1. Lenses 2

2. CALCULATIONS USING THE LENS FORMULA
Calculate the distance of the image to the converging lens in the following situations
object distance /cm
Focal length /cm a
30.0
40.0 b
25.0
20.0 c
50.0 d
In each case say whether the image is real or virtual

3. Solved Example
object distance /cm
Focal length /cm
Image dist ance cm a
30.0
40.0
-120 (virtual) b
25.0
20.0
100 (real) c
50.0
33.3cm (real) d
No image (rays emerge parallel)
V= -120 cm
The image is 120 cm from the lens and it is virtual (- sign)

4. The power of a lens
By definition the power of a lens is the reciprocal of its focal length in metres:
The unit of lens power is m-1. These units are often referred to as dioptres (D).
Lens power
Therefore a lens with a power of 3D has a focal length of 1/3 m
The dioptre is not a standard SI unit
This of course also means that the power of a lens is given by:

5. Calculations With Lens Power
Calculate the power of each of these lenses.
object distance /cm
Image distance/cm
Focal length /cm
40.0
30.0 30
-120
20.0
50.0

6. Determination Of The Focal Length Of A Converging Lens
The distant object method:
To make a rough estimate of the focal length of a convex lens. Produce a sharply illuminated image of a distant object (e.g. a distant window).
To a rough approximation the rays of light from the object will be arriving parallel at the screen and therefore the distance between the lens and the screen is approximately the focal length of the lens.

7. Lens formula method
Using an illuminated object (O) change u, systematically measuring the distance v in each case O u v
A graph of 1/v against 1/u is plotted

8. Extrapolate the line to meet the x and y axes
Because of the relationship
1/u= 1/f
Where 1/u falls to zero
1/u
m-1
Where 1/v falls to zero
1/v= 1/f
1/v m-1