OPTICS(ASSIGNMENT)BOARD QUESTIONS

1. Distinguish between linearly polarized and unpolarized light. Show that light waves are transverse in nature. Why does light from a clear blue portion of the sky show a rise and fall of intensity when viewed through a polaroid which is rotated? Explain by drawing the necessary diagram.

2. A ray PQ is incident normally on the face AB of a triangular prism of refracting angle of 600, made of a transparent material of refractive index 2/√3, as shown in the figure. Trace the path of the ray as it passes through the prism.

Qn. No.8

Qn.No8

3. Draw a schematic diagram of a reflecting telescope. Write its two advantages over a refracting telescope.

4. Draw a ray diagram showing the formation of the image by a point object on the principal axis of a spherical convex surface separating two media of refractive indices n1 and n2, when a point source is kept in rarer medium of refractive index n1. Derive the relation between object distance, image distance in terms of refractive index of the medium and radius of curvature of the surface. Hence obtain lens makers formula in the case of thin convex lens.

5. Define the term coherent sources which are required to produced interference pattern in Young’s double slit experiment.

6. Show, giving a suitable diagram, how unpolarized light can be polarized by reflection. Two polaroids P1and P2 are placed with their pass axis perpendicular to each other. Unpolarised light of intensity I0 is incident on P1. A third polaroid P3 is kept in between P1 and P2 such that its pass axis makes an angle of 600 with that of P1. Determine the intensity of light transmitted through P1, P2 and P3.

7. A convex lens of focal length 20cm is placed coaxially with a convex mirror of radius of curvature 20cm. The two are kept 15cm apart. A point object is placed 40cm in front of the convex lens. Find the position of the image formed by this combination. Draw the ray diagram showing the image formation.

8. Two monochromatic rays of light are incident normally on the face AB of an isosceles right angled prism ABC. The refractive indices of the glass prism for the two rays 1 and 2 are respectively 1.38 and 1.52. Trace the path of these rays after entering through the prism.

9. Two polaroid’s P1and P2 are placed with their pass axis perpendicular to each other. Unpolarised light of intensity I0 is incident on P1. A third polaroid P3 is kept in between P1 and P2 such that its pass axis makes an angle of 450 with that of P1. Determine the intensity of light transmitted through P1, P2 and P3.

10. A convex lens of focal length 20cm is placed coaxially with a concave mirror of radius of curvature 20cm. The two are kept 50cm apart. A beam of light coming parallel to the principal

axis is incident on the convex lens. Find the position of the image formed by this combination. Draw the ray diagram showing the image formation.

11. The ratio of the intensities at minima to the maxima in the Young’s double slit experiment is 9:25. Find the ratio of the width of two slits.

12. In Young’s double slit experiment, describe briefly how bright and dark fringes are obtained on the screen kept in front of a double slit. Hence obtain the expression for the fringe width.

13. Describe briefly how a diffraction pattern is obtained on a screen due to a single narrow slit illuminated by a monochromatic source of light. Hence obtain the conditions for the angular width or secondary maxima and secondary minima.

14. Two wavelengths of sodium light of 590nm and 596nm are used in turn to study the diffraction pattern taking place at a single slit of aperture 2×10-6m. The distance between the slit and screen is 1.5m. Calculate the separation between the positions of the first maxima of the diffraction pattern obtained in the two cases.

15. Two polaroids P1and P2 are placed with their pass axis perpendicular to each other. Unpolarised light of intensity I0 is incident on P1. A third polaroid P3 is kept in between P1 and P2 such that its pass axis makes an angle of 300 with that of P1. Determine the intensity of light transmitted through P1, P2 and P3.

16. Two independent monochromatic sources of light cannot produce a sustained interference pattern. Give reason.

17. Light waves each of amplitude ‘a’ and frequency ῳ, emanating from two coherent light sources superpose at a point. If the displacements due to these waves is given by Y1 =a cos ῳt and Y2 =a cos (ῳt +ф) where ф is the phase difference between the two. Obtain the expression for the resultant intensity at that point.

18. In Young’s double slit experiment using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ, is K units. Find out the intensity of light at a point where the path difference is λ/3.

19. A beam of unpolarised light is incident on a glass air interface. Show, using a suitable diagram, that light reflected from the interface is totally polarized, when µ = tan ip, where µ is the refractive index of glass with respect to air and ip is the Brewster’s angle.

20. For a single slit of width a, the first minimum of the interference pattern of monochromatic light of wavelength λ occurs at an angle of λ/a . At the same angle of λ/a we get a maximum for two narrow slits separated by a distance a. explain.

21. A convex lens is placed in contact with a plane mirror. A point object at a distance of 20cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens?

22. In a modified setup of Young’s double slit experiment, its given that SS2 – SS1 = λ/4. Ie the source S is not equidistant from the slits S1 and S2. a)Obtain the conditions for constructive and destructive interference at any point P on the screen in terms of the path difference δ = S2P - S1P. b) Does the observed central fringe lie above or below ‘O’? Give reason to support your answer.

P S1

S O

S2

23. An equiconvex lens of refractive index µ1, focal length f and radius of curvature R is immersed in a liquid of refractive index µ2. For µ2 >µ1 and µ2 <µ1 , draw the ray diagrams in the two cases when a beam of light coming parallel to the principal axis is incident on the lens. Also find the focal length of the lens in terms of original focal lens and refractive index of the glass of the lens and that of the medium.

24. A parallel beam of light 450nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1.5cm away. It is observed that the first minimum is at a distance of 3mm from the Centre of the screen. Calculate the width of the slit.

25. A parallel beam of light 500nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1m away. It is observed that the first minimum is at a distance of 2.5mm from the Centre of the screen. Calculate the width of the slit.

26. State Huygens’s principle. Using this principle draw diagram to show how a plane wave from incident at the interference of the two media gets refracted when it propagates from a rarer to a denser medium. Hence verify Snell’s law of refraction.

27. When monochromatic light travels from a rarer to a denser medium, explain the following by giving reasons. i. Is the frequency of reflected and refracted light same as the frequency of incident light? Ii. Does the decrease in speed imply a reduction in the energy carried by light wave?

28. A convex lens of focal length 25cm is placed coaxially in contact with a concave lens of focal length 20cm. Determine the power of the combination. Will the system be converging or diverging?

29. A convex lens is placed over a plane mirror. A pin is now positioned so that there is no parallax between the pin and its image formed by this lens-mirror combination. How can this observation be used to find the focal length of the convex lens? Give appropriate reasons in support of your answer.

30. You are given three lenses L1,L2 and L3 each of focal length 20cm. An object is kept 40cm in front of L1. The final real image is formed at the focous ‘I’ of L3. Find the separations between L1,L2 and L3 .

40cm 20cm I