
























Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
Key points from this lecture are: Properties of Light, Accelerating Electrons, Electromagnetic Waves, Constant Speed, Electromagnetic Spectrum, Electromagnetic Spectrum, Transparent Materials, Earth’s Atmosphere, Shadows, The Eye Topics covered in Basic concepts of Physics course are: Newton’s Laws of Motion, Linear Motion, Momentum, Energy, Rotation, Gravity, Liquids, Gase, Plasmas, Heat, Waves, Sound, Electrostatics, Electric current, Magnetism, Electromagnetic Induction, Color, Light, Atom an
Typology: Slides
1 / 32
This page cannot be seen from the preview
Don't miss anything!
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
-^
The EM wave moves outwards (
emanates
) from the vibrating
charge.
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.
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
docsity.com
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!
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?
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!
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
opaque
to visible
light, have natural frequencies in therange of visible light.
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
-^
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
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).