My Learning Cart
?
Categories
Account

A concave mirror has a focal length of 20 cm. What is the distance of the object

₹0.0
Login to Download
  • 📥 Instant PDF Download
  • ♾ Lifetime Access
  • 🛡 Secure & Original Content

What’s inside this PDF?

Question: A concave mirror has a focal length of 20 cm. What is the distance of the object from the mirror if the image formed is at a distance of 30 cm from the mirror? (2021)

Options:

  1. 10 cm
  2. 15 cm
  3. 25 cm
  4. 30 cm

Correct Answer: 25 cm

Exam Year: 2021

Solution: 

Using the mirror formula: 1/f = 1/v + 1/u, where f = -20 cm (concave mirror), v = -30 cm (image distance), we find u = 25 cm.

A concave mirror has a focal length of 20 cm. What is the distance of the object

Practice Questions

Q1
A concave mirror has a focal length of 20 cm. What is the distance of the object from the mirror if the image formed is at a distance of 30 cm from the mirror? (2021)
  1. 10 cm
  2. 15 cm
  3. 25 cm
  4. 30 cm

Questions & Step-by-Step Solutions

A concave mirror has a focal length of 20 cm. What is the distance of the object from the mirror if the image formed is at a distance of 30 cm from the mirror? (2021)
  • Step 1: Identify the given values. The focal length (f) of the concave mirror is 20 cm, so we write f = -20 cm (negative for concave mirrors). The image distance (v) is 30 cm, so we write v = -30 cm (negative because the image is on the same side as the object).
  • Step 2: Write down the mirror formula: 1/f = 1/v + 1/u, where u is the object distance we want to find.
  • Step 3: Substitute the known values into the mirror formula: 1/(-20) = 1/(-30) + 1/u.
  • Step 4: Simplify the equation. First, calculate 1/(-30): 1/(-30) = -1/30.
  • Step 5: Rewrite the equation: -1/20 = -1/30 + 1/u.
  • Step 6: To isolate 1/u, add 1/30 to both sides: 1/u = -1/20 + 1/30.
  • Step 7: Find a common denominator for -1/20 and 1/30, which is 60. Rewrite the fractions: -1/20 = -3/60 and 1/30 = 2/60.
  • Step 8: Now combine the fractions: 1/u = -3/60 + 2/60 = -1/60.
  • Step 9: To find u, take the reciprocal: u = -60 cm.
  • Step 10: Since we are looking for the distance of the object from the mirror, we take the absolute value: u = 60 cm.
  • Mirror Formula – The relationship between the focal length (f), image distance (v), and object distance (u) for mirrors, expressed as 1/f = 1/v + 1/u.
  • Sign Convention – Understanding the sign conventions for concave mirrors, where focal length is negative and image distance is also negative for real images.
Soulshift Feedback ×

On a scale of 0–10, how likely are you to recommend The Soulshift Academy?

Not likely Very likely
Home Practice Performance eBooks