Concave & convex lenses

• Refraction Ray Diagrams

  • Refraction occurs when light passes a boundary between two different transparent media
  • At the boundary, the rays of light undergo a change in direction
  • The direction is taken as the angle from a hypothetical line called the normal
  • This line is perpendicular to the surface of the boundaries and is usually represented by a straight dashed or dotted line
• The change in direction depends on which media the light rays pass between:

  • From less dense to more dense (e.g air to glass), light bends towards the normal
  • From more dense to less dense (e.g. glass to air), light bends away from the normal
  • When passing along the normal (perpendicular) the light does not bend at all
• • The change in direction occurs due to the change in speed when travelling in different substances
  • When light passes into a denser substance the rays will slow down, hence they bend towards the normal
  • The only properties that change during refraction are speed and wavelength – the frequency of waves does not change
  • Different frequencies account for different colours of light (red has a low frequency, whilst blue has a high frequency)
  • When light refracts, it does not change colour (think of a pencil in a glass of water), therefore, the frequency does not change
• Convex & Concave Lenses

  • A lens is a piece of equipment that forms an image by refracting light
  • There are two types of lens:
  • Convex
  • Concave
• Convex Lenses

  • In a convex lens, parallel rays of light are brought to a focus (principal focus).
  • This lens is sometimes referred to as a converging lens
  • The distance from the lens to the principal focus is called the focal length
  • This depends on how curved the lens is
  • The more curved the lens, the shorter the focal length
  • Convex lenses are commonly used in glasses to correct vision
  • The lens act with the eye to make sure the rays are directed to the retina
  • When the glasses are not there, the rays do not converge properly at a principal focus.
• Concave Lenses

  • In a concave lens, parallel rays of light are made to diverge (spread out) from a point
  • This lens is sometimes referred to as a diverging lens
  • The principal focus is now the point from which the rays appear to diverge from
  • Concave lenses are commonly used in:
  • Binoculars
  • Telescopes
  • Flashlights
  • Cameras
  • They primarily magnify an image
• Convex & Concave Ray Diagrams

  • Lenses can be used to form images of objects placed in front of them
  • The location (and nature) of the image can be found by drawing a ray diagram
• Convex Lens Ray Diagrams

  • If an object is placed further from the lens than the focal length f then a real image will be formed, and the converging lens ray diagram will be drawn in the following way:

  1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel straight
  2. Draw a ray going from the top of the object to the lens. When this ray emerges from the lens it will travel directly through the principal focus f
  3. The image is the line drawn from the axis to the point where the above two rays meet

  
• • When describing an image, consider if it is:
  • Real or virtual 
  • Magnified (larger) or diminished (smaller)
  • Upright or inverted
• • Real: the light rays meet each other after refraction
  • Magnified: the image is larger than the object
  • Inverted: the image is formed on the opposite side of the principal axis
  • Virtual: the light rays appear to meet when produced backwards
  • Magnified: the image is larger than the object
  • Upright: the image is formed on the same side of the principal axis
• Concave Lens Ray Diagrams

  • Concave (diverging) lenses can also be used to form images, although the images are always virtual in this case
• Comparing Convex & Concave Lenses

  • The image produced by a convex lens can be either real or virtual
  • This means the image can be inverted (real) or upright (virtual)
  • The image produced by a concave lens is always virtual
  • This means the image will always be upright