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L ight. Light is an electromagnetic wave. … its speed is approximately 300 million meters per second. … it travels nearly a million times faster than sound in air. … it does not need a medium to propagate. … it, like all electromagnetic waves, is a transverse wave. Light.
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Light is an electromagnetic wave. … its speed is approximately 300 million meters per second … it travels nearly a million times faster than sound in air. … it does not need a medium to propagate. … it, like all electromagnetic waves, is a transverse wave.
Light • electromagnetic waves created by vibrating electric charges having frequencies that fall within the range of sight
Electromagnetic Nature of Light A situation to ponder… A photographer wishes to photograph a lightning bolt by setting a camera so that it is triggered by the sound of thunder.
A situation to ponder… CHECK YOUR NEIGHBOR Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. • Good idea for all strikes. • Poor idea for nearby lightning strikes. • Poor idea for all strikes.
A situation to ponder… CHECK YOUR ANSWER Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. • Good idea for all strikes. • Poor idea for nearby lightning strikes. • Poor idea for all strikes. Explanation: Light travels about a million times faster than sound. By the time the sound of thunder arrives, the lightning bolt is long gone.
Transparent and Opaque Materials Opaque materials: colored glass is opaque to much of incident white light
Transparent and Opaque Materials Light is transmitted similar to sound … • light incident on matter forces some electrons in matter to vibrate
Transparent and Opaque Materials How light penetrates a pane of glass • electrons in atoms of glass are forced into vibration • energy is momentarily absorbed and vibrates electrons in the glass • a vibrating electron either emits a photon or transfers the energy as heat • light slows due to time delay between absorption and reemission of photons
Transparent and Opaque Materials Average speed of light through different materials • vacuum—c (300,000,000 m/s) • atmosphere—slightly less than c (but rounded off to c) • water—0.75 c • glass—0.67 c, depending on material • diamond—0.41 c
Shadows • are formed when rays of light are blocked • A total shadow is called an Umbra. • The Penumbra is where some light fills in the shadow.
Eclipses • A solar eclipse occurs when the moon’s shadow falls • on the Earth. • A lunar eclipse occurs when the Earth’s shadow falls on the moon.
Rodsprocess only lightness and darkness.Conesare essential for colour vision.
The Purkinje Effect …also known as dark adaptation. … as light intensity decreases, red objects are perceived to fade faster than blue objects of the same brightness. … occurs because rods are more sensitive to blue light, whilst cones are sensitive to red.
Reflection Reflection: the returning of a wave to the medium through which it came when encountering a reflective surface Law of reflection: angle of incidence = angle of reflection
Reflection Virtual image • same size as object, formed behind a mirror, and located at the position where the extended reflected rays converge • as far behind the mirror as the object is in front of the mirror
Reflection Plane mirror • note, the only axis reversed in an image is the front-back axis
Reflection Curved mirrors form a different virtual image • convex mirror (that curves outward): virtual image is smaller and closer to the mirror than the object • concave mirror (that curves inward): virtual image is larger and farther away than the object
Reflection Diffuse reflection • light striking a rough or irregular surface reflects in many directions • an undesirable circumstance is the ghost image that occurs on a non-cable TV set when TV signals bounce off buildings and other obstructions
Refraction Refraction • bending of light when it passes from one medium to another • caused by change in speed of light
Refraction Here we see light rays pass from air into water and from water into air • pathways are reversible for both reflection and refraction
Refraction Illusions caused by refraction • Objects submerged in water appear closer to the surface.
Refraction Illusions caused by refraction (continued) • Objects such as the Sun seen through air are displaced because of atmospheric refraction.
Refraction Illusions caused by refraction (continued) • Atmospheric refraction is the cause of mirages.
Color Color we see depends on frequency of light ranging from lowest (red) to highest (violet). In between are colors of the rainbow. Hues in seven colors: red, orange, yellow, green, blue, indigo, and violet. Grouped together, they add to appear white.
Color Selective Reflection Most objects don’t emit light, but reflect light. A material may absorb some of the light and reflect the rest. Selective Transmission The color of a transparent object depends on the color of the light it transmits.
Color Additive primary colors (red, blue, green): red + blue = magenta red + green = yellow blue + green = cyan
Color Opposites of primary colors: opposite of green is magenta opposite of red is cyan opposite of blue is yellow The addition of any color to its opposite color results in white.
Dispersion Dispersion • process of separation of light into colors arranged by frequency • Components of white light are dispersed in a prism (and also in a diffraction grating)
Dispersion CHECK YOUR NEIGHBOR When white light passes through a prism, green light is bent more than A. blue light. • violet light. • red light. • None of the above.
Dispersion CHECK YOUR ANSWER When white light passes through a prism, green light is bent more than A. blue light. • violet light. • red light. • None of the above
Dispersion Rainbows • a colorful example of dispersion
Polarization Polarization • alignment of transverse electric vectors in electromagnetic waves • property of transverse waves Left: E & M wave is polarized Right: rope vibrations are polarized • In both cases, wave is in the same plane as the plane of vibration.
Polarization Unpolarized light • vibrations producing light are in random directions
Polarization Polarized light • Unpolarized light divided into two internal beams polarized at right angles to each other. One beam is absorbed while the other beam is transmitted.
Polarization Polarized light (continued) • Use your knowledge of vectors and vector components to explain how light that can’t pass through a pair of Polaroids at right angles to each other will pass light when a third Polaroid is sandwiched between them!
