Q: What is the focal length of 2.25 D reading glasses found on the rack in a pharmacy?
🔍 Step 1: Understand the Diopter Definition
The optical power of a lens is measured in diopters (D), which is defined as the reciprocal of the focal length in meters:
D = 1 / f (in meters)
So to find the focal length f, we rearrange the equation:
f = 1 / D
✏️ Step 2: Plug in the Diopter Value
Given the power of the reading glasses is:
D = 2.25
Then the focal length in meters is:
f = 1 / 2.25 ≈ 0.444 meters
📐 Step 3: Convert Meters to Millimeters
Since we want the answer in millimeters (mm), and 1 meter = 1000 mm:
f = 0.444 × 1000 = 444 mm
✅ Final Answer:
The focal length of 2.25 D reading glasses is: