is focused by the pinhole onto the translucent screen. The image is upside down and smaller than the object.
Colour
White light is a mixture of many different colours, each with a different frequency. White light can be split up into a spectrum of these colours using a prism, a triangular block of glass or Perspex.
Light is refracted when it enters the prism, and each colour is refracted by a different amount. This means that the light leaving the prism is spread out into its different colours, a process called dispersion.
Detecting light
The camera
When the film absorbed light, a chemical change produced an image in the film, called the ‘negative’. This was used to produce a photograph on photo-sensitive paper.
the eye
Like the camera, the eye focuses light from an object onto a photo-sensitive material. However, in the eye, this material is the retina. The retina contains cells that are sensitive to light. They produce electrical impulses when they absorb light. These impulses are passed along the optic nerve to the brain, which interprets them as vision.
Sounds
properties
Longitudinal waves
Sound waves are longitudinal waves - the vibrations are in the same direction as the direction of travel.
Loudspeakers
Sound waves are produced by all vibrating objects. Loudspeakers work by converting electrical energy into kinetic energy. This moves the cone which creates the sound waves.
Reflection
Sound waves can reflect off surfaces. We hear sound reflections as echoes. Hard, smooth surfaces are particularly good at reflecting sound.
Detecting sounds
Ears
We can detect sound using our ears. An ear has an eardrum inside, connected to three small bones. The vibrations in the air make the eardrum vibrate, and these vibrations are passed through the three small bones (called ossicles) to a spiral structure called the cochlea. Signals are passed from the cochlea to the brain through the auditory nerve, and our brain interprets these signals as sound.
Microphones
Mobile phones and telephones contain microphones. These devices contain a diaphragm, which does a similar job to an ear drum. The vibrations in air make the diaphragm vibrate, and these vibrations are changed to electrical impulses. In the lab, the electrical impulses can be sent to an oscilloscope, which represents them as a graph on a screen.
Oscilloscope traces
the amplitude is the maximum height of the wave from its resting position – the greater the amplitude, the louder the sound
the wavelength is the distance between the crests (tops) of two waves next to each other (or any other two identical point on waves next to each other)
the frequency is the number of waves per second – the higher the frequency, the closer together the waves are and the higher the pitch
Ultrasound
Sound with a frequency of more than 20,000 Hz is called ultrasound. It is too high pitched for humans to hear, but other animals (such as dogs, cats and bats) can hear ultrasound.
Ultrasound has many applications in medicine, including ultrasound scans to check on the health of unborn babies.
Speed
Clap-echo method
This method involves measuring the time taken for you to hear an echo from a sharp clap. You stand a long distance from a wall, clap, and listen for the echo. The distance travelled is twice the distance from you to the wall (because the sound has to travel to the wall and back).
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