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Light Test - 7

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Light Test - 7
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  • Question 1
    1 / -0
    Two plane mirrors, A and B, are kept at an angle θ with respect to each other.

    The light falling on A gets reflected. Then it falls on B and again gets reflected. The direction of emergent ray is opposite to that of the incident ray. What is the value of angle θ?
    Solution


    From the diagram, we have
    2α + 2β = 180°
    α + β = 90°
    Secondly, we have
    θ + (90° - α) + (90° - β) = 180°
    Hence, θ = 90°
  • Question 2
    1 / -0
    Read the following statements and choose the correct option accordingly:

    Statement A:
    Reflection from the water surface is diffused reflection.
    Statement B: In diffused reflection, a parallel beam, after reflection, does not remain parallel.
    Solution
    Statement A is incorrect, but statement B is correct.
    Reflection from the water surface is specular reflection as water surface acts as a plane surface.
    In diffused reflection, if parallel rays strike a rough surface, then the reflected rays will not be parallel. Hence, it will produce a diffused image.
  • Question 3
    1 / -0
    What will be the total number of images formed if an object is placed symmetrically between two plane mirrors inclined at an angle of 72° with respect to each other?
    Solution
    The number of images formed is given by:



  • Question 4
    1 / -0
    Two plane mirrors are inclined with respect to each other at some angle. A ray of light is incident at 45° on one; after reflection from the second mirror, it retraces its path. The angle between the mirrors is
    Solution


    From the diagram:
    ∠A + ∠B + ∠C = 180°
    θ + 90° + 45° = 180°
    θ = 45°
  • Question 5
    1 / -0
    A beam of light is incident at an angle of 30° on a plane mirror. The mirror is then rotated by an angle of 20°.

    By what angle will the reflected beam rotate?
    Solution
    If the mirror is rotated by an angle θ, the reflected ray will be rotated by an angle 2θ in the sense of rotation of mirror.
    If the mirror is rotated clockwise, the reflected ray will be rotated clockwise and vice versa.
    Hence, the answer is 40°.
  • Question 6
    1 / -0
    An object is moving towards a plane mirror with a velocity of 2 m/s. At what speed does the image approach the object?
    Solution
    Velocity of image with respect to mirror = 2 m/s
    As the observer moves towards the mirror, the image moves closer to the mirror with same speed, but in opposite direction. Hence, their relative velocity adds up.
    So, the image approaches the object with a speed of 4 m/s.
  • Question 7
    1 / -0
    A star is present between two vertical barriers, a little above a plane horizontal mirror, as shown below:



    From which points would an observer be able to see the star?
    Solution
    The diagram below depicts the reflections that would need to occur for each of the images to exist. The red rays run directly into the barriers, which means they are blocked. So, the image of the star will not be visible from points 2 and 4.

  • Question 8
    1 / -0
    Read the following statements and choose the correct option accordingly:

    Statement A:
    Refractive index determines the optical density of a medium.
    Statement B: Refractive index is independent of the wavelength of the light.
    Solution
    Refractive index is the physical property which determines the optical density of a material. The greater the refractive index of a medium, the lesser is the speed of light in that medium. So, statement A is correct.
    Refractive index of a medium is inversely proportional to the wavelength of light. The greater the wavelength of light, the lesser is the refractive index of the medium. So, statement B is incorrect.
  • Question 9
    1 / -0
    Light is travelling from glass into water. The speed of light in glass is 2 × 108 m/s and the speed of light in water is 2.5 × 108 m/s. The refractive index of water with respect to glass is
    Solution
    Refractive index (μ) of a medium is given by the ratio of the speed of light in vacuum (c) to the speed of light in medium (v).


    Refractive index of water,

    Refractive index of glass,

    Refractive index of water with respect to glass,
  • Question 10
    1 / -0
    Read the following statements and choose the correct option accordingly.

    Statement A: During the propagation of light from a denser medium to a rarer medium, light bends away from the normal.
    Statement B: During refraction, there is no change in the frequency of light.
    Solution
    Whenever the light travels from a denser medium to a rarer medium, it always bends away from the normal. So, statement A is correct.
    During the propagation of light from one medium into another medium, there is no change in the frequency of the light, but speed and wavelength of the light do change. So, statement B is also correct.
  • Question 11
    1 / -0
    Read the following statements and choose the correct option accordingly.

    Statement A:
    Two vessels, A and B, are filled to the brim. Vessel A is filled with alcohol and vessel B is filled with water. The base of the vessel B will look deeper.
    Statement B: The refractive index of alcohol is less than that of water.
    Solution
    Apparent depth of an object in a medium is given by the ratio of the real depth to the refractive index.



    Hence, apparent depth is inversely proportional to the refractive index. (Lesser the refractive index, deeper the object in medium appears.)
    Refractive index of alcohol is 1.36, while refractive index of water is 1.33 (less than alcohol).
    Since the refractive index of water is less than that of alcohol, therefore the base of the vessel B will look deeper.
    Hence, Statement A is correct, but statement B is incorrect.
  • Question 12
    1 / -0
    A diver in a swimming pool wants to signal his distress to a person lying on the edge of the pool by flashing his water-proof flashlight. Which of the following would be the best way to give that signal?
    Solution
    If the diver directs the light at an angle less than the critical angle, only then the light will refract through the water surface and reach the person sitting on the edge of the lake. If the angle of incidence is greater than the critical angle, the light will remain in the denser medium (i.e. water).
  • Question 13
    1 / -0
    An object, placed at infinity, starts moving towards a convex lens. As it approaches the centre of curvature, the speed of the image is _____ as compared to that of the object, and the size of the image is _____ as compared to that of the object. As the object moves from centre of curvature towards the focus, the speed of the image is _____ as compared to that of the object, and the size of the image is _____ as compared to that of the object.
    Solution
    When the object is at infinity, its image is formed at focus, and when the object is placed at the centre of curvature, its image is formed at the centre of curvature (2F). So, the object travels a very large distance, whereas the image only travels a distance equal to the focal length (F to 2F). So, the speed of the object is more than the speed of its image.
    The size of the image remains less than the size of the object as the object moves from infinity to 2F. When the object is at 2F, the size of the image is the same as the size of the object.
    If the object is at the focus of the lens, its image is formed at infinity. So, the object travels a distance equal to the focal length (2F to F), whereas its image travels an infinite distance.
    The size of the image becomes more than the size of the object as the object travels from 2F towards F.
  • Question 14
    1 / -0
    An object is placed at a distance of 20 cm from a convex lens of focal length 10 cm. At what distance from the lens is the image formed?
    Solution
    Here, the object distance, u = -20 cm, focal length, f = 10 cm
    Using lens formula,







    Alternate method:
    Here, the object is placed at 2F. So, its image will be formed at 2F on the other side of the lens.
  • Question 15
    1 / -0
    Read the following statements and choose the correct option accordingly.

    Statement A: A lens forms a virtual image having half the size of an object placed at 2 m.
    Statement B: The lens used is a concave lens of focal length 2 m.
    Solution
    Here, the object distance, u = -2 m, focal length, f = -2 m
    Using lens formula,







    So, the image formed is imaginary.
    As the image distance is half the object distance, so both statements A and B are correct.
  • Question 16
    1 / -0
    A ray of light incidents on a plane mirror at an angle of 30°. The deviation produced in the ray (in degrees) is
    Solution
    Angle of deviation = 180°- 2(Angle of incidence)
    Angle of deviation = 180°- 60°= 120°
  • Question 17
    1 / -0
    Which of the following statements is/are true for irregular reflection?
    Solution
    Irregular reflection is the phenomenon due to which a parallel beam of light travelling through some medium, on striking some rough surface, gets reflected in various possible directions.
  • Question 18
    1 / -0
    In a military training exercise, three teams are stationed at points A, B and C, as shown below, with a hillock between teams A and C. Team A sends a light signal to B, which has to reflect it to team C, so that all the three teams start the exercise simultaneously. What should be the angle of the reflecting mirror, held by team B?

    Solution
    Applying the laws of reflection, for the mirror at B,
    Angle of reflection = Angle of incidence = 30°
    Hence, = 90° – 60° = 30°

  • Question 19
    1 / -0


    Ram stands at 'A' on the side of a plane mirror as shown in the above diagrams. He cannot see himself, but can see Raju at 'P' and Sham at 'Q'. Which of the ray diagrams given above correctly explains the situation?
    Solution
    Ram cannot see himself because he is not standing in front of the mirror. He can see Raju and Sham because reflected rays reach his eyes.
  • Question 20
    1 / -0
    Which of the following is NOT true about an image formed by a plane mirror?
    Solution
    The image is formed on the other side of the mirror and having the same size as that of the object. The image produced is just like the object depicting its shape, structure, size, colour, etc.
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