Thin Lens Equation
Name:
Teammates:
Introduction
Consider a simple lens, light is refracted upon entering the lens, it travels in a straight line within the lens, and then is refracted
again upon leaving the lens. Since the sides of the lens are not necessarily parallel, the light leaves the lens at a different angle
from which it entered. Depending on the shape of the lens, the light is either focused or defocused.
If we approximate both sides of a lens as spherical with radii of curvature r
1
and r
2
, then the focal length (d
f
) of the lens when
surrounded by air is given by the lens maker equation:
f
1
1
1
(n1)(
)
d
r
=
+
2
1
r
where r
1
is positive if the center of the sphere is to the right of the lens and r
2
is positive if the center of the sphere is to the left, n
is the index of refraction of the lens, This formula is extremely useful for creating a lens, it is not very useful for determining the
focal length of a lens that has already been created. A more practical method for finding the focal length (d
f
) of a lens is by using
the thin lens equation
Fig: 1:
Lab set up
o
i
1
1
d
d
d
+
=
f
1
where d
o
is the distance the source (object) is away from the lens and d
i
is
distance the image is behind the lens. This equation is applicable only for a
thin lens approximation. Note that when d
o
is very large, d
f
is equal to d
i
.
Procedure
1.
Set up the optical bench as in Fig 1 using the d
f
= 200 mm focal length lens.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Summer '10
 Adler
 Physics, Light, Thin lens

Click to edit the document details