Lenses And The Eye Flashcards
Is the focal length of a diverging lens positive or negative?
Negative
What does ‘u’ stand for?
The distance between an object and lens
It is always positive
What does ‘v’ stand for?
Distance between image and lens
It is positive if the image is real and negative if the image is virtual
How dos a more powerful lens (thicker) differ from a weaker one in terms of converging/diverging rays and focal length
It will converge/diverge rays more strongly and have a shorter focal length
Cornea
Transparent ‘window’ with a convex shape and a high refractive index. It does most of the eyes focussing
Aqueous humour
Watery substance that lets light pass through the pupil to the lens
Iris
Consists or radial and circular muscles that control the size of the pupil.
Regulates intensity of light
Lens and ciliary muscles
Lens acts as a fine focus, controlled by the ciliary muscles. When the ciliary muscles contract, tension is released and the lens takes a far more superficial shape. When they relax the suspensory ligaments pull the lens into a thin flatter shape
When is the eye said to be accommodated
When the ciliary muscles are not fully relaxed
Vitreous humour
Jelly-like substance that keeps the eyes shape
Retina
Where images are formed, contains rods and cones
Fovea
Centre of the retina with the highest concentration of cones
Optic nerve
Carries signals from the rods and cones to the brain
Far point
Furthest distance that the eye can comfortably focus on. For normally sighted people this is infinity
Near point
Shortest distance the eye can focus on
How do rods and cones send signals to the brain?
They contain chemical pigments that bleach when light falls on them which stimulates the cell to send signals to the brain via the optic nerve
How are rods and cones reset?
By enzymes using vitamin A from the blood
What is spatial resolution?
The ability to form separate images of objects that are close together
Two objects can only be distinguished from each other if there is at least one rod or cone between the light from each of them
What is persistence of vision?
When an afterimage remains for a short amount of time on the retina after a bright image is removed
How long do nerve impulses from the eye take to decay?
1/5th of a second
Myopia • what objects can the eye not focus on? • when does it occur? • where are uncorrected images brought to focus? • type of lens used to correct
- distant objects
- when the lens and cornea are too powerful or the eyeball is too long
- in front of the retina
- diverging
Hypermetropia • what objects can the eye not focus on? • when does it occur? • where are uncorrected images brought to focus? • type of lens used to correct
- near objects
- when the lens and cornea are too weak or the eyeball is too short
- behind the retina
- converging
Astigmatism
• possible causes
• type of lens to correct
• information stated on the prescription
- an irregularly shaped cornea or lens which has different focal lengths for different planes
- cylindrical lens
- power and axis angle for correction
Is the focal length of a converging lens positive or negative?
Positive
