Question: If a concave lens has a power of -5 D, what is its focal length? (2021)
Options:
-20 cm
-25 cm
-15 cm
-10 cm
Correct Answer: -25 cm
Exam Year: 2021
Solution:
Power P = 1/f, thus f = 1/P = 1/(-5) = -0.2 m = -20 cm.
If a concave lens has a power of -5 D, what is its focal length? (2021)
Practice Questions
Q1
If a concave lens has a power of -5 D, what is its focal length? (2021)
-20 cm
-25 cm
-15 cm
-10 cm
Questions & Step-by-Step Solutions
If a concave lens has a power of -5 D, what is its focal length? (2021)
Step 1: Understand that the power (P) of a lens is related to its focal length (f) by the formula P = 1/f.
Step 2: Since the power of the concave lens is given as -5 D, we can write this as P = -5.
Step 3: Rearrange the formula to find the focal length: f = 1/P.
Step 4: Substitute the value of P into the formula: f = 1/(-5).
Step 5: Calculate the focal length: f = -0.2 m.
Step 6: Convert the focal length from meters to centimeters: -0.2 m = -20 cm.
Lens Power and Focal Length – The relationship between the power of a lens and its focal length, where power (P) is the inverse of the focal length (f) in meters.
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