Reflection & refraction of light PHY232 Remco Zegers [email protected] Room W109 – cyclotron...

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reflection & refraction of light

PHY232Remco [email protected] W109 – cyclotron buildinghttp://www.nscl.msu.edu/~zegers/phy232.html

http://www.phys.pe.kr/03_wave/03_wave_menu.html

PHY232 - Remco Zegers reflection & refraction of light 2

light

c=f

light: wavelike oscillation of E and B fields (just like any electromagnetic radiation). The only things special about it, is that we can see it.


light rays and wave-front

isotropic light source (uniform in all directions): the wavefronts indicate maxima in the E/B

oscillations the rays indicate the direction of the wave fronts I=Psource/(4R2) I: intensity (W/m2) Psource :power of

source. R distance from source

wavefront

rays


Huygens principle

All points on a wavefront serve as point sources of spherical secondary wavelets. After a time t, the new positions of the wavefront will be the tangent of these secondary waveletsmotion of a plane wave:

vt1

vt2

t=0


Huygens principle II

For a circular wavefront…

t=0

vt1

vt2


question

consider a square wavefront at t=0. After some time, the wavefront has traveled outward and is now shaped like a

a) circle b) square c) ellipse d) rounded square


from now on…

we usually only draw the light rays and not the wave-front


reflection

consider a light ray that strikes a mirror. It will be reflected.

The angle of incidence (relative to the normal to the plane) equals the the angle of reflectionr=I

of course, the mirror must be very flat for this to be true

mirror

i r

normal


virtual image

if we look at a mirror, we see an image at a location where the object not really is.

the image is formed at a location where there is not light present

this is called a virtual image

see next chapter…


index of refraction and speed of light

In vacuum, the speed of light equals c=3x108 m/s.

In any other medium, light propagates more slowly.. in air: v=c/1.0003 in water: v=c/1.33 in glass: v=c/1.5

in general: v=c/n where n: index of refraction

n also depends on the wavelength ()n is larger for smaller this effect is called dispersion


refraction

if light passes from one medium with refractive index n1 to a medium with refractive index n2 it is bent (refracted):

Snell’s law n1sin1=n2sin2 or n1/n2=sin2/sin1


refraction II

if n1< n2 then the light ray bends towards the normal when passing through the medium

if n1> n2 then the light ray bends away from the normal when passing through the material.


demo consider a light ray traveling

from air to water at an incident angle of 300. Given nwater=1.33, what is the angle of refraction?

Now, consider the light-ray traveling from water to air at the same incident angle. What is the angle of refraction?

n1/n2=sin2/sin1


question

a light ray hits a 1 cm glass plate (n=1.5) at an incident angle of 300. The light emerges from the glass plate at an angle

a) <300

b) =300

c) >300

=300

=???

http://www.phys.pe.kr/bbs/view.php?id=wave&page=1&sn1=&divpage=1&category=3&sn=off&ss=on&sc=on&select_arrange=headnum&desc=asc&no=11


question: harpoon fishing

You are standing on the shore near a lake and see a fish in the water which you want to spear. Should you aim the spear a) directly at the fish, b) just above it or c) just under it?

a b

c


a harpoon fisher

http://www.phys.pe.kr/bbs/view.php?id=wave&page=1&sn1=&divpage=1&category=3&sn=off&ss=on&sc=on&select_arrange=headnum&desc=asc&no=2


total internal reflection

consider a ray of light traveling from water to air.

since nwater > nair a light ray is bent away from the normal

however, if

…this equation has no solution,since sin cannot be larger than

1

2=sin-1(nwatersin1/nair)

nwatersin1/nair>1sin1>nair/nwater

1>sin-1(nair/nwater)1>48.7o


internal reflection II

when traveling from a medium 1 to a medium 2 with n1>n2:

if sinI>n2/n1 refraction is not possible and all light will be reflected; =sin-1(n2/n1) is called the critical angle


transport of light!

demo


question

given the above situation and the fact that the critical angle for water->air is 48.70, would the harpoonist know where to aim at? I.e. does she see the fish?

a) yesb) no

=500


dispersion

we already saw that the index of refraction depends on the wavelength (I.e. color) of the light.

this can be used to separate white light into its component colors. demo


rainbow

depending on the location of the raindrop and the angle a person is looking at it, only light of a certain wavelength (color) is visible.

The angle (location) changes depending on where the drop is, so that one sees different colors at different locations in the sky…


lon-capa

after reviewing this lecture, do questions 1-6 from HW set 8.

to do these problems: be careful with what is the initial and final medium…

Reflection & refraction of light PHY232 Remco Zegers [email protected] Room W109 – cyclotron...

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