SIGHT AND WAVE PHENOMENA

SIGHT AND WAVE PHENOMENA


15

(SL Option A1) The eye and sight

Vitreous humour

A.1.1 Describe the basic structure of the human eye.

Retina

A.1.3 State that the retina contains rods and cones, and describe the variation in density across the surface of the retina. A.1.4 Describe the function of the rods and of the cones in photopic and scotopic vision. A.1.5 Describe colour mixing of light by addition and subtraction. A.1.6 Discuss the effect of light and dark, and colour, on the perception of objects. © IBO 2007

A.1.1 BASIC STRUCTURE OF THE EYE

T

he eye operates like a camera. Just as a camera consists of a lens, an aperture or shutter and a ilm, the eyeball needs a lens to refract and focus light on the retina. his is where an upside-down image is formed and the image is converted to electrical impulses that are sent to the brain via the optic nerve, and translated into the upright position. Figure 1501 shows the horizontal section through a human eye.

Sclera

Iris Pupil

Optic nerve

Lens Conjunctiva

Blind spot Blood vessels

Figure 1501 Horizontal section through the human eye he eyeball lies in a special cavity in the skull that contains fatty tissue to protect the eye. he wall of the eye has 3 layers – the outer wall consisting of the sclera and cornea, the middle wall consisting of a choroid, ciliary muscle and iris, and the inner layer consisting of the retina. he white of the eye is moved about by six muscles and the white ibrous coat at the front of the eye contains the cornea which acts like a window of the eye. Inside the white ibrous coat is a black layer that makes the eye “light-tight”, and further in to the centre is the colorpigmented part called the iris which contains muscles that adjust the amount of light entering the hole in its centre called the pupil. Behind the pupil is a converging lens that is connected to the ciliary muscles. he pupil is very small in bright light and relatively big in dark light. he shape of the lens is controlled by the ciliary muscles, and the lens becomes rounder and shortens its focal length to

353

OPTION

A.1.2 State and explain the process of depth of vision and accommodation.

Cornea

CHAPTER 15 (OPTION A) view close objects, and, for distant objects it becomes less round and increases its focal length. In a normal eye, a real, diminished and inverted image of an object comes into focus on the retina ater refraction in the lens. Figure 1502 lists the main similarities of the components of the eye and the camera that were briely mentioned in the beginning of this section. Component of the Eye Cornea Iris Pupil Lens Retina Choroid Sclera

Component of the Camera Aperture for admitting light Aperture diaphragm Hole in the diaphragm Camera lens Film Black lining Camera case

Figure 1502 Components of the eye and the camera

A.1.2 DEPTH OF VISION AND

focal length of the lexible eye lens. he eye has most accommodation for prolonged viewing when viewing at the far point. he apparent size of an object can be increased by using a converging lens to allow the object to be brought closer to the eye, thus increasing the size of the image on the retina. his is the basis behind the simple magniier.

A.1.3 RODS AND CONES he retina contains two photoreceptors called rods and cones that have complementary properties. Rods have fast response rates, and are sensitive at low light levels but they are insensitive to colour. here are around 120 million of them. On the other hand, cones have slow response rates, and are insensitive at low light levels but are sensitive to particular wavelengths of light, and give us our colour vision. here are around 6.5 million of them. It is believed that the cones can be divided into three colour groups red cones (64%), green cones (32%), and blue cones (2%). he variation in density across the surface of the retina is shown in Figure 1504. Visual axis

he size of any image formed on the retina of the eye depends on the angle subtended by the object at the eye. he closer the object is to the eye the greater will be the angle and thus the angular magniication as shown in Figure 1503. However, if an object is too close to the eye then there is diiculty focusing the image.

80º

80º

far point

60º

60º

Blind spot near point

fp

20º

20º

np

Figure 1503

0º

Near and far point of the eye Fovea

he range over which an eye can sharply focus an image is determined by what are known as the near point and far point of the eye. he near point is the position of the closest object that can be brought into focus by the unaided eye. he near point varies from person to person but it has been given an arbitrary value of 25 cm. he far point is the position of the furthest object that can be brought into focus by the unaided eye. he far point of a normal eye is at ininity. he ability of the eye to focus over this range is called accommodation and this is controlled by the ciliary muscles pulling or relaxing in order to change the

354

40º

40º

Density in thousands per square mm

OPTION

ACCOMMODATION

Optic nerve

200

Cone density

150

100

Rod density 50

0 -80

-60

-40

-20

0

20

40

60

80

Angular separation from fovea (degrees)

Figure 1504 Variation of cone and rod density of the iris


A.1.4 PHOTOPIC AND SCOTOPIC VISION

507 nm

Rods are responsible for scotopic vision which is the ability to see at low light levels or vision “in the dark” or light levels below 0.034 candela per square metre – 0.034 cdm-2. hey do not mediate colour and are sometimes termed “colour blind”. Because they do not mediate colour, they are said to have low spatial resolution (acuity). hey are excellent photoreceptors because of their high sensitivity as shown in Figure 1505. heir single absorption maximum is 1700 lumens per watt (unit for luminous lux) at 507 nm. his is in the blue region of visible light and this is the reason why the rods do not mediate colour, and are more sensitive to blue in the night.

1700 lumens per watt

conditions during the day. he pigments of the cones are of three types – long wavelength red, medium wavelength green and short wavelength blue. he cones are less sensitive to light than the rods with their single absorption maximum is 683 lumens per watt (unit for luminous lux) at 555 nm. he cone vision can adapt to changing levels of light more rapidly than the rods and as such they have high spatial resolution. he response curve of Figure 1506 shows that the pigments of the cones have maximum absorbance values at the wavelengths of 445 nm (blue), 535 nm (green) and 705 nm (red). Each response curves overlaps to provide vision within the visible region of the electromagnetic spectrum. (445 nm)

Absorbance (in %)

From the igure it can be seen that cones are concentrated around the fovea region and then have very low density in other areas of the retina. Rods are not found around the fovea but are in high concentrations in other retinal areas. Both cones and rods are absent at the blind spot. Red and green responsive cones are concentrated in the fovea region and blue responsive cones are found outside the foveal region.

100 90 80 70 60 50 40 30 20 10 0

380

420

(575 nm)

460 500

540 580 620

(705 nm)

660 700

740 780

Wavelength (in nm)

Figure 1506 Absorbance values for the 2 types of cones Colour blindness or colour vision deiciency is most commonly a hereditary condition that can afect up to 12% of males and about 1% of females. About 99% is red-green colour blindness although blue-yellow colour deiciency also exists. It cannot be cured at this stage.

Scotopic vision (dark adapted)

Lumens per watt

555 nm

1000

683 lumens per watt

It occurs as a result of either a reduction of the pigment in the cones or if one of the types of cones is completely missing.

500 Photopic vision (light adapted)

A.1.5 COLOUR MIXING BY ADDITION AND SUBTRACTION

400

500 600 Wavelength (nm)

700

Figure 1505 Absorption maxima for rods and cones Cones are responsible for photopic vision or high lightlevel vision, that is, colour vision under normal light

he three primary colours are red, green and blue. All visible colours can be perceived by the mixing combinations of the primary colours by addition or subtraction. In additive processes, colour is created by adding light to a dark background and in subtractive processes, pigments can be used to selectively block out white light. Red and blue are mixed to form magenta (a dark pink), green and blue are mixed to produce cyan

355

OPTION

1500

CHAPTER 15 (OPTION A) (a light blue) and red and green are mixed to form yellow. When the 3 primary colours are mixed, white light is obtained. Cyan and magenta are mixed to form blue, magenta and yellow are mixed to obtain red, and yellow and cyan are mixed to form green. When all the secondary colours are mixed, black is obtained.

A.1.6 LIGHT, DARK, COLOUR - THE PERCEPTION OF OBJECTS TOK What is perception? ‘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)’.

Exercise

1.

15.1

Match the component of the eye that is similar to the component of the camera in the table below.

Component Of he Eye Component Of he Camera Aperture for admitting light Aperture diaphragm Hole in the diaphragm Camera lens Film Black lining Camera case 2.

(a) (b) (c) (d)

Describe the properties of an image formed in the eye. State the component of the eye where the image is formed. Name the coloured part of the eye. Describe the function of the ciliary muscles.

© IBO 2007

OPTION

Perception is a process of acquiring, interpreting, selecting and organising sensory information. he eyes with a reaction time of around 190 ms can be used in combination to sense aspects of depth, colour and form. Depth is important in scenery background and in architecture. he further you look into the distance of a scenery such as the Grand Canyon, the more blurred it becomes due to the scattering of light. Shadows give objects depth and scope depending on the direction that the sun is coming from. In architecture, deep shadow gives the impression of massiveness. Colour is way of creating a feeling within an environment and perceiving what an object looks like against diferent colours. For example, if the eye concentrates on the central dot of four coloured dots set against a diferent coloured matrix, and the three other dots are made to rotate, the three dots disappear. he colour green evokes an impression of calmness while blue evokes a sense of depression. Red glow can give an impression of warmth while blue glow gives an impression of cold. Size is another way in which the brain operates as to how we perceive things. For example, light-coloured ceilings seem to heighten a room whereas soter colours tend to make a room smaller.

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3.

Deine the terms near point and far point and state the arbitary value of each term.

4.

Distinguish the diference between the 2 types of photoreceptors that are found in the retina.

5.

Describe the function of the rods and of the cones in photopic and scotopic vision.