Reflection of Light from Spherical Mirrors.
Concave and Convex Mirrors.
Reflecting surface
Reflecting surface
Concave Mirror
Convex Mirror
The two mirrors shown above are examples of what are called
‘spherical mirrors’, since they form part of the surface of a sphere. It is
important to remember which mirror curves in which direction!
Terms used to describe spherical mirrors.
Centre of
curvature
Principal axis
C
F
Focus
Reflecting
surface
P Pole
The centre of the spherical
mirror is called the pole of the
mirror, P.
The center of the sphere of
which the mirror is part, is
called the center of curvature,
C.
The line joining the pole of the
mirror to the center of curvature
is called the principal axis.
The point halfway between the
pole and center of curvature is
called the focal point or focus of
the mirror, F.
The distance from F to P, is
called the focal length f.
Images formed by a concave mirror.
When examining the formation of images by concave mirrors it is
necessary to understand how various incident rays will be reflected.
1.
A ray which strikes the pole of the mirror will be reflected at an equal
angle to the axis.
Incident ray
i
r
Axis
Reflected ray
Pole
i=r
2.
A ray striking the mirror having passed through the centre of curvature is
reflected back along its own path.
centre of curvature
Axis
Pole
C
3.
A ray which comes in parallel to the axis is reflected back through the
focus of the mirror.
Parallel Ray
Axis
C
F
Reflected Ray
4.
A ray which strikes the mirror having past through the focus, is reflected
back parallel to the axis.
Reflected ray
Axis
C
F
When trying to calculate where an image is formed by a concave
mirror, it is only necessary to draw any two of the four rays described
above. The image will be located where the two chosen rays intersect.
Depending on the location of an object in front of a concave
mirror, the image formed will either be a real image or a virtual image.
A real image is one formed by the actual intersection of light rays.
A real image may be located on a screen or by the method of no parallax.
A virtual image is one formed by the apparent intersection of light
rays and may not be located on a screen, but only by the method of no
parallax.
For a concave mirror:
If the object is outside the focus, the image will be real and found in
front of the mirror.
If the object is at or inside the focus, the image will be virtual and
behind the mirror.
The mirror formula.
The location of the image formed by a concave mirror will vary
according to the location of the object. The nature of the image as in
whether it will be magnified or diminished, upright or inverted, will also
depend on where the object is placed in front of the mirror.
When performing calculations involving spherical mirrors the following
formula is very useful;
1 = 1 + 1
f
u
v
where,
f = focal length of the mirror involved ( = half of C)
u = distance of the object from the mirror, i.e. the object distance
v = the distance of the image from the mirror, i.e. the image distance
N.B.
‘u’ is always positive
‘f’ is + for a concave mirror and – for a convex mirror
‘v’ is + for a real image and v is – for a virtual image.
Magnification.
Magnification = height of image = image distance = v
height of object
object distance
u
m=v/u
Uses of Concave Mirrors.
 Floodlighting, searchlights.
 Make-up / shaving mirrors
 Dentists’ mirror
Placing a light source at the focus of a concave mirror
produces a very bright parallel beam.
Placing an object inside the focus of a concave mirror
produces a magnified and upright image.
Page 20 exercise 3.1!