Chapter 8  (Aberration of optical system)

1. The following are true about chromatic aberration:

a. it results from the fact that longer wavelength is deviated 
    more than shorter wavelength.

b. it does not occur with light of same wavelength

c. it results from the dispersive power of a material

d. it is dependent on the refractive index of a material

e. it can be reduced by combining crown and flint glass
 

Answera.F   b.T   c.T   d.F   e.T
Chromatic aberration results from the dispersive power of a material 
and results from the fact that different wavelength deviates differently 
when passing through an optical medium (refraction). Longer wavelengths such as red deviates less than shorter wavelengths such as blue. Chromatic aberration is independent of the refractive power of a material. Reduction of chromatic aberration can be achieved by combining different materials in the optical system to reduce the aberration such as crown and flint glass. 
 
 

2. Duochrome test::

a. uses letters or numbers of different colours against the same background

b. uses blue and red colours

c. is sensitive to changes in refraction of 0.25D or less

d. is useful in myopic patients to avoid undercorrection

e. can not be used in colour blind patients
 

Answera.F   b.F   c.T   d.F   e.F
Duochrome test is a subjective test using the principle of 
chromatic aberration. It uses letters or numbers of the same
colour usually black against different backgrounds: red and 
green. It is sensitive changes in refraction of 0.25D or less.
It is useful in myopic patients to avoid overcorrection, in which case the eye needs to accommodate causing eyestrain. 
 
 

3. Spherical aberration in human eye is reduced by the following:

a. the anterior surface of the cornea is flatter peripherally than centrally

b. the cortex of the lens has a higher refractive index than the nucleus

c. the lens has variable anterior surface curvature

d. the retina has a radius of curvature which is shorter centrally than 
    peripherally

e. the vitreous reduces spherical aberration
 

Answera.T   b.F   c.F   d.F   e.F 
Spherical aberration is reduced by the following in human eye:
the anterior surface of the cornea is flatter peripherally than centrally.
the nucleus of the lens of the eye has a higher refractive index than the lens cortex.
the presence of  iris reduces light coming from the peripheral lens.
the retinal cones are more sensitive to light entering the eye paraxially than to light entering obliquely.
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4. The following are true about spherical aberration:

a. it is responsible for reduced vision in dilated pupil

b. it results from the prismatic effect of the peripheral parts of a spherical lens

c. light passing through the periphery of a spherical lens is deviated more 
    than those through the paraxial zone of the lens

d. it can be reduced by using a lens with less dispersive power

e. it can be reduced with a doublet.
 

Answera.T   b.T   c.T   d.F  e.T
Spherical aberration is the result of prismatic effect of a spherical lens. 
A convex lens can be looked upon as a series of prism placed base to
base and the prismatic power gets progressively stronger towards the centre. The reverse applies to concave lens.
A dilated pupil causes blurred vision mainly due to chromatic aberration.
The dispersive power of a lens is related to chromatic aberration.
It can be reduced with a doublet.
 
 

5. Oblique astigmatism

a. occurs when light passing through the lens obliquely

b. is more troublesome the higher the power of the lens

c. is most troublesome in the reading section of the varifocal glasses 

d. is worse with meniscus lenses than biconvex or biconcave lens

e. of spectacle can be reduced with pantoscopic tilt
 

Answera.T   b.T   c.T   d.F   e.T
Oblique astigmatism is worse with stronger lens, biconvex or biconcave lens (as against meniscus lens) and in the reading section of the varifocal glasses. 
It can be reduced by using meniscus lens and pantoscopic tilt of the spectacles.
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6. Best form lenses is used to reduce:

a. chromatic aberration

b. oblique astigmatism

c. spherical aberration

d. coma

e. glare
 

Answera.F   b.T   c.T   d.F   e.F
Best form lenses are lenses that have been made to reduce both spherical and oblique aberrations.
 

7. Oblique astigmatism in the eye is reduced by::

a. aplanatic curvature of the cornea 

b. the vitreous

c. the variable refractive indexi of the lens

d. the spherical surface of the retina

e. the peripheral retina has less resolving power than the central cornea

 

Answera.T   b.F   c.F   d.T  e.T
The aplanatic curvature of the cornea reduces oblique astigmatism. 
The astigmatism is also reduced by the spherical surface of the retina 
so that the circle of least confusion fell on the retina. The peripheral retina has less resolving power than the central cornea.
 
 

8. The following are true about image distortion through strong lenses:

a. it is mainly the result of chromatic aberration

b. it is mainly the result of spherical aberration

c. pin-cushion distortion occurs with high minus lens

d. barrel distortion occurs with high plus lens

e. it can be reduced by getting the patient to use contact lenses
 

Answera.F   b.T   c.F   d.F   e.T
Image distortion through strong lenses is caused mainly by spherical 
aberration. Pin-cushion distortion occurs with high plus lens and barrel 
distortion occurs with high minus lens. Contact lens can reduce the problem in aphakia and high myopes.