Q22 of 23 Page 65

The optical properties of a medium are governed by the relative permittivity (εr) and relative permeability (μr ). The refractive index is defined as For ordinary material εr > 0 and μr > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with εr < 0 and μr < 0. Since then such ‘metamaterials’ have been produced in the laboratories and their optical properties studied. For such materials As light enters a medium of such refractive index the phases travel away from the direction of propagation.

(i) According to the description above show that if rays of light enter such a medium from air (refractive index = 1) at an angle in 2nd quadrant, them the refracted beam is in the 3rd quadrant.


(ii) Prove that Snell’s law holds for such a medium.


(i) Let’s assume the given postulate is true, then if A and B be two parallel rays entering to such a medium from air the refracted diagram will be FIG (1)


Here incidence angle is θi and refracted angle is θr . AB shows the incident wave front and ED shows the refracted wave front, then all the points on ED will have the same phase.


We know all the points with the same optical path length must have the same phase.



FIG (1)


Given n2=


n2AE=BC+ n2CD




If BC>0,


CD should be greater than 0, which is true according postulate diagram, which means rays of light enter such a medium from air (refractive index = 1) at an angle in 2nd quadrant, then the refracted beam is in the 3rd quadrant.


Let us consider the normal case where light entering 2nd quadrant refracting in 4rth quadrant as in FIG (2), then


n2AE=BC+ n2CD



,


which means if AE>CD, BC <0 which is not possible


the postulate is true



FIG (2)


(ii) From FIG (1)







--Hence Snell’s law is proved


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19

Figure 10.5 shown a two-slit arrangement with a source which emits unpolarised light. P is a polarizer with axis whose direction is not given. If I0 is the intensity of the principal maxima when no polarizer is present, calculate in the present case, the intensity of the principal maxima as well as of the first minima.


 20

A small transparent slab containing material of μ =1.5 is placed along AS2 (Fig.10.6). What will be the distance from O of the principal maxima and of the first minima on either side of the principal maxima obtained in the absence of the glass slab.


AC = CO = D, S1C = S2C = d << D


 21

Four identical monochromatic sources A, B, C, D as shown in the (Fig.10.7) produce waves of the same wavelength and are coherent. Two receiver R1 and R2 are at great but equal distances from B.


(i) Which of the two receivers picks up the larger signal?


(ii) Which of the two receivers picks up the larger signal when B is turned off?


(iii) Which of the two receivers picks up the larger signal when D is turned off?


(iv) Which of the two receivers can distinguish which of the sources B or D has been turned off?


 23

To ensure almost 100 per cent transitivity, photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediated between that of air and glass (which makes the optical element of the lens). A typically used dielectric film is MgF2 (n = 1.38). What should the thickness of the film be so that at the center of the visible spectrum (5500 Å) there is maximum transmission.