The Eye
How we see…
• Light from an object enters the eye through the cornea.
• The curved cornea and the lens focus the light and produce an image on the back of the eye – the retina.
• The light receptor cells in the retina sends
Rod cells and Cone cells
• There are two types of receptor cells in the retina – rod cells and cone cells.
• Rod cells are sensitive to low light but do not respond to colour.
Two major functions of the eye:
• Controlling how much light that enters the eye. This is controlled by the muscles in the iris which controls the size of the pupil.
• Focussing the light that enters the eye. The majority of the focussing is done by the cornea. The fine focusing of near and distant objects is done by the lens.
Focusing Light on the Retina
• For the brain to see a clear image it needs to be focused on the retina. The light rays need to be refracted (bent) in order to form this image.
• The cornea is responsible for most of the
refraction. The lens makes fine adjustments.
Focusing
Light rays from individual point sources on an object are refracted
and focused onto the retina
When light rays from all points on an object within the field of view are focused onto the
retina, an inverted image is formed; when the brain interprets the image we see the
Focusing light on near and
distant objects: Accommodation
• Light rays coming from a near object are going away from each other- they therefore need to be bent strongly to focus an image on the retina. To achieve this the lens needs to bulge.
• Light rays coming from a distant object are almost parallel to one another so they do not need to be bent so much to be focused on the retina. To achieve this the lens needs to be
Focusing on a Distant Object (Accommodation)
Parallel rays of light from a distant object
Ciliary muscle relaxed
Suspensory ligaments pulled taut
Lens becomes less convex
Focusing on a Near Object (Accommodation)
Diverging rays of light from a near object
Ciliary muscle contracted
Suspensory ligaments relaxed
Lens becomes more convex
Focusing and the Lens
Ciliary Body; circular ciliary muscle fibres form a ring around
the lens
Suspensory Ligaments; attach the lens
to the ciliary muscles
Iris Lens Lens
Cornea
The ciliary muscles
are responsible for changing
the shape of the lens
during
Focusing on distant objects
• To focus on a distant object the lens needs to be pulled thin.
• To achieve this the ciliary muscles relax and the suspensory ligaments connected to the lens are
Focusing on a near object
• To focus on a near object the lens needs to bulge.
When the eye focuses on near objects, the ciliary muscles
contract and thus the
suspensory ligaments become
less taut allowing the elastic lens to become more convex
When the eye focuses on distant objects, the ciliary muscles relax
and thus the suspensory ligaments are pulled taut
Controlling light entering the eye
• The muscular iris (the coloured part of our eyes) controls the amount of light that is allowed to enter our eyes
through the pupil.
• In dim conditions the iris allows more light to enter
through the pupil so that more light can enter the eye.
This is so that a clear image can be formed on the retina.
• The size of the pupil is controlled by the circular and radial muscles of the iris.
• In bright light the circular muscles contract
(radial muscles relax) narrowing the pupil size. • In dim light the radial muscles contract (circular
Controlling the amount of light entering the eye
Bright Light Dim Light
Controlling the amount of light entering the eye
The actions of the iris circular and radial muscles modify the aperture of the pupil and hence regulate the amount of light entering the eye
Circular muscles contract during pupil constriction
and relax during pupil dilation
Radial muscles contract during
pupil dilation and relax during
pupil constriction
The control of pupil size is a reflex
action involving motor neurons of
Bright light Dim light
At high light intensities, a large number of retinal
photoreceptors are stimulated
Circular muscles contract and radial muscles relax constricting the pupil
At low light intensities, fewer retinal photoreceptors are stimulated
Radial muscles contract dilating the pupil; circular muscles relax
Controlling the amount of light entering the eye
