Light and the Eye Flashcards
What is visible light?
a specific band of energy within the electromagnetic energy spectrum.
what is electromagnetic energy?
energy that is produced by electrical charges and radiated in waves.
how is electromagnetic energy measured?
wavelengths - the distance between the peaks of the electromagnetic waves.
what are gamma waves?
waves that are a ten-billionth of a metre, or one nanometer (= 10^-9 meter)
what re long-wavelength radio waves?
waves that are 10,000 metres long.
what types of light are harmful to our DNA?
x-rays and ultra-violet rays are harmful for our DNA. Gamma rays are very harmful to DNA, can cause radiation sickness which was seen after the Chernobyl disaster.
most perceived light is reflected light. t/f?
true.
what is accommodation?
in vision, bringing objects located at different distances into focus by changing the shape for he lens.
why can’t humans perceive things that other animals can?
other animals can perceive different frequencies to humans e.g. snakes can see infrared and bees can see UV rays.
what is a ‘camera obscura’?
latin for ‘dark room’ a camera obscura is when the rays of light reflected from an object are focussed through a small hole which produces an inverted but to scale image of the object.
what does a large aperture mean?
the image is brighter however more blurry and less clear.
what does a small aperture mean?
the image is more clear but dimmer.
what is camera obscura used by artists for?
tracing scale images of objects in the environment.
what are three mains structures in a human eye?
Iris, pupil and sclera.
what is the cornea?
the transparent tissue at the very front of the eye.
what is the sclera?
the tough white tissue that coats the rest of the eye.
what is the iris?
the muscle tissue that controls the size of the pupil and, thereby, the amount of light that enters the eye.
what does the sclera do?
holds the structures in the eye together and holds the liquid in place.
where is the retina and what is it?
a complex network of cells covering the inside of the back of the eye, includes photoreceptors, horizontal, bipolar, amacrine and ganglion cells.
what does the cornea do?
the cornea is a fixed shape, it accounts for about 80% of focusing light.
what does the lens do?
adjusts its shape according to object distance, accounts for about 20% of focusing light.
what do the ciliary muscles do?
increase the focussing power of the lens by increasing its curvature. they tighten which causes the lens to thicken and focus on close objects.
how does the cornea bend?
the area just behind the cornea is filled with liquid which means the cornea can bend to different light.
what is the near point?
the distance at which the lens cannot adjust any further for close objects.
what is presbyopia?
the hardening of the lens and the weakening of the ciliary muscles causing the distance of the near point to increase with age. therefore, correctiveness lenses are needed for close activities, such as reading.
what is hyperopia?
farsightedness.
what is myopia?
nearsightedness - the inability to see distant objects clearly, the image is focussed in the front of the retina when the lens is relaxed.
what causes myopia?
differences in the shape of the eye.
what are the two types of myopia?
refractive and axial.
does refractive myopia mean?
the cornea or lens bends too much light.
what is axial myopia?
the eyeball is too long.
what re the three cells of the eye?
rods, cones and visual neurons.
what are photoreceptors?
rods and cones.
what are the three main sections of the rods and cones?
outer segments, inner segments and synaptic terminal.
what do the outer segments do?
the outer segments contain visual pigment, this is where the light is transduced into electricity.
what do the inner segments do?
house the important cellular structures including the ribosomes and mitochondria.
what does the synaptic terminal do?
electrical signal propagates here and is passed onto visual neurons.
what do the outer segments contain?
pigment molecules.
what are the two components of pigment molecules?
opsin and retinal.
what is opsin?
a large protein that crosses the disc membrane.
what is retinal?
a light sensitive molecule within the disc membrane.
when does visual transduction occur?
when the retinal absorbs light.
what is isomerisation?
a process in which retinal changes its shape due to the absorption of light.
what angle is our blind spot at?
20°.
what is the distribution of rods and cones in the retina?
the number of rods increases as the angle gets closer to 0° however there are no rods at 0° so the number decreases sharply at 10°. the number of rods is consistently low until 0° in which there is a spike. there are no cones or rods at 20° where the blind spot is.
describe the cones?
cones are small and numerous in the fovea and larger but fewer in the periphery. the cones contribute to day-time spatial, high acuity vision. cones are responsible for colour vision.
describe rods.
rods are absent in the fovea, however are far more numerous than cones in the peripheral retina. rods are more sensitive to low light and are therefore responsible for night vision. rods have little role in colour vision which is why colours are less apparent in dim light.
what is dark adaptation?
the process of increasing sensitivity to light in the darks, measured by a dark adaptation curve.
explain the three stages of a dark adaptation study.
light-adapted sensitivity is measured by establishing the threshold for seeing the test light while in a lit room.
the light in the room is turned off and the participant must adjust the intensity of to keep it barely visible as their eyes adapt to the dark.
test light is focused onto either rods or cones.
describe the neural convergence in the retina.
126 million photoreceptors.
1 million ganglion cells.
rods converge more than cones.
120 million rods = about 120 rod inputs per 1 ganglion cell.
6 million cones = about 6 cone inputs per ganglion cell.
are rods or cones more sensitive to light?
rods.
how are rods more sensitive to light?
individually rods require less light to respond. convergence of rod activity on bipolar/ganglion cells increases the likelihood of neural firing. convergent rod pathways cannot resolve fine detail.
what is convergence?
when many cells synapse onto a single neuron, increases light sensitivity and decreases spatial resolution of rod cells.
do rods or cones provide more visual acuity?
cones.
how do cones provide more visual acuity?
all-cone foveal vision involves one-to-one wiring with ganglion cells. provides information to the brain about spatial position of the light stimulus, leading to the ability to discriminate fine details.
cones need far more light to cause response in ganglion ells than rods do.
what is spatial summation?
simultaneous signals coming from multiple presynaptic neurons and received by a single postsynaptic neuron.
what is temporal summation?
a single presynaptic neuron rapid firing signals to a postsynaptic neuron.
