The Lens Equation (Edexcel A Level Physics): Revision Note

A student investigates the focal length of a thin lens by using it to project an image onto a screen.

The object is set 50.0 cm from the centre of the lens, and the screen is 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.

Answer:

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

$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

$\frac{1}{f}=\frac{1}{50}+\frac{1}{75}=\frac{1}{30}$

$f=30\mathrm{cm}$

It is easy to forget the last step in a calculation like this one. Remember that the lens equations gives you $\frac{1}{f}$, so you need to take the reciprocal of your answer to calculate$f$.