Test 29 1. A lens has the following power +4.00/+2.00 X 90: a. the spherical equivalent is +5.00D b. the horizontal focal length is 25cm  c. the vertical focal length is 50cm d. the circle of least confusion is midway between     the two focal lengths e. is similar to one with the power +6.00/-2.00 X180   Answer a.T   b.T   c.F   d.T   e.T The lens can be considered as made up of two powers +6.00DS and +4.00DS. The horizontal focal length is 25cm and the vertical focal length is 100/6 = 16.67cm. 2. A 15 year old boy was given glasses for the time with the following      prescriptions:     OD: -0.50  / +1.75 X  150      OS: -0.50 / +1.50  X  22      He returns few days later says he cannot wear them due to ocular      discomfort. The power and the interpupillary distance are checked      and found to be accurate:   a. the problem is likely to be distortion due to meridional      magnification b. the discomfort can be reduced by reducing the      cylindrical power while maintaining the spherical      equivalent c. the discomfort can be reduced by rotating the cylinder      towards 1800 d. the discomfort can be reduced by increasing the vertex     distance e. the discomfort can be reduced by changing the      cylindrical lens to minus power   Answer a. T   b.T   c.T   d.F   e.T The discomfort can be reduced by reducing the vertex distance.    3. The following are true about light: a. the range of visible wavelength is from 100 to 700nm  b. light with a wavelength of more than 700nm is      in the ultraviolet spectrum c. infrared is not visible to the eye d. shorter wavelength is bent more than longer wavelength     when passes through a plus lens e. green light is bent more than red light when passing     through a prism   Answer a.F   b.F   c.T   d.T   e.T The range of visible wavelength is between 400 to 700nm. Ultraviolet has a wavelength of less than 400nm and infrared beyond 700nm. Shorter wavelength is bent more than longer wavelength by a prism of a plus lens. 4. Wavefront analysis:  a. measures higher order optical aberration b. measures chromatic aberration  c. measures monochromatic aberration d.  is affected by the pupil size e. can be measured with Tscherning aberrometry   Answer a.T   b.F   c.T   d.T   e.T Wavefront analysis does not measures chromatic aberration. It measures higher optical aberration and is useful in laser vision correction. 5. The following are true about the Stiles-Crawford effect : a. the central retinal cones are maximally stimulated by      light entering the eye through the pupil parallel to the      visual axis b. light entering the pupil obliquely has a significant effect     on scotopic stimulus c. light entering the pupil 3 mm from the physiologic      centre of the eye significantly affect the photopic      stimulus d. it reduces the effect of spherical aberration of the     lens on the fovea e. it can be used to evaluate photoreceptor misalignment     in retinal disease   Answer a.T   b.F   c.T   d.T   e.T The Stiles-Crawford effect has a greater effect on cones than rods and therefore affects the photopic stimulus more than the scotopic stimulus.