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A lens forms a real image of an object placed at a distance of 60 cm from it. If

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Question: A lens forms a real image of an object placed at a distance of 60 cm from it. If the image distance is 15 cm, what is the focal length of the lens?

Options:

  1. 10 cm
  2. 12 cm
  3. 20 cm
  4. 25 cm

Correct Answer: 10 cm

Solution: 

Using the lens formula 1/f = 1/v - 1/u, we have 1/f = 1/15 - 1/60. Solving gives f = 10 cm.

A lens forms a real image of an object placed at a distance of 60 cm from it. If

Practice Questions

Q1
A lens forms a real image of an object placed at a distance of 60 cm from it. If the image distance is 15 cm, what is the focal length of the lens?
  1. 10 cm
  2. 12 cm
  3. 20 cm
  4. 25 cm

Questions & Step-by-Step Solutions

A lens forms a real image of an object placed at a distance of 60 cm from it. If the image distance is 15 cm, what is the focal length of the lens?
  • Step 1: Identify the given values. The object distance (u) is -60 cm (negative because it is on the same side as the object) and the image distance (v) is 15 cm.
  • Step 2: Write down the lens formula: 1/f = 1/v - 1/u.
  • Step 3: Substitute the values into the lens formula: 1/f = 1/15 - 1/(-60).
  • Step 4: Calculate 1/15, which is 0.0667, and 1/(-60), which is -0.0167.
  • Step 5: Now, calculate 1/f: 1/f = 0.0667 + 0.0167 = 0.0834.
  • Step 6: To find the focal length (f), take the reciprocal of 0.0834: f = 1/0.0834.
  • Step 7: Calculate f, which gives approximately 12 cm.
  • Lens Formula – The lens formula relates the focal length (f), image distance (v), and object distance (u) of a lens, expressed as 1/f = 1/v - 1/u.
  • Real Image Formation – A real image is formed on the opposite side of the lens from the object and can be projected onto a screen.
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