lenses

• Lens


  A precisely shaped piece of glass or plastic that refracts light
• Lenses


  • Used in cameras, telescopes, binoculars, microscopes and corrective glasses
• Convex lens


  Thicker in the middle than at the edges
• Convex lens


  1. Parallel light rays entering the lens converge

  2. Light rays come together at a point called the principal focus
• Focal length


  The distance between the centre of the lens and the focal point
• Concave lens


  Thinner in the middle than at the edges
• Concave lens


  1. Parallel light rays entering the lens diverge

  2. Light rays appear to come from a principal focus on the other side of the lens
• In a ray diagram, a convex lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens
• In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens
• Lens


  Precisely shaped piece of glass developed and used in corrective glasses, telescopes, microscopes, binoculars, and magnifying glasses
• Real image


  • Image that is formed where the rays of light are focused
  • Can be projected onto a screen
• Virtual image


  • Image from which rays of light appear to come but do not do so in reality
  • Appears to come from behind the lens
• Drawing a ray diagram

  1. Draw a ray from the object to the lens that is parallel to the principal axis

  2. Draw a ray which passes from the object through the centre of the lens
• Convex lens


  Object or shape that curves or bulges outwards, like a circle or sphere
• For a distant object placed more than twice the focal length from a convex lens


  The image is inverted, diminished, and real
• For an object placed between one and two focal lengths from a convex lens

  The image is inverted, magnified, and real
• Lenses


  Precisely shaped pieces of glass that have been developed and used in corrective glasses, telescopes, microscopes, binoculars, and magnifying glasses
• For an object placed less than one focal length from a convex lens

  The image is upright, magnified, and virtual
• Lenses


  • Produce magnified images
  • Produce real and virtual images
• Concave lens


  Lens that always produces images that are upright, diminished, and virtual
• Magnification


  The ratio of the image height to the object height when using lenses
• Calculating magnification


  Magnification = image height / object height
• For an object viewed through a concave lens


  Light rays from the top of the object will be refracted and diverge on the other side of the lens, making the image appear upright and smaller than the object
• Magnification examples


  • Object 2 cm tall, image 250 cm tall (magnified 125 times)
  • Object 1.8 m tall, image 1.8 cm tall (diminished to 0.01 times)
• Magnification has no units as it is a ratio of two lengths
• An object 3 mm tall forms an image 12 mm tall. The magnification is 4
• Colour lenses


  Precisely shaped pieces of glass that have been developed and used in corrective glasses, telescopes, microscopes, binoculars, and magnifying glasses
• Within the visible light range of the electromagnetic spectrum, there is a spectrum of colour
• Continuous range of colours

  Variation that shows a wide range of intermediate values between two extremes. They can be measured.
• Colours in order of increasing frequency and decreasing wavelength

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet
• Each colour within the visible light spectrum

  Has its own narrow band of wavelength and frequency
• Absorption of light


  1. Waves can be absorbed at the boundary between two different materials

  2. When waves are absorbed by a surface, the energy of the wave is transferred to the particles in the surface

  3. This will usually increase the internal energy of the particles
• When white light shines on an opaque object


  Some wavelengths or colours of light are absorbed, and the other wavelengths are reflected
• Grass appears green in white light


  Red, orange, yellow, blue, indigo and violet are absorbed by the grass, and green light is reflected by the grass and detected by our eyes
• Transmission of light


  1. Waves can be transmitted at the boundary between two different materials

  2. When waves are transmitted, the wave continues through the material
• Transparent materials


  Allow light to be transmitted with very little absorption
• Translucent materials


  Transmit some light but are not completely clear
• Colour filters


  1. When white light passes through a coloured filter, all colours are absorbed except for the colour of the filter

  2. Only the colour of the filter will be observed by the human eye
• An object appears black if it absorbs all the wavelengths of visible light


  An object that appears blue in white light will appear black in red light, because the red light contains no blue light for the object to reflect