King Saud University  College of Applied Medical Sciences  Optometry Department  OPTO-271: Second Mid-Term Exam                     May 24th, 2003

                              Time Allowed:            One & Half (1.5H) Hour

                               Student’s Name:      .................................................       

                               Student’s Number:     ...............................................                 

                               Course Instructor:          Dr. Idriss M. Bedja

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 QUESTION 1 (3 Marks)

If you are asked to study the absorption/transmission characteristics of a pigment like a visual purple, using graphics or text, enumerate the different steps for the study.

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Answer:

We dilute first the non homogeneous grains of visual purple into a solvent giving both a transparent and homogeneous solution. To study the absorption characteristics of the pigment we use shine first a monochromator with a white light.  With monochromator we can select continuously a single wavelength of intensity Io from 400 nm up to 800 nm to pass through the pigment solution. The transmitted light after the cell solution will have intensity I and a photocell will convert it into electrical current to be analyzed by computer.  The difference Io – I is the light intensity absorbed. To remove the contribution of  the solvent in the absorption characteristics graph, we use the same experiment but without the pigment.  The computer will subtract the solvent absorption from the solution absorption.

 

QUESTION 2 (1.5 Marks)

 For which cases below, radiation has an effect on a substance when traveling through it:

1)      radiation is transmitted partially by the substance

2)      radiation is absorbed partially by the substance

3)      radiation is totally transmitted by the substance

4)      none of them

Answer:  (2)

 

QUESTION 3 (2.5 Marks)

A)                If a person is exposed directly to the sun for a quiet long time, mention

briefly the most harmful effects may occur on his eyes:

(1)    .Thermal effect

(2)    .Photochemical effect

(3)    Photoluminescence effect.

 

B)                To what is due the lens yellowing occurred with age?

UV absorption will accelerate the production of yellow pigemts which cause yellow coloration of the lens. …

 

QUESTION 4 (2 Marks)

(a)    Exposing to high level of visible radiation may cause:

Photochemical and thermal effect of the retina, also dark adaptation will be longer

(b)   How to preserve the eye ability to adapt to low levels of illumination.

By wearing filtering glasses all day and reduce the intensity and duration exposure.

 

QUESTION 5 (3 Marks)

            The cones are responsible for discrimination of very fine details

                        True___X___                          False______

            The rods are responsible for the motion and edge detection

                        True__X____                          False______  

            Dark adaptation is referred to as transition zone from vision

            mediated by rods to vision Mediated by cones.

                        True______                             False___X___

 

QUESTION 6 (1.5 Marks)

The spectral color associated with a single wavelength in visible light is defined by the term:  …HUE………..

 

QUESTION 7 (2.5 Marks)

The human retina has about 120 millions rods and about 6 millions cones.  Why so it has about the same concentration (about 150 000/mm²) at their respective higher concentration areas?

Answer: Because total area of rods >> total area of cones

 

QUESTION 8 (3.5 Marks)

a)  If you know the base of foveal cone, compute its angular extent to  

     the nodal point.  Make a draw for help.

Answer:   

          2.5 mm (Base) per 16.67 mm to Nodal point

Tan q = (2.5x10-3 mm)/(16.67 mm) 

          =  0.00015   so    q = tan^-1 0.00015= 0.0085926  X 3600

                                                       q = 30.9 ² @ 31²

 

b)  Also, determine the theoretical resolving power of the perfect eye

     in radians and in seconds of arc.

Answer:    We know that the linear size of the theoretical resolving power of a perfect eye is 3.8 mm, or a cone of a base 2.5 mm subtends an angle of 31’’ at the nodal point.

2.5 mm  ------------------   31”

3.8 mm ------------------ q = 3.8x31/2.5 = 47 ²  …./3600  = 0.01306°   …/57.3 = 0.000227 rad

 

QUESTION 9 (2 Marks)

According the formula, RP=1.22λ/d,  if d becomes becomes larger, the VA increases respectively, which is not always the case.  Mention what happens when d becomes larger.

Answer:   

When pupil size d becomes larger, aberrations also increases and so on for the illumination which force the VA to decrease.

 

QUESTION 10 (2 Marks)

Give the different types of visual acuity:

Answer:   

1/ Light discrimination

2/ Spatial discrimination

3/ Temporal discrimination

 

QUESTION 11 (1.5 Marks)

The ability of humans eyes to detect a break in a line is reported as a performance reaching a value as low as 2 sec of arc in the substended angle.  This exceptional ability may be explained on the basis of (give one or more answers)

 a)      brightness differences

b)      spatial localization

c)      both of them   (X)