Properties of Light - Lecture Slides - Basic Concepts of Physics, Slides of Physics

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Today:

Chapter 26 (Properties of Light)

Light: preliminaries

•^

Light is the only thing we actually see – e.g. When I “see”you, I am actually seeing light reflected off you.

-^

Light is a transverse wave (recall earlier), whose origin is accelerating electrons,

e.g. in the sun

•^

Accelerating electrons not only can produce light, but alsoradio waves, microwaves, x-rays…. Grouped together as electromagnetic waves

•^

Different types of electromagnetic waves differ in theirfrequency (and wavelength): light is just a small part of theelectromagnetic spectrum with certain frequency range

Electromagnetic Waves cont.

-^

The EM wave moves outwards (

emanates

) from the vibrating

charge.

Electromagnetic Wave Velocity

•^

An electromagnetic wave travels at

one constant speed

through space. Why?Inherently due to wave nature

(e.g. objects like spacecrafts can

change speed, and go at different constant speeds);

specifically,

induction and energy conservation:

-^

Recall (last chap): the strength of the induced fields dependson the

rate of change

of the field that created it.

So, if light traveled slower, then its electric field would change

slower, so would generate a weaker magnetic field, that inturn generates a weaker electric field, etc

wave dies out.

Similarly, if light sped up, would get stronger fields, with ever-

increasing energy.

-^

Both cases violate energy conservation.

The Electromagnetic Spectrum

•^

In vacuum, all electromagnetic waves move at the samespeed

c

, but differ in their frequency (and wavelength).

-^

Visible light:

4.3 x 10

14

Hz to 7 x 10

14

Hz

i.e. red is at the low-freq end of light (next lowest is infrared)violet is the high-freq end (next highest is ultraviolet)

and

long-wavelength

short wavelength

recall speed

c

f

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Clicker Question

Which of the following is fundamentally different fromthe others?A) sound wavesB) X-raysC) gamma raysD) light wavesE) radio wavesAnswer: AThe others are all electromagnetic waves: transverseoscillations of electric and magnetic fields.

Is it correct to saythat in every case,without exception,any radio wavetravels faster thanany sound wave? 1. Yes^ 2. No

Any radio wave travels at the speed of light.A radio wave is an electromagnetic wave,like a low-freq light wave.A sound wave, on the other hand, isfundamentally different. A sound wave is amechanical disturbance propagated througha material medium by material particles thatvibrate against one another. Sound is notpart of the EM spectrum!In air, the speed of sound is about 340 m/s,about one millionth the speed of a radiowave. Sound travels faster in other media,and light is slower in other media, but, thesefactors are usually more like factor of 2 or 3,at most, not a million…

Answer: 1,

_until very recently:_*

* In 1999, however, researchers in carefully controlled expts managed to

slow light down to 17m/s!

The electromagnetic spectrum cont

•^

Recall Ch 18 on waves: frequency of wave = freq ofvibrating source.Applies to EM waves too, where source is oscillatingelectrons

-^

Note that EM waves are everywhere! Not just in air, but ininterplanetary “empty space” - actually a dense sea ofradiation. Vibrating electrons in sun put out EM waves offrequencies across the whole spectrum.

-^

Any

body at any temperature other than absolute zero, have

electrons that vibrate and (re-)emit in response to the EMradiation that permeates us, even if very low frequency.

Which of these lamps is emittingelectromagnetic radiation?

1. Lamp A2. Lamp B3. Both4. Neither

Clicker Question

1. Lamp A2. Lamp B 3. Both 4. Neither Which of these lamps is emittingelectromagnetic radiation?

Answer: 3 All bodies with any temperature at all continually emitelectromagnetic waves. The frequency of these waves varies withtemperature. Lamp B is hot enough to emit visible light. Lamp A iscooler, and the radiation it emits is too low in frequency to bevisible—it emits infrared waves, which aren’t seen with the eye. Youemit waves as well. Even in a completely dark room your waves arethere. Your friends may not be able to see you, but a rattlesnake can!

Transparent materials

Simple model of atom: think of electrons attached to nucleuswith springs. Light makes these springs oscillate.• Different atoms/molecules have different “spring strengths”- so different natural frequencies.•^

If this natural freq = that of impinging light

, resonance

occurs (recall ch 20) i.e. vibrations of electrons build up tohigh amplitudes, electrons hold on to the energy for “long”times, while passing it to other atoms via collisions, finallytransferred to heat.

Not

transparent, i.e opaque

• So materials that are

opaque

to visible

light, have natural frequencies in therange of visible light.

Transparent materials cont.

•^

Glass is transparent: its natural freqs are higher, in theultraviolet range. So glass is not transparent to ultraviolet.But

is

transparent to lower freqs i.e. visible spectrum.

•^

What happens in this off-resonance case?Atoms are forced into vibration but at less amplitude, sodon’t hold on to the energy long enough to transfer muchto other atoms through collisions. Less is transferred toheat; instead vibrating electrons

re-emit

it as light at same

frequency of the impinging light. (see more next slides)

-^

Infrared waves – frequencies lower than visible – canforce vibrations of

atoms/molecules

as well as electrons in

glass. Increases internal energy and temperature of glass.Often called

heat waves

Transparent materials cont.

-^

So: Light-transparent materials (like glass) have natural frequencies that don’t coincide with those of light. The atoms re-emit after absorbing.This re-emission is

time-delayed

-^ This leads to speed of light being different in different media:In vacuum, it’s

c

In air, only slightly less than

c

In water, it’s 0.

c

In glass, 0.

c^

(but depends on type of glass)

When light emergesback into air, it travelsagain at original

c

Questions

Why in the sunlight is a black tar road hotter to thetouch than a pane of window glass?

Sunlight is absorbed and turned to internal

energy in the road surface, but transmitted through theglass to somewhere else.

(2)

Can you get sunburned through a glass window?

Glass is opaque to ultraviolet light, so won’t

transmit it, so you won’t get sunburned (although youmight get hot! as light in the visible-range istransmitted and then you absorb it).