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 and sightDescribe the basic structure of thehuman eye. The structure should be limited to those features affecting the physical operation of the eye.

Main focussing

Fine tuning of focus

Controls amount of light

aka - ciliary fibers...Changes the shape of the lens

Ciliary muscle changes the lens

Sends the signal to the brain

Main concentration of cones

The “viewing screen” of the eye

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

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.

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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.

Photopic vision is the vision of the eye under well-lit 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.

S,M,L = short, medium and long wavelengths of cones

R = rods

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

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Cones and ColourThere 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.

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Describe colour mixing of light byaddition and subtraction.Students should be able to “identify” primary and secondary colours.

Hyperlinks

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Questions

E.g. Shining white light on a yellow book.

1. The graph below shows the overall relative light absorption curve for the light-sensitive cells involved in scotopic vision. The relative light absorption is expressed as a percentage of the maximum.

1 0 0

8 0

6 0

4 0

2 0

0

re la tiv e lig h t ab so rp tio n

7 0 06 0 05 0 04 0 0

(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)

wavelength / nm

Discuss the effect of light and dark,and colour, on the perception ofobjects.

Students should consider architectural effects oflight and shadow (for example, deep shadow givesthe impression of massiveness). Glow can be usedto give an impression of “warmth” (for example,blue tints are cold) or to change the perceivedsize of a room (for example, light-coloured ceilingsheighten the room).TOK: This can contribute to a discussion onperception.