Concepts of Physics B Name __________________________________________

Unit 4.1 - LightShake the end of a stick back and forth in still water and you will produce waves on the water surface. Similarly shake a charged rod back and forth in empty space and you will produce ______________________ waves in space. The shaking charge can be considered an electric current.• A magnetic field _________________ an electric current. • A changing magnetic field surrounds a ______________ electric current.• A changing magnetic field creates a _______________ electric field.• The changing electric field creates a _______________ magnetic field.An electromagnetic wave is composed of oscillating electric and magnetic fields that regenerate each other. No medium is required.

Electromagnetic WavesThe oscillating fields come from the vibrating charge. At any point on the wave, the electric field is perpendicular to the magnetic field. Both are ______________________ to the direction of motion of the wave.For electromagnetic radiation, there is only one speed - the speed of light – no matter what the frequency or wavelength or intensity of the radiation. The changing electric field ____________ a magnetic field. The changing magnetic field acts back to ____________ an electric field. Only one speed could preserve this harmonious balance of fields.

If the wave traveled at ______ than the speed of light, the fields would rapidly die out. The electric field would induce a weaker magnetic field, which would induce a still weaker electric field. If the wave traveled at ______ than the speed of light, the fields would build up in a crescendo of ever greater magnitudes. At some critical speed, however, mutual induction continues indefinitely, with neither a loss nor a gain in energy. From his equations of electromagnetic induction, Maxwell calculated the value of this critical speed and found it to be 300,000 km/sec, which happened to be the speed of __________! Maxwell quickly realized that he had discovered the solution to one of the greatest mysteries of the universe - the nature of light; light was an _________________________ wave!!

Electromagnetic SprectrumThe electromagnetic spectrum consists of radio waves,microwaves, infrared, light, ultraviolet rays, X-rays, and gamma rays. Light is a _______________ of the family of electromagnetic waves that includes radio waves, microwaves, and X-rays. The range of electromagnetic waves is called the electromagnetic _________________.

The lowest frequency of light we can see appears _______. The highest visible light, ___________, has nearly twice the frequency of red light. Electromagnetic waves of frequencies lower than the red of visible light are called __________________. Heat lamps give off infrared waves. Electromagnetic waves of frequencies higher than those of violet are called __________________________. They are responsible for sunburns.

Transparent, Opaque and TranslucentMaterials that transmit light are ______________. Glass and water are transparent. Glass is transparent to all the frequencies of visible light. The frequency of the reemitted light is identical to that of the light that produced the vibration to begin with. The main difference is a slight time delay between absorption and reemission (see image below). This time delay results in a lower average speed of light through a transparent material.

In a vacuum, the speed of light is a constant 300,000 km/s; we call this speed of light c.• In water, light travels at 75% of its speed in a vacuum, 0.75c. • In glass, light travels at about 0.67c, depending on glass type. • In a diamond, light travels at only 0.40c. When light emerges from these materials into the air, it travels at its original _____________, c.

Materials that absorb light without reemission and thus allow no light through them are ____________. Wood, stone, and people are opaque. Metals are also opaque. When light shines on metal it is reemitted as visible light. This reemitted light is seen as a reflection. That’s why metals are shiny.Some materials are _________________; they transmit light (like transparent materials) but don’t transmit details of the objects involved. Shower curtains and light ceiling panels are examples of translucent objects. They both let light pass through them but you cannot see the _____________ of what is behind the curtain (you see a silhouette of the person but not details) or the ceiling panels (the light passes through to the room, but the wires in the ceiling are not visible).

Polarized LightLight travels in waves. The fact that the waves are transverse - and not longitudinal - is demonstrated by the phenomenon of polarization.• If you shake the end of a horizontal rope, a transverse wave travels along the rope.• The vibrations are back and forth in one direction.• The wave is said to be ___________________.

If the rope is shaken up and down, a vertically polarized wave is produced. The waves traveling along the rope are confined to a ________________ plane. If the rope is shaken from side to side, a horizontally polarized wave is produced. A vibrating electron emits a polarized electromagnetic wave. A vertically vibrating electron emits vertically polarized light. A horizontally vibrating electron emits horizontally polarized light.

An incandescent or fluorescent lamp, a candle flame, or the sun all emit light that is _______ polarized. The electrons that produce the light vibrate in random directions.

When light shines on a polarizing filter, the light that __________ _____________is polarized. The filter is said to have a polarization axis that is in the direction of the vibrations of the polarized light wave. Light passes through __________ polarizing filters when the polarization axes are aligned but not when they are crossed at right angles. A rope analogy illustrates the effect of crossed sheets of polarizing material.

a. Light is ____________________ when the axes of the polarizing filters are aligned (image on the left).

b. Light is ___________________ when they are at right angles to each other (image on the right).