(Spherical surface / Plane surface / Spherical mirror)
In this
section, Feynman discusses the formation of an image through a single spherical surface, plane surface, and spherical mirror.
1. Spherical
surface:
“The
farther ones may deviate if they want to, unfortunately, because the ideal
surface is complicated, and we use instead a spherical surface with the right
curvature at the axis. (Feynman et al., 1963, section 27–2 The focal length of a spherical surface).”
Feynman says that the simplest situation is a
single refracting surface and we desire to arrange the curved surface such that
every ray from O which hits the surface, at any point P, will be
bent to reach the focal point O′. That is, the time it takes for the
light rays to go from O to P plus n×O′P is
equal to a constant independent of the point P. However, the curved surface
is a fourth-degree curve sometimes known as the Cartesian ovals (or oval of Descartes) that may
be used for lens design. We may derive the curved
surface using simple analytic geometry based on the equation OP + n×O′P
= Ö[(x – o)2 + y2] + nÖ[(x – o′)2 + y2] = c in which o and o′ are the x
co-ordinate of O and O′ respectively. The fourth-degree curve can
be obtained after expanding the equation Ö[(x – o)2 + y2] + nÖ[(x – o′)2 + y2] = c.
According to Feynman, the law that connects the
distances s and s′, and the radius of curvature R of a
single spherical surface is (1/s) + (n/s′) = (n−1)/R.
The main point of the equation is: the excess time (due to extra path)
along route OP (h2/2s) and the other
route (nh2/2s′) is balanced by the excess time
(due to extra refractive index) needed for (n−1)VQ = (n−1)(h2/2s). The equation derived is known as the Gaussian formula for a
single spherical surface. We can also derive the formula (27.3) by assuming the angles of incident light rays are
so small such that the sines of angles can be replaced by the angles (in
radians). In section 27-3, he adds that most of the lenses have two surfaces,
but this is not a common definition of a lens.
2. Plane
surface:
“… if we look into a plane surface at an object that
is at a certain distance inside the dense medium, it will appear as though the
light is coming from not as far back (Feynman
et al., 1963, section 27–2
The focal length of a spherical surface).”
In the case of a plane surface, Feynman let R
equal to infinity and obtain (1/s) + (n/s′) = 0. It can be
simplified as s′ = −ns, which means that if we can look from a
dense medium into a rare medium and see a point in the rare medium, the image
appears to be deeper by a factor n. When we look at the bottom of a pool
from above, it is not exactly correct to say that the apparent depth of the
pool is ¾ of the real depth just because the reciprocal of the refraction index
of water is ¾. Specifically, the
apparent depth would vary with the angle of incidence or any point in the pool
with respect to the location of an observer’s eye. The factor of ¾ is approximately
correct for paraxial rays that are
small enough if the cosine of the incident angle is almost equal to one or the
sine of the incident angle is almost equal to the same incident angle (in
radians).
To illustrate the meaning of negative s′ (or
negative image), Feynman uses a ray diagram (Fig. 27–3) whereby the light rays
which are diverging from O, will be bent at the refracting surface, and
they will not come to a focus. That is, O is too close to the refracting
surface such that the image is formed “beyond infinity.” Feynman explains that
the image is a virtual image if the rays appear to come from a fictitious
point that is different from the original location. Although the virtual
image cannot be directly observed on a screen, it is still possible for
the light rays to converge and
form an observable image if a suitable lens is chosen. On the other hand, a real image is not only observable by
using a screen, its aerial image through the lens axis (or optical axis) can be
seen directly by naked eyes.
3. Spherical mirror:
“… we
leave it to the student to work out the formula for the spherical mirror, but
we mention that it is well to adopt certain conventions concerning the
distances involved s (Feynman et al.,
1963, section 27–2 The focal length of a spherical surface).”
According to Feynman, the object distance s
is positive if the point O is to the left of the surface, whereas the
image distance s′ is positive if the point O′ is to the right of
the surface. In addition, the radius of curvature of the surface is considered positive
if the center is to the right of the surface. However, there should be
clarifications on the freedom of adopting certain conventions in optics. For
example, we have the freedom to let the object distance to be positive
or negative if the object O is to the left of the spherical mirror. On
the other hand, we also have the freedom to let the image distance to be
positive or negative if the image O′ is to the right of the spherical
mirror. Thus, we have at least four conventions based on the object distance
and image distance.
Feynman mentions that if we had used a concave
surface, our formula (27.3) would still give the correct result if we merely
make R a negative quantity. The convention is commonly known as “Real is
Positive” convention, but some students may not be able to determine the
location of an image whether it is at the left or right of the spherical mirror.
To have a better understanding of the nature of the image, we can use ray
diagrams to show whether the image is upright or inverted. One may explain that
the concave mirror behaves like a convex lens in the sense that the light rays
refracted to the right, are instead reflected to the left. This is possible
provided the concave
mirror is small compared with its radius of curvature such that parallel rays
are focused to a common point (or focal point).
Review Questions:
1. How would you explain the derivation of the formula of a single spherical surface, (1/s)
+ (n/s′) = (n−1)/R?
2.
How would you explain the apparent depth of a swimming
pool does not look as deep as it really is?
3. What are the pros and cons of “real is positive”
convention for a spherical mirror?
The moral of the lesson: the equation (1/s) + (n/s′) = (n−1)/R
can be explained by how the excess time of two diagonal rays
(due to extra path) is balanced by the excess time of a horizontal ray (due
to extra refractive index).
References: