Edexcel A Level Physics:復習筆記5.22 The Lens Equation
The Lens Equation
- This equation can be applied to all?thin converging?and?diverging?lenses
- The equation relates the?focal length?of the lens to the?distances?from the?lens?to the?image?and the?object
- ?Where:
- f?= focal length (m)
- v?= image distance from lens (m)
- u?= object distance from lens (m)
- This equation only works for?thin?converging or diverging lenses
- The values are?positive?if the image is?real
Worked Example
A student investigates the focal length of a thin lens by using it to project and image onto a screen.
The object is set 50.0 cm from the centre of the lens and the screen moved back and forth until the inverted image is sharp. This position is found to be 75.0 cm from the lens.
Determine the focal length of the lens.
Step 1: Write the known values
-
- Distance from object to lens,?u?= 50.0 cm
- Distance from image to lens,?v?= 75.0 cm
Step 2: Write the equation and substitute in the values
f?= 30 cm
Exam Tip
It is easy to forget the last step in a calculation like this one. Remember that you are calculating? not?f, and that you need to take the?reciprocal?of your answer.
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