Fig. 16 – 3B Scientific Light Box User Manual
Page 15
15
Focus - Concave Lens:
Allow a set of parallel rays to fall on the concave lens parallel to its axis of
symmetry. The rays diverge after refraction. The point from which they appear to diverge is the focus
(F) of the lens and OF is the focal length (f).
Theoretically, the focal length of a concave lens can be calculated by many methods. Some of them are
shown in Appendix 1. However, these methods are not very reliable with the light-box apparatus.
• Which lens would you use to remedy myopia (short/near-sightedness)? Which one for
hypermetropia (long/far-sightedness)? Explain.
Focal Plane:
Allow two parallel rays to fall on a convex lens near the lens center and mark their focus.
Changing the angle of incidence, but keeping the points of incidence the same, mark the foci for
several sets of two parallel rays. Join these loci to obtain a straight line, referred to as FOCAL LINE.
(In spherical lenses, a plane is obtained called the FOCAL PLANE).
The concept of the focal plane is used in designing cameras and telescopes.
Radius of Curvature:
Every lens has two radii of curvature, r
1
and r
2.
By tracing the curve of one side of the lens, r
1
can be obtained. Shift the curved surface along this
tracing and trace further. Repeat this till a complete circle is formed. The radius of this circle is r
1
. By
doing the same for the other curved surface, r
2
can be obtained.
Having found r
1
and r
2
for a lens, find its focal length by the LENSMAKER’S EQUATION :
(
)
+
−
=
2
1
1
1
1
1
r
r
f
µ
Calculate the focal length for both the convex lens and the concave lens by this equation (take the
mean value of
µ obtained in the prism and slab experiments - the lenses are also made of acrylic
plastic).
• Are the calculated focal lengths in agreement with what you observed in previous experiments?
O
F
Fig. 16