PhysicsWhen an object is placed in front of a concave mirror at a distance 30cm from an image is obtained on a screen at a distance of 20 cm from the mirror. Find the focal length of the mirror.

When an object is placed in front of a concave mirror at a distance 30cm from an image is obtained on a screen at a distance of 20 cm from the mirror. Find the focal length of the mirror.


  1. A
    -6cm
  2. B
    +6cm
  3. C
    -12cm
  4. D
    +12cm 

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    Solution:

    Concept: A convex mirror has a negative focal length, while a concave mirror has a positive one. A concave mirror is a spherical mirror that satisfies the rules of light reflection and has an internally curved reflective surface. It is possible to imagine that the light beams coming from a distant object are parallel to one another.
    u=- 30cm
    =- 20cm
    Utilising the mirror formula
    1/v+1/u=1/f
    So now change the settings.
    1/-20+1/-30=1/f
    =- 12cm
    Hence, option 3 is the correct answer.
     
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