The eye

Question 1

Cornea

Answer 1

protective transparent layer at the front of the eye. It has a fixed convex curvature and therefore acts as a ‘fixed focus’ lens.

Question 2

Eye lens

Answer 2

flexible and attached to ciliary muscles

Question 3

Ciliary muscles

Answer 3

muscles change the thickness of the eye lens which alters its optical power.

Question 4

Retina

Answer 4

form an image on the retina of the eye of any object within a range of distances.

Question 5

ciliary muscle fibres lie along concentric circles round the rim of the eye lens

Answer 5

-To view a near object, the eye muscles must become taut so that the muscle fibres shorten
and make the eye lens thicker and more powerful.
-To view a distant object, the eye muscles must relax so that the muscle fibres lengthen,
allowing the eye lens to become thin and less powerful.

Question 6

Iris job

Answer 6

controls the amount of light entering the eye

Question 7

iris consists of concentric and radial muscle

fibres

Answer 7

• In bright light, the concentric fibres contract and the radial fibres relax so the iris expands, making the eye pupil narrower so less light passes through it.
• In dim light, the concentric fibres relax and the radial fibres contract so the iris contracts, dilating (i.e. widening) the eye pupil so more light passes into the eye.

Question 8

Retina

what is it?

Answer 8

layer of light-sensitive cells at the back of the eye

Question 9

Fovea

Answer 9

The retinal cells are most dense at the fovea which is the region of the retina near the principal axis of the eye lens. The fovea consists mostly of cones whereas rods predominate near the periphery of the retina.

Question 10

Rods

Answer 10

sensitive to low levels of light intensity but cannot distinguish between colours

Question 11

Cones

Answer 11

three types, each sensitive to a different range of wavelengths (different colours)

Question 12

what do rods do?

Answer 12

rods predominate at the periphery of the retina, dim objects viewed in dark conditions can often be seen at the edge of your field of view (but not at the centre) but you cannot tell their colour.

Question 13

How do rods work?

Answer 13

Rods contain rhodopsin, also known as visual purple, which consists of complex molecules that each can be split in two by light photons. This causes a change in the cell potential which helps to ‘trigger’ the nerve fibre to which the cell is connected. Up to ten photons need to be absorbed to trigger a rod. Several rods connected to the same nerve fibre need to be triggered to send an electrical impulse to the brain.

Question 14

what do cones do?

Answer 14

(These ranges correspond broadly to red, green or blue light ) Cones do not respond to very low levels of light intensity and automatically become less sensitive at very high intensities.

Question 15

Resolution

Answer 15

The resolution of the eye is determined by the size and closeness of the retinal cells.

Question 16

angular seperation

Answer 16

Ɵ =d/u
d= separation
u = distance from the eye

Question 17

separation of the image centres

Answer 17

Y = Ɵv
v= distance from he eye lens to the retina

Question 18

converging lens

Answer 18

parallel rays converge to a focus. The point where parallel rays are focused to is called the principal focus or the focal point of the lens.

Question 19

diverging lens

Answer 19

parallel rays diverge (spread out). The point where the rays appear to come from is the principal focus or focal point of this type of lens.

Question 20

Principal axis

Answer 20

lens is the straight line that passes normally through both surfaces at their centres.

Question 21

Focal plane

Answer 21

the plane on each side of the lens perpendicular to the principal axis containing the principal focus is called the focal plane.