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. Main focussing Controls amount of light Main concentration of cones Fine tuning of focus Sends the signal to the brain Ciliary muscle changes the lens 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 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. 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 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 Cones and Colour 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

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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. 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. ......................................................................................................................... (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.

Questions Tsokos page 478 Q’s 1-3,6-9, 12-19.