Convex & Concave Lenses (OCR Gateway GCSE Physics)

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

How to construct a ray diagram showing the refraction of light as it passes through a rectangular block

• 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
  • This point is called the 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 a focal point behind the retina
  • When the glasses are not there, the rays do not converge properly at a principal focus so the person cannot seen an image very well

The focal length is the distance from the lens to the 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

Parallel rays from a concave lens appear to come from the principal focus

Representing Lenses

• In diagrams, the following symbols are often used to represent each type of lens:

Concave and convex symbols

Step 1: Draw a parallel ray on the left

Step 2: Draw the refracted ray at the first surface

• • As the ray enters the block it bends towards the normal since it is going into a denser material
  • In this case, the angle of refraction is smaller than the angle of incidence

Step 3: Draw the refracted ray at the second surface

• • As the ray leaves the block it bends away from the normal
  • In this case, the angle of refraction is larger than the angle of incidence