A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear?
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Optics 1 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Question 1. A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear? (Delhi 2008) Answer: In order to make the lens disappear the refractive index of liquid must be equal to 1.5 i.e. equal to that of glass lens. Question 2. A converging lens of refractive index 1.5 is kept in a liquid medium having same refractive index. What would be the focal length of the lens in this medium? (Delhi 2008) Answer: The lens in the liquid will act like a plane sheet of glass ∴ Its focal length will be infinite (∞) Question 3. How does the power of a convex lens vary, if the incident red light is replaced by violet light? (Delhi 2008) Answer: According to Lens Maker’s formula
Optics 2 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana ∴ power of the lens will be increased. Question 4. How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced with red light? (All India 2008) Answer: We know that λ red > λ violet, therefore µ red < µ violet and hence δ red < δ violet. When incident violet light is replaced with red light, the angle of minimum deviation of a glass decreases. Question 5. Why does the bluish colour predominate in a clear sky? (All India 2008) Answer: Blue colour of the sky : The scattering of light by the atmosphere is a colour dependent. According to Rayleigh’s law, the intensity of scattered light I∝1λ4, blue light is scattered much more strongly than red light. Therefore, the colour of sky becomes blue. The blue component of light is proportionately more in the light coming from different parts of the sky. This gives the impression of the blue sky. Question 6. How does the angle of minimum deviation of a glass prism of refractive index 1.5 change, if it is immersed in a liquid of refractive index 1.3? (All India 2008) Answer:
Optics 3 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Hence angle of deviation is decreased. Question 7. You are given following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope? (Delhi 2009) Lenses Power (P) Aperture L1 3D 8 cm L2 6D 1 cm L3 10D 1 cm Answer: Objective – Less power and more aperture. So L1 Eyepiece – More power and less aperture. So L3.
Optics 4 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Question 8. Two thin lenses of power + 4D and – 2D are in contact. What is the focal length of the combination? (All India 2009) Answer: Question 9. Two thin lenses of power + 6D and – 2D are in contact. What is the focal length of the combination? (All India 2009) Answer: Question 10. A glass lens of refractive index 1.45 disappears when immersed in a liquid. What is the value of refractive index of the liquid? (Delhi 2010) Answer: The value of refractive index of the liquid should be 1.45 so that the glass lens of refractive index 1.45 disappears when immersed in a liquid. Question 11. State the conditions for the phenomenon of total internal reflection to occur. (Delhi 2010) Answer: Two essential conditions for total internal reflection are : 1. Light should travel from an optically denser medium to an optically rarer medium. 2. The angle of incidence in the denser medium must be greater than the critical angle for the two media.
Optics 5 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Question 12. Calculate the speed of light in a medium whose critical angle is 30°. (Delhi 2010) Answer: ∴ Speed of light, v = 1.5 × 108 ms-1 Question 13. A converging lens is kept coaxially in contact with a diverging lens — both the lenses being of equal focal lengths. What is the focal length of the combination? (Delhi 2010) Answer: Focal length of the combination is Infinity. Question 14. When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer. (All India 2010) Answer: No, the energy carried by the lightwave remains the same. Reason : As energy E = hv Here frequency remains same. Question 15. When monochromatic light travels from one medium to another its wavelength changes but frequency remains the same. Explain. (Delhi 2011) Answer: If v1 and v2 denote the velocity of light in medium 1 and medium 2 respectively and λ1 and λ2 denote the wavelength of light in medium 1 and medium 2. Thus
Optics 6 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana The above equation implies that when a wave gets refracted into denser medium (v1 > v2) the wavelength and the speed of propagation decreases but the frequency v (=v/λ) remains the same. Question 16. Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid? (Delhi 2012) Answer: When the refractive index of glass of biconvex lens is equal to the refractive index of the liquid in which lens is immersed or µ1 = µg Question 17. For the same value of angle of incidence, the angles of refraction in three media A, B and C are 15°, 25° and 35° respectively. In which medium would the velocity of light be minimum? (All India 2012) Answer: ∴ Velocity of light is minimum in medium A. Question 18. How would a biconvex lens appear when placed in a trough of liquid having the same refractive index as that of the lens? (Comptt. Delhi 2011)
Optics 7 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Answer: A biconvex lens appears plane glass when placed in a trough of liquid having the same refractive index as that of the lens. Question 19. Two thin lenses of power -4D and 2D are placed in contact coaxially. Find the focal length of the combination. (Comptt. All India 2011) Answer: Power of combination = – 4D + 2D = – 2D ∴ Focal length, f = – 50 cm Question 20. Two thin lenses of power -2D and 2D are placed in contact coaxially. What is the focal length of the combination? (Comptt. All India 2011) Answer: Power of combination = -2D + 2D = 0 Question 21. Write the relationship between angle of incidence ‘i’, angle of prism ‘A’ and angle of minimum deviation for a triangular prism. (Delhi 2013) Answer: where [δm is angle of minimum deviation] Question 22. When red light passing through a convex lens is replaced by light of blue colour, how will the focal length of the lens change? (Comptt. All India 2013)
Optics 8 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Answer: Focal length of lens will decrease μv>μr Question 23. If the wavelength of light incident on a convex lens is increased, how will its focal length change? (Comptt. All India 2013) Answer: Question 24. A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens? ‘ (Delhi 2014) Answer: Focal length of lens = 20 cm (Hint: Rays coming out of lens are incident normally on plain mirror and hence reflected rays will trace the path of incident ray, hence forming image on the object itself, thus object and image overlapping each other at F of convex lens.) Question 25. A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. (All India 2014) Answer: The lens will behave as a diverging lens, because -1)
Optics 9 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana The value of (µ – 1) is negative and ‘f’ will be negative. Question 26. A biconvex lens made of a transparent material of refractive index 1.5 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. (All India 2014) Answer: The lens will behave as a converging lens because Hence value of ‘f’ will be positive Question 27. A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens? (Delhi 2015) Answer: Converging. Question 28. Why does bluish colour predominate in a clear sky? (All India 2015) Answer: Blue colour of the sky : The scattering of light by the atmosphere is a colour dependent. According to Rayleigh’s law, the intensity of scattered light I∝1λ4, blue light is scattered much more strongly than red light. Therefore, the colour of sky becomes blue. The blue component of light is proportionately more in the light coming from different parts of the sky. This gives the impression of the blue sky. Question 29. When an object is placed between f and 2f of a concave mirror, would
Optics 10 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana the image formed be (i) real or virtual and (ii) diminished or magnified? (Comptt. Delhi 2015) Answer: (i) Real (ii) magnified Question 30. How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light? Give reason. (Delhi 2017) Answer: The angle of minimum deviation decreases, when violet light is replaced by red light because refractive index for violet light is more than that for red light. Question 31. An object is kept in front of a concave lens. What is the nature of the image formed? (Comptt. Delhi 2017) Answer: When an object is kept in front of a concave lens, the nature of image formed is virtual, erect and diminished. Question 32. When light travels from a rarer medium to denser medium, the speed of light decreases. Does the reduction in speed imply a reduction in the energy? (Comptt. Delhi 2017) Answer: The reduction in speed, due to light travelling from a rarer to denser medium does not imply reduction in the energy. Question 33. The objective lenses of two telescopes have the same apertures but their focal lengths are in the ratio 1: 2. Compare the resolving powers of the two telescopes. (Comptt. All India 2017) Answer: Ratio of resolving power = 1 : 1 Resolving power is same because it does not depend on focal length of the objective.
Optics 11 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Question 34. Why must both the objective and the eye piece of a compound microscope have short focal lengths? (Comptt. All India 2017) Answer: For getting higher magnification in compound microscope, both objective and eyepiece must have short focal length, because Question 35. Draw a ray diagram of a reflecting type telescope. State two advantages of this telescope over a refracting telescope. (Delhi 2008) Answer: (ii) Advantages of reflecting telescope over a refracting telescope: 1. Due to large aperture of the mirror used, the reflecting telescopes have high resolving power. 2. This type of telescope is free from chromatic aberration (formation of coloured image of a white object). 3. The use of paraboloidal mirror reduces the spherical aberration (formation of non-point, blurred image of a point object). 4. Image formed by reflecting telescope is brighter than refracting telescope. 5. A lens of large aperture tends to be very heavy and therefore difficult to make and support by its edges. On the other hand, a mirror of equivalent optical quality weights less and can be supported over its entire back surface. Question 36. Draw a ray diagram of an astronomical telescope in the normal
Optics 12 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana adjustment position. State two drawbacks of this type of telescope. (Delhi 2008) Answer: (i) Magnifying power m = −f0fe. It does not change with increase of aperature of objective lens, because focal length of a lens has no concern with the aperature of lens. (ii) Drawbacks : Images formed by these telescopes have chromatic aberrations. Lesser resolving power. The image formed is inverted and faintes. Question 37. Draw a ray diagram of a compound microscope. Write the expression for its magnifying power. (Delhi 2008) Answer:
Optics 13 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana When the final image is formed at the least distance of distinct vision Question 38. Draw a labelled ray diagram of an astronomical telescope in the near point position. Write the expression for its magnifying power. (All India 2008) Answer: Question 39. Draw a labelled ray diagram, showing the image formation of an astronomical telescope in the normal adjustment position. Write the expression for its magnifying power.(All India 2008) Answer: (i) Magnifying power m = −f0fe. It does not change with increase of aperature of objective lens, because focal length of a lens has no concern with the
Optics 14 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana aperature of lens. (ii) Drawbacks : Images formed by these telescopes have chromatic aberrations. Lesser resolving power. The image formed is inverted and faintes. Question 40. Draw a ray diagram for the formation of image in a compound microscope. Write the expression for its magnifying power. (All India 2008) Answer: When the final image is formed at the least distance of distinct vision Question 41. A ray of light passing through an equilateral triangular glass prism from air undergoes minimum deviation when angle of incidence is 3/4th of the angle of prism. Calculate the speed of light in the prism. (Delhi 2008)
Optics 15 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Answer: Question 42. Calculate the distance of an object of height h from a concave mirror of focal length 10 cm, so as to obtain a real image of magnification 2. (Delhi
Optics 16 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana 2008) Answer: Given : f = -10 cm; Magnification, m = 2 To calculate : u = ? Question 43. Define refractive index of a transparent medium. A ray of light passes through a triangular prism. Plot a graph showing the variation of the angle of deviation with the angle of incidence. (All India 2009) Answer: Refractive index of a transparent medium is the ratio of the speed of light in free space to the speed in the given medium. Question 44. (i) What is the relation between critical angle and refractive index of a material? (ii) Does critical angle depend on the colour of light? Explain. (All India 2009)
Optics 17 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Answer: Question 45. The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. If focal length of the lens is 12 cm, find the refractive index of the material of the lens. (Delhi 2010) Answer: Given : R1 = 10 cm, R2 = -15 cm, f = 12 cm Using lens maker’s formula, we have Refractive index of the material of the lens : Question 46. (a) The bluish colour predominates in clear sky. (b) Violet colour is seen at the bottom of the spectrum when white light is dispersed by a prism. State reasons to explain these observations. (Delhi 2010) Answer:
Optics 18 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana (a) The scattering of light by the atmosphere is colour dependent. According to Rayleigh’s law, the intensity of scattered light, I∝1λ4 Blue light is scattered much more strongly than red light. The blue component of light is proportionately more in the light coming from different parts of the sky. This gives the impression of the blue sky. (b) As refractive index of prism is different for different colours, therefore, different colours deviate through different angles on passing through the prism. As λviolet < λred therefore µviolet > µred. Hence δviolet > δred maximum deviation is of violet colour. That is why violet colour, is seen at the bottom of the spectrum when white light is dispersed by a prism. Question 47. A biconvex lens has a focal length 2/3 times the radius of curvature of either surface. Calculate the refractive index of lens material. Answer: Question 48. (i) Why does the Sun appear reddish at sun-set or sun-rise? (ii) For which colour the refractive index of prism material is maximum and minimum? (Delhi 2009). Answer: (i) During Sunrise or sunset, the Sun is near the horizon. Sunlight has to travel a greater distance. So shorter waves of blue region are scattered away by the atmosphere. Red waves of longer wavelength are least scattered and reach the observer. So the Sun appears reddish. (ii) Refractive index of prism material is maximum for violet colour and refractive index of prism material is minimum for red colour.
Optics 19 Address:-In Front of Sonu Bakers,Hisar,7497033292 | By:-Er.Naveen Sardana Question 49. Find the radius of curvature of the convex surface of a plano-convex lens, whose focal length is 0.3 m and the refractive index of the material of the lens is 1.5. (Delhi 2009) Answer: ∴ Radius of curvature = -15 cm. Question 50. (i) Out of blue and red light which is deviated more by a prism? Give reason. (ii) Give the formula that can be used to determine refractive index of material of a prism in minimum deviation condition. (Delhi 2009) Answer:
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