Vision & Optical Instruments Flashcards
What anatomical feature of the eye has the greatest density of light receptors and the greatest acuity (sharpness) in the visual field?
The fovea
What does the cornea and lens do as a unit?
Form a system that acts as a single thin lens.
A real image must be projected onto the light sensitive ______, which lies at a fixed distance from the lens.
Retina
What allows the eye to detect light intensities from the lowest observable to 10^10 times greater (without damage)?
The variable opening (pupil) of the eye along with chemical adaptation.
Which nerve conveys signals received by the eye to the brain?
The optic nerve.
Where does the biggest change in the refractive index, and bending of rays, occur in the eye?
The cornea, rather than the lens.
The image formed in the eye is much like what kind of lens?
A single convex lens.
The image must fall precisely on the retina to produce clear vision. How else might we might phrase this using physics terms?
The image distance, di, must equal the lens-to-retina distance
Because the lens-to-retina distance does not change, what must happen for the image distance, di, to accommodate for objects at various distances?
The image distance, di, must be the same for objects at all distance.
The eye manages this by varying the focal length of the lens to accommodate for objects at various distances.
The process of adjusting the eye’s focal length is called?
Accommodation
Light rays from a nearby object can diverge and still enter the eye. How must the lens accommodate this?
The lens must be more converging (more powerful) for close vision. To be more converging, the lens is made thicker by the action of the ciliary muscles.
When is the eye most relaxed?
When viewing distant objects.
Vision of very distance objects is called? While close vision is termed?
Totally relaxed
Accommodated
What is the thin lens equation?
P = 1/do + 1/di
What is the magnification equation?
m = -di/do = hi/ho
Define nearsightedness or myopia.
The inability to see distance objects clearly while close objects are clear.
Why does nearsightedness occur?
The eye over-converges the nearly parallel rays from a distance object and the rays cross in front of the retina.
The distance to the farthest object than can be clearly seen is called?
The far point of the eye (normally infinity).
Define farsightedness or hyperopia.
The inability to see close objects clearly while distant objects may be clear.
Why does farsightedness occur?
A farsighted eye does not converge sufficient rays from a close objec to make the rays meet on the retina.
The distance to the closest object that can be seen clearly is called?
The near point of the eye.
How do we correct nearsightedness?
Place a diverging spectacle lens in front of the eye. This reduces the power of the eye.
How do we correct farsightedness?
Plac ea converging spectacle lens in front of the eye. This increase the power of an eye that is too weak.
What is astigmatism?
An unevenness or asymmetry in the focus of the eye. This is mostly due to irregularities in the shape of the cornea.
Define microscope.
A microscope is a multiple-element system having more than a single lens or mirror.
How is a microscope made?
From two convex lens.
The image formed by the first element becomes the object for the second element.
How does ray tracing goes hand-in-hand with microscopes?
They help to visualize the image formed.
A microscope is composed of thin lenses or mirrors, does it obey the thin lens equations?
Yes and it is not difficult to describe their behavior numerically
Define compound microscope.
The simplest microscope, which is constructed from two convex lenses.
What is the objective lens?
This is the first lens and has a typicall magnification value from 5x to 100x
What is the eyepiece?
The second lens and has several lenses which slide inside a cylindrical barrel
What is the purpose of a microscope?
to magnify small objects, and both lenses contribute to the final magnification.
What is the equation for overall magnification, m, the product of individual magnifications? Define variables.
m = mo x me
where mo = magnification of the objective and me = magnification of eyepiece
How high can normal optical microscopes magnify to?
Up to 1500x with a theoretical resolution of -0.2um.
Why are microscope objective lenses particuarly important?
They primarily gather light from the specimen.
What three parameters describe microscope objectives?
The numerical aperture (NA), the magnification (m), and the working distance.
How is NA related to light gathering ability of a lense?
It is obtained using the angle of acceptance, theta, formed by the maximum cone of rays focusing on the specimen
What is the equation to calculate the numerical aperture (NA)? Define variables.
NA = nsin(alpha)
where n = refractive index of the medium between the lens and specimen, and alpha = theta/2
As the angle of acceptance given by theta increases, NA becomes?
Larger and more light is gathered from a smaller focal region giving higher resolution.
What does the numerical aperture not do?
It does not indicate how far the lens could be from the specimen or the “working distance”.
What is the working distance?
The distance from the front lens element of the objective to the specimen.
The higher the NA the ______ the lens ill be to the specimen.
closer
The working distance ______ as the NA and magnification both increase.
decreases
What is the term f/# used to denote?
It denotes the light per unit area reaching the image plane
If the acceptance angle is small, the NA of the lens can also be used in what equation?
f/# = f/D = 1/2NA
As the f-number decreases, the camera is able to?
Gather more light from a larger angle
A greater f/# means?
Less light reaches the image plane
Can the NA be larger than 1.00?
The answer is “yes” if we use immersion lenses in which a medium such as oil, glycerine or water, is placed between the objective and microscope cover slip.
This minimizes the mismatch in refractive indices.
What are telescopes used for?
Viewing distant objects, producing an image that is larger than the image that can be seen with the unaided eye.
What does the two-lens telescope use in order to be efficient?
Uses two convex lenses
What is the angular magnification, M, equation? What does the minus sign indicate?
M = -fo/fe
That the image is inverted
How can we obtain the greatest angular magnification?
Have a long focal length objective and a short focal length eyepiece.
The greater the angular magnification?
The larger an object will appear when viewed through a telescope.
True or False. A concave mirror acts like a convex lens.
True
