3B Scientific Light Box - Manual (Page 17)

17
In the set-up for spherical aberration, block the two inner rays and observe the colored foci obtained
closely.
• Which color has the shortest focal length? Is this in agreement with the dispersion obtained by a
prism? (Consider the lens as two prisms placed end-on-end).
For more information on aberrations, see Appendix 2.
Appendix 1:
Determination of Focal Length of a Concave Lens
Method 1:
Throw the image of a source of light by means of a converging lens on to a screen and note
its position. Let the source be S
1
and let the image formed by the converging lens at O
1
be at S
2
. Place
the divergent lens at a point O
2
so that the image is displaced from S
2
to S
3
, where it is again located on
the screen. The distances required are O
2
S
2
and O
2
S
3
for S
2
and S
3
are conjugate points for the
diverging lens. The rays are directed to S
2
so that S
2
is a virtual object and in accordance with the
notation described,
Here, L = -O
2
S
2
L’ = O
2
S
3
The formula
f L L
1 1
'
1
= + , then gives the value of f, which in this notation will be of negative sign.
The experiment should be repeated for different positions of O
2
, while S
1
and O
1
remain fixed, and also
for different positions of the convergent lens relative to S
1
.
Method 2:
Another method for the determination of the focal length of the diverging lens requires the
above apparatus with a plane mirror in addition.
The diagram illustrates the method. If the rays from the concave lens strike a plane mirror placed at any
point, M, to the right of it at normal incidence, they are returned to form an image at the source, S
1
. In
these circumstances the rays from the convex lens are directed towards the principal focus, F, of the
S
1
O
1
O
2
S
2
S
3
Fig. 19