Science 8: Unit C

1
Science 8 Unit C Light and Optical Systems

• Topic 4 Lenses and Vision

2
Convergence and Divergence

• Lenses through refraction cause light rays to
  either
• Converge light rays move together. or
• Diverge light rays move apart from each other.
• A concave lens causes light rays to diverge.
• A convex lens causes light rays to converge.

3
Convex Lenses and Focal Points

• Convex lenses cause light rays to converge to a
  single point the focal point. Of course, light
  rays keep moving past this point, but as they do
  they become inverted (upside down). The top rays
  will become the bottom rays, and the bottom rays
  will become the top rays.

4
Concave Lenses and Divergence

• Concave lenses cause light rays to diverge or
  spread apart. The light rays never touch or cross
  over.
• The image through a concave lens is always
  right-side-up.

5
Convex Lenses and Real Images

• A convex lens can focus the image onto a screen.
  Any image that you can form onto a screen or
  surface is called a real image. Any real image
  formed (by a camera, projector, eyeball) will be
  inverted.

6
Concave Lenses and Virtual Images

• A concave lens cannot do this as the light rays
  are not focused, but diverge. You can see the
  image on the mirror, but no image can be formed
  on a screen. This kind of image is called a
  virtual image.

7
The Eye

• Images are formed on the retina of the eye, which
  acts as the film. The focusing is done by the
  lens, controlled by the ciliary muscles.
• The ciliary muscles focus the lens by changing
  its shape and thickness. This process is called
  accommodation.

8
Near and Far Point of the Eye

• The nearest point that an eye can focus on
  comfortably is called the near point and for the
  average human this is 25 cm. The furthest point
  that an eye can focus on comfortably is called
  the far point and for the average human it is
  said to be infinity.

9
The Iris and Iris Reflex

• The amount of light that enters the eye is
  controlled by the size of the pupil, which, in
  turn, is controlled by the size of the iris. The
  iris automatically thickens and shrinks depending
  on the light environment. This process is called
  the iris reflex.

10
The Cornea and the Retina

• The surface of the eye is protected by the
  cornea, a tough surface which keeps the sensitive
  parts of the eye protected.
• The image formed on the retina is sent to the
  brain via the optic nerve. The brain then makes
  sense of the image. The location of the optic
  nerve on the retina creates a blind spot. Because
  we have two eyes with two different blind spots,
  we do not notice as the brain compensates for the
  gaps in vision.

11
Near and Far Sightedness

• If an image is focussed before the retina, then
  the eye is near-sighted. The person sees blurry
  images except for closer objects. The problem is
  corrected with concave lenses which cause the
  rays to diverge and travel further, focusing on
  the retina.
• If an image not focussed by the time it hits the
  retina, then the eye and the person are
  far-sighted. The person see blurry images except
  for far away objects. The problem is corrected
  with convex lenses which cause the rays to
  converge and focus on the retina.

12
Eye Versus Camera

• Eye
• Function to translate light energy into
  electrochemical signals.
• Where does light into eye? The Pupil
• What controls the amount of light entering eye?
  The Iris muscles.
• What Focusses the light onto retina? The lens
• What adjusts the lens to focus images onto
  retina?
• - the ciliary muscles.
• What forms and stores the image? - The retina
• What protects contents of the eye? The cornea
• What provides the eye with structural strength? -
  The vitreous and aqueous humours.

13
Eye Versus Camera

• Camera
• Function to translate light energy into
  chemical signals (Film) or electric signals
  (digital).
• Where does light into eye? The aperture
• What controls the amount of light entering eye?
  The diaphragm.
• What Focusses the light onto retina? The lens
• What adjusts the lens to focus images onto
  retina?
• - the focussing ring.
• What forms and stores the image? - The film
• What protects contents of the eye? The outer
  shell
• What provides the eye with structural strength?
  The outer shell