The eye and sight
Describe the basic structure of the
human eye. The structure should be limited to those features affecting the physical
operation of the eye.
The “viewing
screen” of the eye
Main focussing
Controls amount
of light
Main concentration
of cones
Fine tuning
of focus
Ciliary muscle
changes the lens
Sends the signal
to the brain
aka - ciliary fibers...Changes
the shape of the lens
State and explain the process of
depth of vision and accommodation.
The near point and the far point of the eye for normal vision are also included.
(Closest distance without
straining) near point
(25cm)
far point
Hyperlink
Accommodation is the process by which the eye increases its optical power to
maintain a clear image (focus) on an object as it draws near the eye.
State that the retina contains rods and cones, and
describe the variation in density across the surface
of the retina.
Describe the function of the rods and of the
cones in photopic (cones) and scotopic
(rods) vision.
Students should be able to sketch and interpret spectral response graphs and
give an explanation for colour blindness.
S,M,L = short, medium and long
wavelengths of cones
R = rods
Photopic vision is the vision of the eye under welllit conditions. In humans and many animals,
photopic vision allows colour perception, mediated
by the cone cells.
Scotopic vision is the monochromatic vision of the
eye in low light. Since the cone cells are
nonfunctional in low light, scotopic vision is produced
exclusively through the rod cells so therefore there is
no colour perception.
Scotopic and photopic
vision
The sensitivity of the cones in your
eye is known as the photopic
response and refers to colour vision
and the perception of fine detail.
The sensitivity of the rods in your
eye is known as the scotopic
response and refers to vision
under conditions of low level light
intensity – so called ‘night vision’.
Colour blindness
Hyperlink
Hyperlink
Cones and Colour
There are "red," "blue," and "green" cones, which are sensitive
to those colors and combinations of them. You need all three
types to see colours properly. When your cones don't work
properly, or you don't have the right combination, your brain
doesn't get the right message about which colours you're
seeing.
Describe colour mixing of light by
addition and subtraction.
Students should be able to “identify” primary
and secondary colours.
Hyperlinks
Hyperlink
Questions
E.g. Shining white light on a yellow
book.
1. The graph below shows the overall relative light absorption curve for the lightsensitive cells involved in scotopic vision. The relative light absorption is expressed
as a percentage of the maximum.
relative light absorption
100
60
80
40
20
0
400
500
600
700
wavelength / nm
(a)
State the name of the cells involved in scotopic vision.
...................................................................................................................................
(1)
(b)
(i)
On the axes above, sketch a relative light absorption curve for a cell involved in photopic vision.
(2)
(ii)
State the colour to which the cell is most sensitive.
.........................................................................................................................
(1)
(c)
Outline how colour blindness may arise from defects in the retina’s light sensitive cells.
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
...................................................................................................................................
(3)
(Total 7 marks)
Discuss the effect of light and dark,
and colour, on the perception of
objects.
Students should consider architectural effects of
light and shadow (for example, deep shadow gives
the impression of massiveness). Glow can be used
to give an impression of “warmth” (for example,
blue tints are cold) or to change the perceived
size of a room (for example, light-coloured ceilings
heighten the room).
TOK: This can contribute to a discussion on
perception.