Mirrors and Lenses | AP Physics B
•
Mirrors reflect light, while lenses
refract light
•
•
(a)
(b)
Mirrors and Lenses
Mirrors (a) reflect light while lenses (b) refract
light.
Types of Mirrors
• Concave mirrors focus light
• Convex mirrors scatter light
(a)
(b)
Concave and Convex Mirrors
(a) Concave mirrors focus light at a point in front
of the mirror. (b) Convex mirrors scatter
incoming light in all directions.
Types of Lenses
• Convergent lenses converge light
to a single point
• Divergent lenses diverge light,
thereby scattering it
(a)
(b)
Convergent and Divergent Lenses
(a) Convergent lenses converge light rays
together, while (b) divergent lenses diverge light
rays away from one another.
A lens with a thicker center
converges light
o That is, if the lens’ center
is thicker than the top and
bottom, it is convergent,
and vice versa
Don’t just look at the concavity
or convexity of the lens to
determine if it is convergent or
divergent
o Sometimes, one side will
be concave and the other
convex
Example of a Deceptive Lens
Since one side is convex and the other is
concave, it’s may be hard to decide if it is a
convergent or divergent lens. Fortunately, if you
remember that a thicker center converges, you’ll
see that the center is thicker than the ends,
meaning this lens must converge light.
Focal Point – the point where all rays of
light appear to come together, after
hitting a mirror or lens
(a)
(b)
Focal Point of a Concave Mirror and
Convergent Lens
The (a) concave mirror and (b) convergent lens
reflect and refract incoming light (pink arrows),
such that the focal point forms to the left of each
(pink circle).
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© 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017
Mirrors and Lenses | AP Physics B
Focal Point of a Convex Mirror and
Divergent Lens
The (a) convex mirror and (b) divergent lens
reflect and refract incoming light (pink arrows).
Unlike the concave mirror and convergent lens,
these rays are scattered, not focused. To find the
focal point, we must trace back the scattered
light rays (yellow dotted line) to the point where
they would theoretically meet up. In both these
cases, that point exists to the right of the mirror
and lens, at the pink circles.
•
Focal Length – distance from the
focal point to the lens/mirror
Calculating the Focal Length
• For a mirror:
o Suppose we have this
mirror, and we draw a
circle whose curve
matches that of the mirror
•
o Written more concisely:
1
f = r
2
f = focal length
r = radius of
curvature
For a lens:
o Calculating the focal
length of a lens is much
trickier
o We’d have to use the lens
makers’ equation
But you won’t
need to know it
Power of a Lens
• Lenses differ in power depending
on the degree to which they
refract light
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• P=
f
• P = power
f = focal length
o Focal power is measured
in diopters
Circle Whose Curve Matches that of the
Mirror
The radius of this circle (green bar) is known as
the mirror’s radius of curvature. The focal length
is half the radius of curvature. So, the focal point
will be located at the blue circle.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017