Eye anatomy and physiology Flashcards
Define: Law of Specular reflection
When a ray of light reverses direction, in an equal angle in reference to the point of perpendicular contact.
Occurs when light enters smoothly polished surfaces like
- Mirrors
- Still pools of liquids
Snell’s law, refraction
When the path of light is deflected from a straight line as it enters a medium where its speed is different.
Light will always bend towards the surface of normal when going from a rarer to denser medium.
Components
- Refractive index of medium: speed of light in vacuum/ speed of light in medium
- This index is always greater in mediums as light travels fastest in a vaccum.
- Angle of incidence
- Angle of emerging ray
Define: Total internal reflection
Occurs when the angle of incidence exceeds the angle of refraction (the critical angle)
Here it cannot fulfil Snell’s law and the ray is reflected at the boundary between two mediums.
Power of a convex lens
Its ability to focus light from a infinite distance to a finite distance (f).
Measured in dioptres (D)= 1/f
Power of a concave lens
Always less than 0 because the distance will always be greater.
Concave lens can be used to adjust the power of convex lens
How do concave lenses correct myopic vision?
A myopic eye has high refractive power of the cornea/ lens, causing image to be formed in from of the retina.
Concave lens (minus power) moves the image further away from the front of the eye, allowing the image to be formed closer to the retina.
How do convex lenses correct hyperopic vision?
Hyperopic eyes have a cornea/ lens with a power too weak for its length, preventing the image from being formed before the fovea.
Convex lens (plus lens power) increases the angle of refraction, bring the focus point shorter, in front of the fovea.
How does stigmatic imagery occur?
Happens when the surface of the lens resembles a rugby ball–> the maximum point of curvature is sufficiently greater than the minimum point.
- Here the form a line that is perpendicular.
The maximum point= greatest power of lens
Minimum point= weakest power
As light is refracted, two focus points are formed as a result of the differing powers, forming a stigmatic image.
What are the properties of an electromagnetic wave.
Contains an oscillating electrical field perpendicular to an oscillating magnetic field.
Both fields are perpendicular to the direction of the wave.
Light interference
Occurs with monochromatic light, where dark and light stripes/ slits are formed from light passing through two slits/ round apertures.
This is due to the differing lengths of the light pathways.
- Constructive interference: when the peaks of two pathways coincide= bright stripe
- Destructive interference: when the peak of one wave coincides with the trough of the other.
Light interference
Occurs with monochromatic light, where dark and light stripes/ slits are formed from light passing through two slits/ round apertures.
This is due to the differing lengths of the light pathways.
- Constructive interference: when the peaks of two pathways coincide= bright stripe
- Destructive interference: when the peak of one wave coincides with the trough of the other.
What colour is the longest and shortest wavelength of visible light.
Red= longest
Shortest= blue/ violet
Define: light dispersion
The ability of a medium (prism) to break up visible light into its colour constituents.
- Due to its ability to transfer light at differing velocities
Describe the anterior segment of the eye
Anterior to posterior:
- Cornea
- Anterior chamber
- Iris
- Pupil
The cornea is responsible for how much refractive power of the eye?
2/3
Light manipulation at cornea
Refraction
10% of light is dispersed
Q factor of cornea
Known a ‘asphericity’
- How much of the cornea is flattened at the periphery, compared to the apex.
Normal is around -0.25
- More negative= steeper cornea
- More positive= flatter cornea
Causes of a more negative Q corneal factor
Central keratoconus (thinning and steeping of central cornea)
What is the relationship between visual acuity and pupil diameter?
As pupil diameter decreases, near distance visual acuity increases.
- Spherical aberrations are reduced with a smaller pupil size, which improves retinal image quality.
Larger pupils decreased diffraction, allowing for better far-sighted vision.
How does the lens accommodate for near-sighted vision
The ciliary body contracts, reduces tension on the zonular fibres which makes the lens more bulbous
- Increased refractive power= greater acuity.
Define: Presbyopia
Loss of near-sight focus due to the hardening of the lens
- Prevents the lens from changing shape and accomodating.
How does light scattering caused by the lens change with age.
Younger lens (age 20) scatters a lot more light (20%) than older lens (60), (>40%).
More blue light is also absorbed with age, which makes colour discrimination harder.
Define accommodation
Occurs when the power of the lens changes to converge the image on the retina.
- Images that are closer causes contraction of the ciliary body and relaxation of the suspensory ligament, forming a more bulbous lens with increased refractory power.
- Relaxation of ciliary muscles reduces refractory power through the increased tension of the suspensory ligaments.
This occurs through sensory and motor inputs.
Define, in terms of accommodation:
- Near point
- Far point
- Range
- Amplitude
Near point
- The closest distance an object can be seen with maximum accommodation.
Far point
- The greatest distance an object can be seen with relaxed/ minimum accommodation
Range
- The range between far and near point.
Amplitude
- Difference in power of the lens at a fully and relaxed accomodation
How to investigate accommodation amplitude
Start off with looking through a lens at an object 40cm away.
Adding lenses until image start to blur and calculating difference between lenses.
Define: Emmetropia
Sharp vision
- Where faraway object at infinity distance is in sharp focus where the lens is relaxed
