Reflection, in
physics, is a phenomenon of wave motion, in which a wave is returned after
impinging on a surface. When energy, such as light or sound, traveling from one
medium encounters a different medium, part of the energy usually passes on while
part is reflected. Regular reflection (in which the direction of the reflected
wave front is sharply delineated) is governed by the law that both the
incident, or striking, rays and the reflected rays travel in directions making
equal angles with the normal, a line perpendicular to the reflecting surface at
the point of incidence; and that the rays lie in the same plane as the normal.
These angles are called the angle of incidence and the angle of reflection.
Rough surfaces reflect in many directions, and such reflection is called
diffuse.
To reflect a wave train, the reflecting surface must
be wider than one-half the wavelength of the impinging waves. For example, a
pile rising above the surface of the ocean may reflect ripples, but long waves
pass around it. A thin windowpane reflects shrill noises, which have very short
wavelengths, but sounds of longer wavelength pass through it. Small particles
of dust in the atmosphere may reflect only the shorter blue wavelengths in
sunlight.
Reflection of Wave Pulses from a
Boundary
A wave pulse
on a string is generated by a quick movement of a hand and travels down the string
toward the left (A). If the end of the string is free to move up and down at
the wall, the pulse will come back down the string on the same side (C1). If
the string is tied to the wall, the pulse will travel back along the string on
the opposite side (C2). For the free end, the pulse will have twice the
original amplitude at the turnaround point (B1); for the fixed end, the pulse
will have no amplitude at the turnaround point (B2).
Concave Mirror
A concave
mirror curves inward like the inside surface of a hollow sphere. Light striking
the surface of a concave mirror reflects inward, or converges. The size,
position, and type of image—real or virtual—depend on the size and position of
the object and the focal point of the mirror, or the place where light rays
converge. In this illustration, an object is placed between a concave mirror
and its focal point. The mirror forms a larger, upright, virtual image.
Convex Mirror
A convex
mirror curves outward like the outer surface of a ball. Light striking the
surface of a convex mirror reflects outward, or diverges. In this illustration,
parallel light rays from an object that strike the mirror are reflected as
though they came from an image behind the mirror. These light rays form an
upright, smaller, virtual image; that is, the brain perceives the diverging
rays as though they came from an image behind the mirror.