P7.2

HideShow resource information
• Created by: amy_mair
• Created on: 23-06-16 11:35
• P7.2
• Refraction
• Waves change speed and direction
• 1. The speed of a wave is affected by the density of the substance or medium it is travelling in
• 2. So when a wave crosses a boundary between two substances it changes speed
• 3. Since wave speed = frequency x wavelength
• 4. The change in speed and wavelength can cause to change direction too this is called refraction
• If a light wave hits the boundary face on it slows down but carries on in the same direction. It now has a shorter wavelength but the same frequency
• But if a wave metts a different medium at an angle, part of the wave hits the bounday first and slows down while another part carries on at the first, faster speed for a while. So the wave changes direction it has been refracted
• 5. Converging or convex lenses use refraction to focus light waves to form an image
• 6. .As a light ray hits the surface of the lens and passes from air to glass, it slows down. This causes the light ray to bend towards the normal
• 8. The curvature of the lens means all the parallel rays hitting different parts of the lens bent towards the same focal point, where an image is formed of whatever the light is coming from
• Spectrums
• A triangular prism can refract light
• 1. .Different wavelengths of light refract by different amounts. So white light disperses into its different colours as it enters the prism
• 2. A rectangular prism has parallel boundaries
• 3. But with a triangular prism, the boundaries are not parallel which means the differernt wavelengths do not recombine
• 7. When it hits the glas to air boundary on the other side it speeds up and bends away from the normal
• Lenses
• Converging lenses
• 1. A converging lens is convex- it gets fatter towards the middle. It causes rays of light to converge (come together) to a focus
• 2. All lenses have a principal axis a line which passes straight through the middle of the lens
• 3. The focal point of the lens is where rays are initially parallel to the principle axis meet.
• 4. The focal length of the lens is just the distance between the middle of the lens and its focal point
• 5. Focal length is related to power. The more powerful the lens, the more strongly it converges parallel rays of light, so the shorter the focal length
• 6. To make a more powerful lens from the same material, you just make it with a more strongly curved surface
• Power (D) = 1 / Focal Length
• A lot of objects like stars are so far away that you can think of them as just dots of light. Light rays from them are effectively parallel by the time they reach earth
• Telescopes
• Simple refracting telescopes
• Use 2 converging lenses
• 1. A simple refracting optical telescope is made up of two convex lenses with different powers
• An objective lens and a more powerful eyepiece. The objective lens collects the light from the object being observed and forms an image of it, and the eyepiece magnifies this image so we can view it
• 2. The lenses are aligned to have the same principle axis and are placed so that their focal points are in the same place
• 3. Many objects in space are so far away that by the time their light arrives on earth the light rays are effectively parallel
• 4. The objecive lens onverges these parallel rays to form a real image beteween the two lenses
• 5. The eye piece lens is much more powerful then the objective lens as it is more curved.
• It acts as a magnifying glass on the real image and makes a virtual image- where the light entering the eye lens appears to have come from
• 6. The angular magnification, M, of the telescope can be calculated from the focal lengths of the objective lens
• Magnification = Focal length of the objjetive lens / focal length of the eye lens
• Astronomical telescopes
• 1. Most astronomial telescopes use a concave mirror instead of a obejctive lens
• 2. Concave mirrors are shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge
• 3.  Convcave mirrors are like a portion of a sphere. The centre of the sphere is the centre of curvature
• 4. The centre of the mirrors surface is called the vertex
• 5. Halfway between the centre of curvature and the vertex is the focal point
• 6. Rays parallel to the mirrors axis, those from a distant star reflect and meet at the focal point
• 6. Rays, parallel to the mirrors axis
• 7. By putting a lens near the focal point of the mirror to act as an eyepiece, you can form a magnified image
• Diffraction and Telescopes
• Diffraction
• 1. All waves spread out at the edges when they pass through a gap or past an object
• 2. The amount of diffraction depends on the size of the gap relative to the wavelength of the wave.
• The narrower the gap, or the longer the the wavelength, the more the wave spreads out
• 3. A narrow gap is one about same size as the wavelength of the wave
• 4.So whether a gap counts as narrow or not depends on the wave in question
• 5. Light has a very small wavelength- it can be diffracted but its only when you have a really small gap that you notice any effects
• Aperture
• 1. Some objects in the sky are distant and faint, only a tiny amount of radiation from them reaches us
• 2. To collect enough of the radiation from these objects to see them, you need to use a telescope with a huge objective lens the more radiation can get into the telescope and the better the image formed
• 3. The only way round this problem is to have an aperture that is much wider than the wavelength of radiation you want to look at. This way the radiation passes through the aperture and into your telescope with little diffraction and you get a sharp image
• Diffrction Grating
• 1.A dffraction grating has very narrow slits- smalll enough  to diffact light
• 2. When white light passes through the gaps in a diffraction grating, the different wavelngths of coloured lights are all diffracted but by different amounts
• 3. This creates a spectrum of different coloured light
• 4. Astronomers can use these spectra to analyse the light coming from stars