Chapter 25 Flashcards
(16 cards)
What is depth of field?
What is spherical aberration?
Depth of field is the size of the aperture (f-number) also determines the range of o next distances over which adequate focusing occurs
When aperture is large diameter, depth of field is small
Spherical aberration is how it is not possible to obtain a perfectly focused image with a spherical lens
What’s the difference between a camera and the eye?
Difference focus mechanisms
Camera uses movable lens-sensor distance
The eye has a fixed lens-retina distance so it focuses by adjusting the focal length of lens
What are the eyes 2 photoreceptors?
125 million photoreceptors
Rods and cones
Rods are much more sensitive but only respond to one wavelength so in realt dim light we don’t see colour (monochromatic)
3 types of cones that have different wavelength responses so we see colour
Slides 7-9 March 5
What is accommodation?
Action of ciliary muscle causes changes in the focal length of the eye lens
When the muscle is relaxed the lens is relatively flat and has a longer focal length
When the muscle is tense the lens is more rounded and has a shorter focal length
Normal vision corresponds to being able to accommodate object distances of 25cm to infinity
What is the near and far point of the eye
Near point- closest distance for which the lens can accommodate to focus light in the retina. The normal near point is 25cm but changes with age(from 18cm at 10 to 500cm or more at 60)
Far point- farthest distance for which the lens of the relaxed eye can focus light on the retina. Person with normal vision can see very distant objects so the normal far point is infinity
What is refractive power?
Corrective lens prescriptions are written in terms of refractive power, P, rather than focal length:
P (in diopters) = 1/f (in meters)
If thin lenses are placed together:
Ptot=P1 + P2 + P3…
What is hyperopia?
What lens corrects this?
Farsightedness is a condition where distant objects can be focused but near objects cannot
Uncorrected near point of the eye is greater than 25cm
Corrected with converging lens (forms image of a near object at a further distance from the eye which is can be focused
Slides 15-16 March 5
What is myopia?
Nearsightedness is a condition where nearby objects can be focused but far objects cannot
Uncorrected far point of the eye is less than infinity
Myopia occurs because of an elongated eyeball or overly curve cornea
Myopia is corrected with a diverging lens. The lens forms an image of a distant object at a distance from the eye that can be focused
Slides 18-19 March 5
What is presbyopia?
How is it corrected?
What does person with normal vision do and person with myopic vision?
Condition whereby as the lens ages it loses its flexibility and the eye can no longer accommodate nearby objects
Corrected by converging lens same as hyperopia
Person with otherwise normal vision will need reading glasses as they get older
Person with myopic eyes will require bifocals; negative focal length for distance vision, positive focal length for near vision
What are the two types of laser eye surgery?
LASIK- most procedures where flap is cut in cornea, pulled back and exposed section is reshaped with laser (fast recovery)
PRK- outer surface of cornea is re shaped by the laser wafer than lasik because no need to cut flap in cornea
Long recovery time
What is a simple magnifier?
Produce retinal images larger than can be obtained using the unaided eye
The largest retinal image possible with the unaided eye occurs when the object is at the near point
Closer object= bigger angle= bigger image formed from retina
Angle maximum at near point of eye (25 cm)
Magnifier is converging lens placed so that the object distance is less than the focal length
Image from simple magnifier turns into object for eye
Slides 9-14 March 7
How do you find angular magnification?
m= θ (with instrument) / θo (unaided eye, object at near point)
m= (h/p) / (h/25cm) -> 25cm/p = 25cm(1/f-1/q)
m(max)= 25cm(1/f-1/-25cm) = 25cm/f + 1
Relaxed eye example slides 17-18 March 7
How does a compound microscope work?
Has 2 converging lenses
The objective lens has a very short focal length and the eyepiece lens has a focal length of a few centimetres
The lenses are separated by a distance L that is much greater than either of the focal lengths
Objective lens forms a real, enlarged image which becomes the object for the eyepiece lens. The eyepiece forms a virtual, enlarged final image
Slides 22-25 March 7
How do you get a large magnification with a compound microscope?
Object is placed just outside the focal point of the objective lens
The distance between the lenses (L) is set so that the objective lens image forms at or just inside the focal point of the eyepiece lens
p1~fo, q1~L (approx) so M(1)=-q1/p1 ~-L/fo
For image forming at infinity: m(e)=25cm/fe
m=M(1) x m(e)
m= -L/fo x 25cm/fe
What is resolution and diffractions role?
One of the effects of diffraction is to limit the ability to distinguish between two objects
If an optical instrument is to distinguish between two objects, then it must form separate images of the objects
Resolution is forming two images and distinguishing between them
Slides 31-34 March 7
What is Rayleigh’s criterion?
Says that two sources can just barely be resolved if the centre of the diffraction pattern due to one source occurs at the first minim of the diffraction pattern due to the other source
Sources are solved if:
Sinθ >_ 1.22λ/a
