P5.3 - Wave Interaction

• Ray diagrams are used to show what happens when electromagnetic waves hit a surface, or travel through matter.
• When you construct ray diagrams:

  • draw lines to represent rays.
  • draw a normal at 90 degrees to the surface and the point where the ray hits it.
  • measure the angles from the normal to the rays.

  
• Sound waves refract when speed changes.
• When light goes from air into glass (or another dense medium) at an angle, it slows down and bends towards the normal. If the difference if density is larger, then the change in direction is also larger.

  
• The earth is curved. If you want to send radio waves very long distances you reflect them from a layer of the atmosphere called the ionosphere.
• High-frequency radio waves, or microwaves, have smaller wavelengths so they can be used for satellite communication.
• What happens to electromagnetic waves depends on the wavelength. Walls transmit radio waves and microwaves, so your TV and mobile phone works. Walls absorb visible light, so you need windows.
• The atmosphere absorbs x-rays, and there are no natural x-ray sources.
• Gamma rays from the sun and space are absorbed by the atmosphere, but gamma rays from rocks could be detected in your house.
• Lower frequency radio waves reflect from the ionosphere but higher frequency waves pass through.

  
• A magnifying lens uses a convex lens.
• A convex lens refracts rays to a principle focus, or a focal point. When the rays going into the lens are parallel then the distance from the optical centre of the lens to the principle focus is called the focal length.

  
• If you have normal vision, the cornea and lens of your eye focus light onto your retina to produce a sharp image.
• Beams of light from a distant object are parallel, focus in front of the retina. This can be corrected with a concave lens - it spreads out the light so it focuses on the retina.

  
• Beams from a nearby object are diverging, focus behind the retina. This can be corrected with a convex lens - it refracts light inwards so it focuses on the retina.

  
• You can use ray diagrams to predict the type, size, and position of an image.
• The type of image depends on the position of the object in relation to the lens.
• Drawing ray diagrams:

  1. Draw a ray from the top of the object to the lens through the focal point. (rays parallel to the principle axis go through the focal point, but other rays do not).
  2. Draw a ray from the top of the object through the centre of the lens. By convention, you draw rays straight through a lens at the optical centre.
  3. Where two rays cross (for real) (or appear to cross for virtual) is the top of the image.
  4. When the bottom of the object is also on the principle axis, then the bottom of the image is also on the principle axis.

  
• Convex:

  • magnifying glass - virtual, magnified, upright.
  • camera / eye - real, diminished, inverted.
  • projector - real, magnified, inverted.
  • microscope / telescope - lens near object: real, inverted. Lens near eye: virtual, upright.

  Concave:

  • spy holes - virtual, diminished, upright.
  • back windows - virtual, diminished upright.

  
• Prisms refract light to see ROYGBIV.
• In a prism:

  • each frequency of light travels at slightly different speeds in glass.
  • each frequency of light is refracted by a different amount.
• Colours with a higher frequency are refracted more than those with a lower frequency. This spreads white light into a spectrum, this is called dispersion. The colours associated with the frequencies in white light spectrum are spectral colours.
• The retina of your eye is sensitive to three of the spectral frequencies: red, green and blue. Spectral colours trigger different signals in the red, green, and blue receptor cells. Your brain puts these together to make perceived colour.
• Reflections from a surface depends on the type of surface. You see images in mirrors because the reflection is specular, or regular. But not in a sheet of paper because the scattering is diffuse.
• Specular reflection - see reflection, diffuse scattering - dont
• Milk is white because the particles scatter all wavelengths
• Ink is black because the particles absorb all wavelengths.
• the atmosphere scatters light with short wavelengths - blue