Test 18

1. For a prism in the air, the following will determine the angle of deviation:  a. the position of the apex  b. the refracting angle of the prism c. the refractive index of the material that made up the prism d. the length of the base e. the angle of incidence of the ray   answera.F   b.T   c.T   d.F   e.T Three factors determine the angle of deviation: a. refractive index of the material that made up the prism b. the refracting angle of the prism c. the angle of incidence of the ray     2. The following are true about the image formed by prism:  a. it is virtual  b. it is erect c. it is deviated towards the base of the prism d. red light is deviated more than green light  e. blue light is deviated more than green light answera.T   b.T   c.F   d.F  e.T The image produced by prism is erect, virtual and deviated towards the apex of the prism. Light with longer wavelength is deviated less than light shorter  wavelength. Therefore, blue light is deviated more than green light which is in turn deviated more than red light by a prism.   3. The following are true about prism:  a. the angle of deviation for a prism is always half of its      refracting angle  b. one prism dioptre causes an apparent displacement of      1cm of an object situated 1 metre from the prism  c. the prism power of a prism can be deduced from its refracting     angle alone d. a one centrad power prism deviates a light by the same amount     as a prism with one prism dioptre. e. the angle of minimum deviation occurs when the angle of incidence     equals the angle of emergence. answera.F   b.T   c.F   d.F   e.T The angle of deviation for a prism is half of its refracting  angle only applies to material with a refractive index of 1.5 such as glass. To calculate the prism power of a prism it is necessary to know the refracting power and the refractive index of the material that made up the prism. A prism of one centrad deviates more than a prism with one prism dioptre.     4. For a patient with a 6 D right esotropia, the following can      relieve the deviation:  a. a 6 D prism placed base out over the right eye b. a 2 D prism placed base out over the right eye and      another 4 D prism base in over the left eye c. two 3 D prisms placed both base out over the two eyes d. two 3 D prisms placed both base in over the two eyes e. two 3 D prisms with one placed base in and the other base out answera.T   b.F   c.T   d.F   e.F For esotropia, the apex of the prism is in the direction  of the deviation ie, the base is in opposite direction to the direction of deviation and therefore base out. If the prism is split between the two eyes, both prisms should be base out.   5. In a patient with a 4 D right hypertropia, the following will relieve the diplopia:  a. a 4 D prism placed base down over the right eye b. a 4 D prism placed base up over the left eye c. two 2 D prism both placed base up over both eyes d. two 2 D prisms both placed base down over both eyes e. two 2 D prisms one placed base down and the other base up. answera.T   b.T   c.F   d.F   e.T For a vertical deviation, the prism can be placed either  base down over the higher eye or base up over the lower eye. If the prisms were split, the two prisms should be placed with bases over each other.   Return to the main page