Light and the eye Flashcards
What is visible light?
Visible light is a specific band of energy within the electromagnetic spectrum.
How is electromagnetic energy measured?
Distance between the peaks of each electromagnetic wave. Measured un meters and nanometers.
What is one nanometer?
10(-9) or 0.000000001 meter. (One billionth of a metre)
What does the visible light spectrum for humans range from?
400 to 700 nanometers.
What happens when there is a aperture in a wall?
The image will appear inverted on the other side.
What happens when the size of an aperture increases?
The image quality decreases.
Why are lenses better than apertures?
Because they bend incoming light rays to ensure sharp and bright images.
What is the cornea?
The transparent tissue at the very front of the eye.
What is the sclera?
Tough white tissue that coats the rest of the eye.
What is the iris?
Muscle tissue that controls the size of the pupil and thereby, the amount of light that enters the eye.
What do the cornea and the lens form?
An inverted image on the retina at the back of the eye.
How much of focusing does the cornea account for?
80%
What does the lens do?
Adjusts its shape according to the viewed object distance, accounts for the other 20% of focusing.
What is accomodation?
Occurs when the ciliary muscles tighten which causes the lens to thicken and focus on objects closer.
What is myopia?
Near sightedness, the inability to see far objects clearly.
What is refractive myopia?
cornea or lens bends too much light.
What is axial myopia?
Eyeball is too long.
How is myopia corrected?
Concave lens.
What is the near point?
The distance where the lens can no longer adjust for close objects.
What is prebyopia?
The process of gradually losing the ability to focus on near objects due to age.
Why does presbyopia occur?
Hardening of the lens and weakening of the ciliary muscles, corrective lens are needed for close activities, such as reading.
What is hyperopia?
Inability to see nearby objects clearly.
Why does hyperopia occur?
Usually eyeball is too short or focus point is behind retina.
What do the outer segments of photoreceptors contain?
Light sensitive visual pigment, light is transduced into electricity.
What happens to the electrical signal in the photoreceptors?
It is propagated from synaptic terminal via a set of neurons to the brain.
What are the two components of the visual pigment molecules on photoreceptors?
- Opsin: large protein.
- Retinal: a light sensitive molecule.
When does visual transduction occur?
When the retinal absorbs light.
What is isomerization?
When the retinal changes its shape after absorbing light.
Where does the transduction of light to electricity take place?
In every photoreceptor, including rod and cone cells.
What is fovea?
The place on the retina that has the highest visual acuity.
How many rod and cone cells does a person haave?
120 million rods and 5 million cone.
What is the blind spot?
The place where the optic nerve leaves the eye.
Why don’t we see the blind spot?
Because one eye covers the blind spot of the other, it is located away from the centre of the visual field, the brain fills in the spot.
How are cone cells spaced?
Small and numerous in the fovea, few in periphery .
What time of vision do cone cells do?
Colour and day time spatial vision.
What are the shape differences between rod and cone cells?
- Rod: Large and cylindrical
- Cone: Small and tapered
How are rod and cone cells distributed on the retina?
Fovea consists solely of cone cells, peripheral has both rod and cones but more rod than comes.
What was the basic method used to test for dark adaptation curve in both rod and cone cells?
Observer is light adapted, light is turned off, observer adjusts the intensity of a test light until they can just see it. (the detection thresholds) threshold values are recorded at different times after the light was turned off.
How does sensitivity split into stages in the dark adaptation experiment?
Sensitivity increases in two stages.
- Stage 1 takes place for 3-4 minutes.
- Then sensitivity levels off for 7-10 minutes, rod cone break
- Stage 2 shows increased sensitivity for another 20-30 minutes.
How large is the sensitivity of the rod cells after the dark adaptation?
100,000 times greater.
What important role does the rod system serve?
It extends the range of illumination over which we can operate, an evolutionary advantage.
What are bipolar cells?
They have two poles and convey information from one nerve to another.
What are ganglion cells?
They pass information to optic nerve fibres in the eye from the bipolar cells.
How many rod cells converge to one ganglion cell?
120
How many cone cells converge to one ganglion cell?
6
How many cone cells converge to one ganglion cell in the fovea?
It is one to one.
What is the advantage of having a one to one connection between photoreceptor and ganglion or bipolar cell?
It gives maximal spatial resolution.
What is gained from many rod or cone cell connections to a bipolar cell?
Spatial summation and an increased sensitivity to light.
However at the expense of spatial resolution.
What is spatial summation?
When the stimuli is activated from multiple different rod or cone cells at once causing the threshold to be met.
Why is summation at the cost of resolution?
Because the more information combined the less detailed information is held as a lot of information is being converged.
What are the pros of rod cells being more sensitive to light than cones?
They take less light to respond, they have a greater convergence which leads to more summation as there is more rods being inputted to one ganglion or bipolar cell increasing the liklihood of response.
What are the cons of rod cells being more sensitive to light?
The rod pathway to the brain cannot resolve detail and resolution as well as cone pathway.
What are the pros of cone cells having one on one connections?
They produce higher visual acuity.
What are the cons of cone cells having one to one connections?
They require more light to respond than rod cells.
What is the spatial resolution in the fovea?
150,000 cones/mm squared
