Test 7 (Optical prescriptions, spectacle lenses)

1. The following transpositions are correct:

a. PL / -2.00 X 180 = -2.00 / +2.00 X 90

b. +2.00 / -0.50 X 60 = +2.50 / +0.50 X 150 

c. -1.25 / +0.75 X 4 = -0.50 / -0.75 X 86 

d. +7.00 / -2.00 X 78 = +5.00 / +2.00 X 168

e. -0.25 /-3.00 X 50 = -3.25 / +3.00 X 140
 

Answera.T   b.F   c.F    d.T   e.T
To transpose an astigmatic lens, first add the spherical 
and the cylindrical power and then alter the sign of the cylinder power and finally add 90 degrees to the existing axis. If the axis is more than 180 degrees, takes 180 degrees from the total degrees to get the new axis.
Therefore question b should be +1.50 / +0.50 X 150 and b should be -0.50 / -0.75 X 94.
 
 

2. When transposing +3.00/-1.00X 90 to the base curve -5.00, 
    the correct answer should be:

a. +8.00 DS/-5.00DC axis 180/-6.00DC axis 90

b. +8.00 DS/-5.00DC axis 180/-6.00DC axis 180

c. +8.00 DS/ -5.00DC axis 90/-6.00DC axis 180

d. -2.00DS/ -5.00DC axis 90/-6.00 DC axis 90

e. -2.00DS/ -5.00DC axis 180/-6.00DC axis 90
 

Answera.T  b.F  c.F  d.F  e.F
 The steps for toric transposition is as follow:
1. Make sure the cylinder has the same sign as the base curve
2. Obtain the required power of the spherical surface by subtracting the base curve power from the spherical power. Here the result is +3-(-5) = +8.00DS
3. Specify the axis of the base curve and this axis is 90 degrees to that of the required cylinder. The result in this case is: -5.00DC axis 180.
4. Finally add the required cylinder tot he base curve power with its axis. The result is -5 (-1.00) = -6.00DC axis 90.
 
 

3.  The following are true about the identity of an unknown lens when 
     viewed through a cross made up of two lines crossed at 90^0:

a. there is no distortion of the cross if the lens is spherical

b. an astigmatic lens always causes distortion of the cross

c. 'scissors' movement of the cross when the lens is 
    rotated suggests the lens is astigmatic

d. if the cross moved in opposite direction to the lens, 
     the lens is likely to be concave

e. displacement of the line on  cross suggests the presence
    of prism within the lens.
 

Answer
a.T   b.F  c.T   d.F  e.T
If the axis of the astigmatic lens coincide with the cross 
lines, there is no distortion. However, if the lens is rotated 
against the cross, scissors movement of the cross always occurs. If the cross moved in opposite direction to the lens, the lens is convex. A prism has no optical centre and therefore one line of the cross is always displaced regardless of the lens position.
 

4. Regarding the Geneva lens measure:

a. it can only be used to measure crown glass

b. it measures the base curve of a lens

c. it makes use of the principle that the total power of 
    a thin lens is equal to the sum of its surface power

d. it can be used to measure the thickness of a lens 

e. it can be used to measure the diameter of a lens

Answer
a.F   b.T  c.T  d.T  e.T
The Geneva lens measure is calibrated for crown glass but can be used for other materials with a correction 
factor. It measures the base curve of lens and makes 
use of the principle that the total power of a thin lens is equal to the sum of its surface power. It can be used for lens diameter or thickness measurement.
 
 

5. The focimeter:

a. contains a convex collimating lens 

b. contains a collimating lens that converge the incoming 
    light

b. uses green light to eliminate spherical aberration

c. contains a telescope system for viewing

d. gives a ring of dots if the lens has no cylindrical power
 

Answer
a.T   b.F  c.F  d.F  e.T
The focimeter contains a convex collimating lens but the light passing through it are rendered parallel. Green light is used to eliminate chromatic aberration. 
a
a

6. The focimeter can measure:

a. back vertex power of the lens

b. prism

c. axis of the cylinder

d. optic centre of the lens

e. refractive index of the lens
 

Answer
a.T   b.T  c.T  d.T  e.F
A focimeter is used to measure the vertex power of 
a lens, the axes and major powers of an astigmatic
lens and the power of a prism. For spectacle lens, 
the back surface of the lenses is placed against 
the rest so that the back vertex power is measured . 
a
a

7. Regarding tinted lenses:

a. the lenses always change colour with light

b. they work by either absorbing the light or reflecting 
    the light

c. transmittance curve of a tinted lens measures the 
    percentage of light transmission for each wavelength

d. the protective goggles used in laser room are made up 
    of tinted lenses.

e. lenses with solid tint have uniform light absorption 
    throughout.
 

Answer
a.F   b.T  c.T  d.T  e.T
Tinted lenses may be of fixed colour or photochromic (the colour changes with light). They work by either absorbing the light or reflecting the light. 
Transmittance curve gives the performance of the lens.
Laser protective goggles are tinted lenses. Light absorption in a lens with solid tint depends on the thickness of the lens; therefore, the absorption is higher in area where the lens is thicker. 
a
a

8. True statements about anti-reflective coatings include:

a. the principle of destructive interference applies to
    anti-reflective coatings

b. the thickness of the coating is a quarter of the 
    wavelength of the incident light

c. they cause the lenses to grow dark in bright light

d. they absorb ultraviolet light

e. they can only be used on plastic lenses
 

Answer
a.T   b.T  c.F  d.F  e.F
Anti-reflective coatings do not change the colour of 
the lens. They use the principle of destructive interference to reduce the reflection. The thickness of the coating is a quarter of the wavelength of the incident light. The coatings can be used on either glass or plastic. The coating materials are made usually made up metal oxides.
a

9. Photochromic lenses:

a. usually have a more rapid darkening process than 
    lightening process

b. give different tints according to the wavelengths

c. tend to become darkened with repeated use

d. that made of glass usually employed organic 
    photochromic compounds

e. tend to darken more rapidly if the environmental 
    temperature is low
 

Answer
a.T   b.T  c.F  d.F  e.T
Photochromatic lenses change colour in the presence of light and the process of darkening is more rapid than 
the process of lightening. The tint that develops depends on the wavelengths of the light. Silver halide is used in glass and organic photochromic compounds are used in plastic. With constant use, the lens eventually 
become darkened. Darkening tends to occur more rapidly in low temperature because heat reduces the effect of light on photochromic lenses..

 

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