Week 12: Lenses and Mirrors
- The distance from a mirror (or lens) to an object one is viewing in (or through) it
iss, theobject distance. Object distances are positive if the object is on the side
of the mirror (or lens) that the light is comingfrom. Object distances are obviously
‘always’ positive, unless the object is avirtual objectformed out of the image of a
previous mirror or lens, which can be either positive or negative.
- The distance from a lens or mirror to the image one is viewing iss′, theimage
distance. Image distances are positive if the image is on the side of the mirror (or
lens) that the light is goingto.
- The focal lengthfof a mirror (or lens) is the point where incident parallel rays
are focusedto(for positive focal lengths) or appear to be defocusedfrom(for
negative focal lengths).fis typically measured in meters (SI) or centimeters (for
convenience). However, the strength oflensesis usually given indiopters, where:
d=^1
f
(958)
withfin meters. This a one diopter (1.00d) lens has a focal length of 1 meter. A
10.00d lens has a focal length of 0.1 meter. A diverging lens with a focal length of
one centimeter is -100.00d.
- The mirror (or thin lens) equation relatings,s′, andfis:
1
s+
1
s′=
1
f (959)
- The transverse magnification of a simple mirror (or lens) is defined bythe ratio of
the image heighty′to the object heighty:
m=y
′
y
=−s
′
s
(960)
- Areal imageis one where the rays of light that appear to the eye to diverge from
a point on the image actually pass through that point. Avirtual imageis one
where the rays of light that appear to the eye to diverge from a point on the image
donotactually pass through the image.
- In addition to being real or virtual, an image can beerect(oriented the same way
as the object) orinverted(oriented the opposite way from the object.
- For a spherical mirror, the focal length is given by:
f=
r
2
(961)
whereris positive when it is on the side of the mirror reflected light is goingto.
393
