THEORY OF OPTICS Flashcards
WHAT DOES THE DUALISM OF LIGHT MEAN?
-light is continuous transverse waves of electromagentic field AND a stream of particles
WHAT IS THE SPEED OF LIGHT?
-in a vacuum is 3.108 ms-1. In a medium is always lower
WHAT IS THE FREQUENCY OF LIGHT?
-the number of oscillations per unit time, independent of the environment and gives the color of light. With increasing frequency, increasing the light energy
WHAT IS THE WAVELENGTH OF LIGHT (λ)?
-distance between the corresponding points of the sinusoid. The wavelength of light
is 390nm (violet) to 790nm (red)
WHAT IS THIS RELATIONSHIP?
λ = c / f
wavelength = speed / frequency
WHAT IS THE AMPLITUDE OF LIGHT?
-indicates the maximum deviation from zero sinusoid. The amplitude depends on the intensity of light
MONOCHROMATIC
light waves with one wavelength
POLYCHROMATIC
light waves with more than one wavelength
WHAT IS REFLECTION?
-If the objects surface is smooth and impermeable, all the light is reflected back into space (mirror).
-The angle of incidence of light on the surface equals the angle of reflection.
-If the surface is uneven and impermeable, the rays reflect to space at different angles (the scattered light).
WHAT IS REFRACTION?
-bending of light when it passes from one transparent substance to another.
-caused by changing the speed and wavelength of light
at the interface of 2 objects with different densities
WHAT IS THE DISPERSION OF LIGHT?
-lights of different colours refract at different angles
RAINBOWS = decomposition of white light on
the prism or water droplets
WHAT IS THE INTERFERENCE OF WAVES?
-when two waves meet or superimpose each while traveling along the same medium. This resultant interference of two or more waves causes the medium to take a new shape that results from the net effect of the two individual waves upon the particles of the medium (even multiples of λ/2 = maxima interference)
WHAT IS DIFFRACTION?
-spreading of a wave into regions where it would not be seen if it moved only in straight lines after passing through a narrow split or past an edge
WHAT IS THE DIAMETER OF THE HUMAN EYE?
-24mm
HOW MANY MUSCLES CONTROL EYEBALL MOVEMENTS?
-6 muscles
WHAT ARE THE LAYERS OF THE WALL OF THE EYEBALL?
-Belimo, central choroid (choroidea) and inner retina
WHERE ARE THE GREATEST CONCENTRATION OF CONES LOCATED IN THE RETINA?
-the optic axis (the yellow spot = point of sharpest vision)
WHAT IS THE WHITE OF THE EYE?
-sclera
WHAT IS THE COLOURED PART OF THE EYE?
-the iris
WHAT IS THE OPTICAL POWER OF THE EYE?
-Optical power of the eye is approximately +60 diopters, which accounts for 42 D cornea and 17 - 20 D lens
WHAT IS THE ROLE OF THE CORNEA?
-Separates the internal environment of the eye from ambient air environment (with a refractive index 1
-transparent
-refractive index of the cornea is 1.37 and is the strongest refractive power of 40 diopters
-supplies 70% of eye
-fixed focus power due to this refraction
WHAT DOES ACCOMODATION OF THE EYE MEAN?
- adaption of the eye for near vision
- properly functioning circular muscle in ciliary body, intact suspension apparatus and flexible lens for good accommodation
-15D accommodative width + near point distance of 7cm from eye (10 YEARS)
-10D accommodative width + near point distance of 10cm from eye (20 YEARS)
-4D accommodative width + near point distance of 25cm from eye
WHAT IS PRESBYOPIA?
-decreasing accomodation ability with age (age of 70 - 0D)
WHAT IS EMMETROPIA?
-the ideal state of the eye, which is in equilibrium length of the eye, diffraction media and their optical power, parallel rays are refracted by the eye so that they converge precisely on the retina
-no refractive defects
(opposite of this = AMMETROPIC)
WHAT IS THE OPTICAL POWER OF THE CORNEA AND THE LENS?
CORNEA = 40/45 D
LENS = 20D
WHAT IS THE OPTICAL POWER OF THE CORNEA AND THE LENS?
CORNEA = 40/45 D
LENS = 20D
WHAT DOES THE ANGLE OF 1arc MEAN?
-represents the limit of resolution of the eye and was taken as the basis for evaluation of visual acuity with
optotypes
WHAT IS SPHERICAL AMETROPIA?
(REFRACTIVE ERRORS)
-refractive error in the eyes
-hyperopia farsightedness
-shorter axial length
-focus of parallel rays lies behind retina so cannot provide a clear image
EG = myopia, hyperopia and astigmatism
WHAT IS VISUAL ACUITY (SNELLEN ACUITY) ?
-clearness of vision which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain
-black symbols on white background (maximal contrast)
-20ft / 6m (optical infinity in which lens attempts to focus (0.164diopters) )
WHAT DOES 20/20 MEAN?
-20/20 means you can see objects clearly from a distance of 20 feet / 6m
-Someone with 20/20 or 6/6 vision (visual acuity) is able to decipher a letter that subtends a visual
angle of 5 minutes of arc no matter what distance they are from the letter
-NUMERATOR: the distance in feet between the subject and the chart
-DENOMINATOR: the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute (the less this bottom number the better the acuity)
- a person with 20/20 vision can just detect a ship which is 25 miles away out on the ocean. A person with
20/10 acuity could be 50 miles away from the ship and still be able to just detect it.
HOW DO YOU TEST NEAR VISUAL ACUITY?
-holding a nearpoint Snellen acuity card at 40 centimeters (about 16 inches).
-MYOPIA = short sightedness (
HOW DO YOU FIND THE SIZE OF A 20/20 LETTER?
-0.4433 mm/ft × d ft = height of 20/20 letter in mm
d = viewing distance in feet (should be long enough to create no more than 1/8D accommodative demand on patients eye)
WHAT IS OPTICAL AFFINITY?
- object be far away from the patient so that the incoming rays of light are as close to parallel as possible, and the amount of accommodation (increased curvature) of the crystalline lens of the eye will be negligible.
WHAT INSTRUMENT MEASURES THE REFRACTIVE STATE OF THE EYE?
-optometers
SUBJECTIVE = rely on subjects judgement of sharpness of blurredness of an object
OBJECTIVE = optical system which determines the vergence of light reflected from the subjects retina
WHAT IS AN AUTOREFRACTOR?
-a computer-controlled machine used during an eye
examination to provide an objective measurement of a person’s refractive error and prescription for
glasses or contact lenses
-measures how light changes as it enters the eye
WHAT ARE THE DIFFERENT TYPES OF LENS?
CONVEX = makes beams converging and image enlarged
CONCAVE = makes beams diverging and image smaller
PLANE CONVEX = focus light onto a single point
PLANE CONCAVE = bend parallel input rays to diverge from one another on the output side of the lens and hence have a negative focal length
WHAT IS THE PRINCIPLE OF THE LENS?
-produces its focusing effect because light travels more slowly in the lens than in the surrounding air, so that refraction, an abrupt bending, of a light beam occurs both where the beam enters the lens and where it emerges from the lens into the air
-because of the curvature of the lens surfaces, different rays of an incident light beam are refracted through different angles, so that an entire beam of parallel rays can be caused to converge on, or to appear to diverge from, a single point. This point is called the focal point.
-refraction of the rays of light reflected from or emitted by an object causes the rays to form a visual image of the object
WHAT WAVELENGTHS OF LIGHT DO HUMANS RESPOND TO?
-390 to 700nm
-max sensitivity around 555nm / 540 THz (green region of spectrum)
-430 to 790 THz in terms of frequency
WHAT IS THE STRONGEST PART OF THE REFRACTING POWER OF THE EYE?
-the cornea providing 70% of the power of the eyes optical system, refracting power is 40 dP
-focus is fixed due to a constant refracting power
-index of refraction is 1.376
DESCRIBE THE LENS
-helps focus light on the back of the eye
-9mm in diameter and 4mm thick
-crystalline
-20% of refracting power of the eye
-index of refraction between 1.406 - 1.386 (gradient index)
-changes shape to accomplish accommodation for close focusing
WHAT IS THE FOVEA?
-the center of the eye’s sharpest vision and the location of most color perception
CONES
WHAT ARE CONES AND RODS?
CONES: daylight vision (bright light)
RODS: twilight vision (dark light)
WITHIN WHAT FIELD DOES THE EYE RECEIVE DATA FROM?
-from a field of about 200 degrees, the acuity over most of that range is poor. T
-to form high resolution images, the light must fall on the fovea, and that limits the acute vision
angle to about 15 degrees. I
-in low light, this fovea constitutes a second blind spot since it is exclusively cones which have low light sensitivity. At night, to get most acute vision one must shift the vision slightly to one side, 4 to 12 degrees so that the light falls on some rods.
EXPLAIN MYOPIA - NEARSIGHTEDNESS
-close objects are seen clearly, but objects farther away appear blurred - light entering retina not focused
-occurs when the eyeball is too long (axial myopia), where the axis from cornea to retina is too long or the cornea has too much curvature (refractive myopia)
-affects 30% of US and EU
-CORRECTION = diverging eyeglasses or contact lenses that correct nearsightedness by bending the visual images that enter the eyes, focusing the images correctly at the back of the eye
-laser can also work to remove a small amount of the eye tissue
EXPLAIN HYPEROPIA - FAR SIGHTEDNESS
-distant objects usually seen clearly, but close objects do not come into proper focus
-occurs if the eyeball is too short or the cornea has too little curvature , so light entering the eye is not focused properly and focal point lays beyond the retina
CORRECTION = converging eyeglasses or contact
lenses - to optically correct farsightedness by
altering the way the light enters eyes
EXPLAIN PRESBYOPIA
-the crystalline lens of your eye loses its flexibility,
which makes it difficult for you to focus on
close objects
-noticeable in early 40s and natural part of ageing
EXPLAIN ASTIGMATISM
-vision condition that causes blurred vision due either to the irregular shape of the cornea or sometimes the curvature of the lens
-optical power of the eye is not the same in all meridians
CORRECTION = glasses with cylindrical lens, contact lenses or refractive surgery
EXPLAIN CATARACTS
-when the inner lens becomes darkened or opaque
-surgically replaced with a plastic lens
EXPLAIN GLAUCOMA
-group of diseases which damage the eyes optic nerve and can result in vision loss and blindness
-slowly lose peripheral vision
EXPLAIN COLOUR BLINDNESS
-inability or decreased ability to see colour or perceive colour differences under normal lighting conditions
-no actual blindness but deficiency of colour vision
-fault in the development of one or more retinal cones that perceive colour in light and transfer to the optic nerve
WHAT IS A DEFLECTION ABOVE THE ISOELECTRIC POINT CALLED?
-positive
WHAT IS AN OPHTHALMIC TONOMETER USED FOR?
-intraocular pressure
WHAT TISSUE HAS THE LOWEST ABSORPTION OF ULTRASOUND?
-bone
WHAT IS A DIVERGING LENS?
-converts parallel lines into diverging
-thicker at the edges than in the middle
WHAT IS A FOCOMETER USED FOR?
-measuring the value of dioptric lenses
WHAT IS EXAMINED DURING AN OCULAR TEST?
-both monocular and binocular vision
-visual acuity in far and near vision
WHAT IS THE DENSITY OF CONES IN THE PERIPHERAL OF THE RETINA?
-decreases
-highest in macula
WHAT IS CHANGED DURING AN AUDIOMETRY TEST?
-sound intensity and frequency
WHAT DOES THE PRE-PRINTED CURVE OF AN AUDIOGRAM MEAN?
-normal threshold of hearing
WHAT IS SCOTOPIC VISION?
-night vision
WHAT ARE THE BEST AND WORST SCORES A PATIENT CAN SCORE ON SNELLEN OPTOTYPE?
WORST = 6/60
BEST = 6/6
WHAT IS USED TO TEST COLOUR PERCEPTION?
-pseudo isochromatic tables
WHAT IS USED TO TEST FAR SIGHT ACTIVITY?
-optotypes
WHAT IS USED TO TEST FOR NEAR SIGHT ACTIVITY?
-jaegar chart
WHAT DOES THE EYE PRODUCE ON THE RETINA?
-a real, inverted image
WHAT IS SNELLS LAW?
-relationship between the angles of incidence and refraction, when a ray of light travels, from a rarer medium of refractive index, to a denser medium of refractive index
ULTRAVIOLET
-400nm and 10nm
INFRARED
-700nm and 1mm
WHAT IS LUMINESCENCE?
the return of excited electrons to basic levels and emission of excess energy in the form of photons
EG: fluoresence
WHAT IS FLUORESCENCE?
secondary radiation characterized by the emission of energy in a very short time
-phosphorescence lasts longer due to electrons being in metastable state (harder for them to return)
BLINDSPOT
part of optic nerve which has no cones or rods
WHAT IS THE CRITICAL ANGLE?
the angle at which the refracted ray is at 90ᵒ. It is the largest angle where refraction can still occur; if it is larger than this the ray will undergo total internal reflection
WHAT IS THE UNIT OF FREQUENCY?
hertz Hz
WHAT IS THE UNIT OF TENSION?
pascal Pa
WHAT IS THE UNIT OF POWER?
watt W
WHAT IS THE UNIT OF SURFACE TENSION?
newton per min
HYPERMETROPIC EYE
-focal point of incident rays lay beyond the retina
SNELLEN OPTOTYPE NUMERATOR NUMBER
distance between patient and optotype
PSEUDO-ISOCHROMATIC TABLES
used to examine colour perception
OPHTHALMIC TONOMETER
measures intraocular pressure
WHAT IS THE OPTICAL POWER OF THE CORNEA?
40-45 D
WHERE ARE RODS CONCENTRATED?
-outer edges of retina
-lowest in macula
-increases towards the periphery of retina
WHERE ARE CONES CONCENTRATED?
-fovea
-quickly reduce towards periphery of retina
WHAT TYPE OF IMAGE DOES THE EYE PRODUCE ON THE RETINA?
-real
-inverted
