UNIT-5 EM WAVES UNIT-6 RAY OPTICS

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Jemimah McDaniel
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1 UNIT-5 EM WAVES 2 Marks Question 1. To which regions of electromagnetic spectrum do the following wavelengths belong: (a) 250 nm (b) 1500 nm 2. State any one property which is common to all electromagnetic waves. 3. Arrange the following electromagnetic waves in increasing order of their frequencies(i.e. begin with the lowest frequency): Visible light, gamma rays, X-rays, microwaves, radio waves, infrared radiations and ultraviolet radiations. 4. Name any two properties which are common to all parts of electromagnetic spectrum. UNIT-6 RAY OPTICS 1 Marks Question 1. The colour of light that travels slowest in glass is : (a) red (b) green (c) yellow (d) violet 2. In a regular prism, what is the relation between angle of incidence and angle of emergence when it is in the minimum deviation position? 3. A Converging lens of focal length 40cm is kept in contact with a diverging lens of focal length 30 cm. Find the focal length of the combination. 4. When a light wave tavels from air to glass:

2 (A) Its wavelength decreases. (B) Its wavelength increases. (C) There is no change in wavelength. (D) Its frequency decreases. 5. Two thin lenses having optical powers of -10D and +6D are placed in contact with each other. The focal length of the combination is : (a) 0.25 cm (b) cm (c) m (d) m 6. Name the principle on the basis of which optical fibres work. 7. Calculate the dispersive power of a transparent material given: n = 1.56, n = 1.54 n = What is the optical power in dioptre of a concave lens of focal length 50 cm? 9. What is meant by resolving power of a telescope? 10. What is meant by resolving power of a telescope? 11. State any one method of increasing the resolving power of an astronomical telescope. 12. Calculate the critical angle for glass and water pair? 13. Two thin lenses of powers +15 D and -15D are put in mutual contact. Find the power, focal length and nature of this combination. 2 Marks Question 14. A ray of light, LM, incident normally on one face AB of a prism ABC having refracting angle A = 50 o grazes the adjacent face AC (See Figure 6 below). What is the refractive index of its material?

3 15. With regard to an astronomical telescope of refracting type, state how you will increase its: (i) magnifying power (ii) Resolving power. 16. Draw a labelled ray diagram showing the formation of an image by a refracting telescope when the final image lies at infinity. 17. You are provided with two convex lenses having focal lengths 4 cm and 80 cm, respectively, to form an astronomical telescope. (i) Which lens would you use as an objective of an astronomical telescope and which one as an eyepiece? (ii) If the telescope is in normal adjustment, what is its:(1) Magnifying power? (2) Length? 3 Marks Question 18. A compound microscope consists of two convex lenses 2cm and 5 cm. When an object is kept at a distance of 2.1 cm from the objective, a virtual and magnified image is formed 25cm from the eye piece. Calculate the magnifying power of the microscope. 19. A convex lens of focal length 5 cm is used as a simple microscope. Where should an object be placed so that the image formed by it lies at the least distance of distinct vision (D=25cm)? 20. A point object O is placed at a distance of 15 cm from a convex lens L of focal length 10 cm as shown in figure below. On the other side of the lens, a convex mirror M is placed such that its distance from the lens is equal to the focal

4 length of the lens. The final image formed by this combination is observed to coincide with the object O. Find the focal length of the convex mirror. 21. A convex spherical surface having radius of curvature of 20cm separates air from glass. When a point object O is kept in air, on its axis, at a distance of 50 cm from its pole, (see Figure ), a real image I is formed in glass at 300 cm from the pole P. Calculate the refractive index of glass. 22. An optical system consists of a thin convex lens L of focal length f = 15 cm and a convex mirror M having radius of curvature R=36 cm, arranged coaxially at a distance of 24 cm. (See Figure below). Where should an object O be kept so that its inverted image I formed by the lens mirror combination coincides with the object itself? 23. Draw a labelled ray diagram of an image formed by a compound microscope, when the final image lies at the least distance of distinct vision (D).

5 24. Find critical angle for glass and water pair, given refractive index of glass is 1.62 and that of water is Starting with an expression for refraction at a single spherical surface, obtain Lens Maker s Formula. 26. Show that the deviation produced by an acute angled prism is (μ-1)a, where A is angle of prism and μ is refractive index of prism material. 27. The focal length of a concave lens is 20 cm. The focal length of a convex lens is 25 cm. These two are placed in contact with each other. What is the power of combination? Is it diverging, converging or undeviating in nature? 28. Draw a neat labeled ray diagram to show how the image of a distant object is formed by an astronomical refracting telescope in normal adjustment. Write (do not derive) an expression for its magnifying power. 29. An astronomical telescope is adjusted to form the final image at infinity. The separation between the lenses is 80 cm. The angular magnification is 15. Calculate the focal lengths of the objective lens and the eyepiece. 4 Marks Question 30. Find the distance between the two lenses of a Compound Microscope if the final image formed by the microscope is virtual and lies at a distance of 25 cm to the left of the eye-piece. Magnifying power of the microscope is 30 and focal lengths of objective and eyepiece are 2cm and 5cm, respectively. WAVE OPTICS 1 Marks Question 1. With what type of source of light are cylindrical wave fronts associated? 2. What is meant by diffraction of light?

6 3. In the field of view of Young s experiment using light of wavelength 6600A, 60 fringes are observed. How many fringes are observed in the same region when light of 4400A is employed? 4. For observing Fraunhofer diffraction from a single slit, what type of wave front should be incident on this slit? 5. How is fringe width of an interference pattern in young s double slit experiment affected if the two slits are brought closer to each other? 6. What is the difference between polarized light and unpolarised light? 7. What type of wave front is associated with a line source of light? 8. Calculate the polarizing angle for glass whose refractive index is State any one difference between interference of light and diffraction of light. 10. A ray of ordinary light is travelling in air. It is incident on air glass pair at a polarizing angle of 56 o. Find the angle of refraction in glass. 11. In fraunhofer diffraction, what kind of source of light is used and where is it situated? 12. State Brewster s law. 13. Find Brewster angle for a transparent liquid having refractive index Marks Question 14. State how focal length of a glass lens(refractive index 1.5) changes when it is completely immersed in : (i) water (refractive index=1.33)(ii) A liquid ( Refractive index=1.65) 15. Why the amplitude of two sources must be almost equal to observe interference? The ratio of amplitude of two waves is 3:5. What would the ratio of their maximum and minimum intensities? 16. Derive the expression for the point to be bright in Young s double slit experiment.

7 17. A slit of width d is illuminated by light of wavelength 5500 A. What will be the value of slit width a when (i) first minimum falls at an angle of diffraction 30 o? (ii) first maximum falls at an angle of diffraction 30 o? 18. State Brewster s law. Prove that reflected ray and refracted ray are perpendicular at Brewster angle. 3 Marks Question 19. In young s double slit experiment using monochromatic light of wavelength 600nm, 5 th bright fringe is at a distance of 0.48 mm from the center of the pattern. If the screen is at a distance of 80 cm from the plane of the two slits, calculate; (i) Distance between the two slits, (ii) Fringe width. 20. In Young s double slit experiment, using light of wavelength 600 nm, 10th bright fringe is obtained on a screen, 3mm from the centre of the pattern. If the screen is 120 cms away from the slits, calculate: (i) Distance between the two slits (ii) Fringe width, i.e. fringe separation. 21. What is meant by diffraction of light? What is an optical grating? State its use. 22. On the basis of Huygen s wave theory of light, show that angle of reflection is equal to angle of incidence. You must draw a labelled diagram for this derivation. 23. Laser light of wavelength 630 nm is incident ona pair of slits which are separated by 1.8 mm. If the screen is kept 80cm away from the two slits, calculate: (i) fringe separation i.e. fringe width. (ii) distance of 10 th bright fringe from the center of the interference pattern. 24. Derive snell s law of refraction using Hugyen s wave theory.

8 25. Monochromatic light of wavelength 650nm falls normally on a slit of width cm and the resulting Fraunhofer diffraction is obtained on a screen. Find the angular width of the central maxima. 26. In Young s double slit experiment, show that: β =, where the terms have their usual meaning. 27. Find the angle of incidence at which a ray of monochromatic light should be incident on the first surface AB of a regular glass prism ABC so that the emergent ray grazes the adjacent surface AC. (Refractive index of glass =1.56)

Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi , Ph. : ,

Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi , Ph. : , 1 O P T I C S 1. Define resolving power of a telescope & microscope and give the expression for its resolving power. 2. Explain briefly the formation of mirage in deserts. 3. The radii of curvature of