Friday, June 08, 2007

IIT-JEE 2007 Linked Comprehension Type MCQ on Optics

Physics is a subject with great scope for setting questions with numerous variations conforming to the prescribed syllabus. Often the questions may look lengthy and too much descriptive. Don’t be scared by lengthy questions. Most of them will be simple as for instance, the following linked comprehension type multiple choice questions which appeared in IIT-JEE 2007 question paper:

Paragraph for Question Nos. 1 to 3:

The figure shows a surface XY separating two transparent media, medium-1 and medium-2. The lines ab and cd represent wavefronts of a light wave traveling in medium-1 and incident on XY. The lines ef and gh represent wavefronts of the light wave in medium-2 after refraction.

(1) Light travels as a

(a) parallel beam in each medium

(b) convergent beam in each medium

(c) divergent beam in each medium

(d) divergent beam in one medium convergent beam in the other medium

Light rays have to be perpendicular to the wave front. Since the wave fronts are shown as parallel, the rays are parallel. So, the correct option is (a).

(2) The phases of the light waves at c, d, e and f are Фc, Фd, Фe and Фf respectively.

It is given that ФcФf .

(a) Фc cannot be equal to Фd

(b) Фd can be equal to Фe

(c) (Фd Фf ) is equal to (Фc Фe)

(d) (Фd Фc ) is not equal to (Фf Фe)

All points on a wave front will have the same phase. The lines cd and ef represent the wave fronts of the same light wave at two instants. Therefore, points ‘c’ and ‘d’ have the same phase. Points ‘e’ and ‘f’ also have the same phase, but these points have the same phase lag with respect to points ‘c’ and ‘d’. So, the correct option is (c).

(3) Speed of light is

(a) the same in medium -1 and medium -2

(b) larger in medium -1 than in medium -2

(c) larger in medium -2 than in medium -1

(d) different at b and d

The angle of refraction is evidently less than the angle of incidence at XY and hence the speed of light is larger in medium-1 [Option (b)].

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