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Optics questions

 VERY SHORT ANSWER QUESTIONS: CBSE

  1. When light undergoes refraction, what happens to its frequency? (unchanged)

  2. What is the ratio of velocities of light of wavelength 4000 A0 & 8000 A0 in free space/vacuum? (1:1)

  3. For same angle of incidence, the angles of refraction in three different media A, B, C are 150, 250 and 350 respectively. In which medium is the velocity of light minimum? 

Answer: A

  1. Not in syllabus

Why is the bluish color predominant in a clear sky? Hint: Scattering α14

  1. An object is placed at principal focus of a concave lens of focal length f. where will its image be located? Answer: At - f2

  2. The refractive index of a material of a convex lens is n1. It is immersed in a liquid of refractive index n2. A parallel beam of light is incident on the lens. Trace the path of the emergent ray when n2>n1.

Answer

   

  1. How does the power of a convex lens vary, if the incident red light is replaced by violet light? Answer: Increases

  2. A diverging lens of focal length f is cut into two identical parts. Each forming a Plano convex lens. What is the focal length of each part? Answer: f/2

  3. A concave lens is placed in water. How will its focal length change from air ? Answer: Increases

  4. Two thin lenses of power +7D and -3D are in contact. What is the focal length of the combination? Answer: 25 cm

  5. A prism of refracting angle 600 gives minimum deviation of 300. What is its refractive index? Answer:  2

  6. What type of wave front will emerge from a point source and a distant source of light? Answer: Spherical and plane respectively.

  7. Can two identical and independent sodium lamps act as coherent sources? Answer: No

  8. Sketch the variation of intensity of the interference pattern in Young’s double slit experiment.

Answer:                               

  1. How does the angular separation of interference fringes change in Young’s double slit experiment, if the distance between the slits is increased? Answer: Decreases

  2. How does the interference pattern change in Young’s double slit experiment, if the (i) width of the slits is doubled (ii) screen is moved away from the slits? Answer: Answer: i) Maxima becomes 16 times, ii) increases

  3. How the angular separation and visibility of fringes in Young’s double slit experiment does change when (i) screen is moved away (ii) width of source slit is increased. Answer: (i) angular separation is Independent, visibility increases (ii) s/S < λ/d is not satisfied, pattern disappears.

  4. How does the fringe width of interference fringes change, when the whole apparatus of Young’s double slit experiment is kept in a liquid of refractive index 1.3? Answer: decreases to 1/1.3

  5. In Young’s double slit experiment, the width of fringes obtained with light of wavelength 6000 A0 is 2 mm. What will be the fringe width if the entire apparatus is kept in a liquid of refractive index 4/3? Answer: 1.5 mm

  6. In Young’s double slit experiment, slits are separated by 0.28 mm and the screen is 1.4 m away. Distance between central and 4th bright fringe is 1.2 cm. Calculate wavelength of light used. Ans 6x10-7m.

  7. Red light of wavelength 6500 A falls on a slit 0.5 mm wide. What is the distance between two dark bands on each side of the central bright diffraction pattern on a screen 1.8 m away? Answer: 4.7mm

  8. Not in the syllabus. Estimate distance for which ray optics is a good approximation of an aperture of 4 mm and wavelength 400 nm. Answer: 1.5 cm

  9. How does the intensity of the central maximum change if the width of the slit is halved in a single slit diffraction experiment? Answer:  ¼

  10. Not in the syllabus. The polarizing angle of a medium is 600. What is the refractive index of the medium? Answer: 3

  11. Not in the syllabus. At what angle of incidence should a light beam strikes a glass slab of refractive index3, such that reflected and refracted rays are perpendicular to each other? Answer: 600

  12. Not in the syllabus. If the angle between planes of polarizer and analyzer is 600, by what factor does the intensity of transmitted light change when passing through the analyzer? Answer:  ¼

  13. Not in the syllabus. Draw a graph showing the variation of intensity of polarized light transmitted by an analyzer.

              

  1. Two coherent waves of equal amplitude produce an interference pattern in Young’s double slit experiment. What is the ratio of intensities k1 and ke. The intensity at a point where phase difference between the two waves is π/2 is k1. The intensity at centre is k2? Answer: ½

  2. Calculate the speed of light in a medium for which the critical angle for total internal reflection is 450. Answer: 2.12x108 m/s

  3. Draw the graph showing the variation of u & v in case of a convex lens.    Answer:                                                                                   

  4. What change in the focal length of a (i) concave mirror and (ii) convex lens occurs, when the incident violet color light  is replaced by red coloured light? Answer: (I) no change in case of mirrors ii) f increases

SHORT ANSWER QUESTIONS:

  1. Draw a ray diagram to show the image formation in a refracting type astronomical telescope in the near point adjustment. Write down the expression for its magnifying power. Why should the diameter of the object be larger? Answer:To increase intensity of image.

  2. Show a ray diagram of image formed by compound microscope. Write the expression for its magnifying power.

  3. Draw a labeled ray diagram of a refrecting type telescope in normal adjustment. Derive expression for its magnifying power. State two advantages of reflecting type telescopes over refracting type telescopes.

  4. Not in syllabus. Define resolving power of a compound microscope. How does the resolving power of a compound microscope changes when (i) refractive index between the object and objective lens increases. (ii) Wavelength of radiation used increases; (iii) diameter of objective is decreased? Increases, decreases, decreases

  5. Not in syllabus. The aperture of objective lens of an astronomical telescope is doubled. How does it affect (i) its resolving power (ii) intensity of the image? Doubled,4 times

  6. Draw the diagrams to show the behavior of plane wave fronts as they pass through I) a thin prism II) a convex lens.

  7. What are coherent sources of light?

  8. State two conditions for sustained interference of light.

  9. Draw the diffraction pattern due to a single slit.

  10. Draw geometrical shape of the wavefronts when light diverges from a point source.

  11. Give two differences between double slit interference and single slit diffraction Patterns of light.

  12. Not in syllabus. If a light beam shows no intensity variation when transmitted through a Polaroid which is rotated, does it mean that the light is unpolarized? Yes

  13. Not in syllabus. What is a plane polarized light? What is a Polaroid? Write its two uses.

  14. Not in syllabus. State Malus law. Write Brewester’s law.

LONG ANSWER QUESTIONS: CBSE

  1. Derive these relations: mirror formula: 1/f = 1/v + 1/u;  

Thin lens formula:

1/f = 1/v - 1/u

Formula for the refraction at spherical surface:

n/v - 1/u = (n-1)/R

Lens maker's formula:

1/f =(n-1)( 1/R1 - 1/R2 )

Formula to find refractive index by a prism:

n = [sin(A+Dm)/2]/sinA/2 

  1. State Huygen’s principles. Verify Snell’s law of refraction using his wave theory.

  2. Not in syllabus. Derive a mathematical expression for the width of interference fringes obtained in Young’s double slit experiment with the help of a suitable diagram.

  3. Write an expression for the width of central maximum due to diffraction at a single slit.

  4. A compound microscope consists of an objective lens of focal length 2.0 cm and an eyepiece of focal length 6.25 cm separated by a distance of 15 cm. How far from the objective should an object be placed in order to obtain the final image at (a) the least distance of distinct vision (25 cm), and (b) at infinity? What is the magnifying power of the microscope in each case? Answer: 2.5 cm, 2.59 cm, -20, -13.5

  5. A giant refracting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece of focal length 1.0 cm is used, what is the angular magnification of the telescope? If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.48 × 106m, and the radius of lunar orbit is 3.8 × 108m. Answer: -1500, 13.7 cm

  6. Calculate the distance of an object of height ‘h’ from a concave mirror of focal length 1 cm, so as to obtain a real image of magnification 2. Answer: – 15 cm

  7. A lens forms a real image of an object. The given graph shows the variation of v with u. what is the nature of lens? Using graph, find focal length of the lens.

         

  1. A double convex lens of glass of refractive index 1.6 has its both surfaces of equal radii of curvature of 30 cm each. An object of height 5 cm is placed at a distance of 12.5 cm from the lens. Calculate the size of the image formed. Answer: f= 25, v = 25, hi= 10 cm

  2. A converging lens of focal length 20 cm is made of a material of refractive index 1.5. If it is immersed In water of refractive index 4/3, what will be the new focal length? Answer: 80 cm

  3. A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16cm? Answer: 7.5 cm, 48 cm

  4. A compound microscope with an objective of 1 cm focal length and an eye-piece of 2 cm focal length has a tube length of 20 cm. Calculate the magnifying power of the microscope, if final image is formed at the near point of the eye. Answer: – 270

  5. A ray of light passing through an equilateral triangular prism from air, undergoes minimum deviation when the angle of incidence is 3/4th of the angle of the prism. Calculate the speed of light in the prism. Answer: 2.13x108 m/s.

  6. Find the ratio of intensities at two points on a screen in Young’s double slit experiment, where the waves from the two slits have path difference of 0 and λ/4. Answer: 2/1

  7. A slit is illuminated by light of wavelength 700 nm. What will be the value of slit width ‘a’ if the first minimum falls at an angle of diffraction 300? Answer: 1.4 µm

  8. A parallel beam of light of wavelength 600 nm is incident normally on a slit of width ‘a’. if the distance between slit and screen is 0.8 m and the distance of 2nd order maximum from the centre of the screen is 1.5 mm, calculate the width of the slit. Answer: 0.8 mm

  9. A double convex lens of refractive index 1.5 has its both surfaces of equal radii of curvature 20 cm each. An object of height 5 cm is placed at a distance of 15 cm from the lens. Calculate size of image formed. Answer: 20 cm

  10. A diverging lens of refractive index 1.5 and of focal length 20 cm in air has the same radii of curvature for both sides. If it is immersed in a liquid of refractive index 1.7, calculate focal length in liquid. Answer: -8.5

  11. The magnifying power of an astronomical telescope in the normal adjustment position is 100. The distance between the object and the eyepiece is 101 cm. Calculate focal lengths of objective and eyepiece. Answer: 32 cm, 1 cm

  12. A converging lens of focal length 50 cm is placed coaxially in contact with another lens. If combination behaves as diverging lens of focal length 50 cm, find power and nature of 2nd lens. Answer: -25 cm, -4D

  13. Not in syllabus. Calculate resolving power of astronomical telescope assuming diameter of the objective to be 6 cm and wavelength of light 540 nm. Answer: 9.11 x 104

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