reflection refraction of light 1 light cf light

reflection & refraction of light 1

light Øc=f Ø 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. reflection & refraction of light 2

light rays and wave-fronts rays Ø isotropic light source (uniform in all directions): Ø the wavefronts indicate maxima in the E/B oscillations Ø the rays indicate the direction of the wave fronts Ø I=Psource/(4 R 2) I: intensity (W/m 2) Psource : power of source. R distance from source reflection & refraction of light 3

from now on… Ø We often only draw the light rays and not the wave-fronts. Ø But sometimes it is easiest to think in terms of the wavefronts. reflection & refraction of light 4

reflection Ø Consider a light ray that strikes a mirror. It will be reflected. Ø The angle of incidence (relative to the normal to the plane) equals the angle of reflection Ør = i normal i r mirror Ø The mirror must be very flat for this to be true reflection & refraction of light 5

virtual image Ø If we look at a mirror, we see an image at a location where the object not really is. Ø The image is formed at a location where there is no light present. Ø This is called a virtual image. reflection & refraction of light 6

index of refraction and speed of light Ø In vacuum, the speed of light equals c = 3 x 108 m/s. Ø In any other medium, light propagates more slowly. . Ø in air: v = c/1. 0003 Ø in water: v = c/1. 33 Ø in glass: v = c/1. 5 Ø in general: v = c/n where n = index of refraction Ø n also depends on the wavelength ( ) n is larger for smaller this effect is called dispersion reflection & refraction of light 7

refraction Ø if light passes from one medium with refractive index n 1 to a medium with refractive index n 2 it is bent (refracted): Snell’s law: n 1 sin 1 = n 2 sin 2 reflection & refraction of light 8

refraction II Ø if nnew > nold then the light ray bends towards the normal when crossing the boundary. Ø if nnew< nold then the light ray bends away from the normal when crossing the boundary. reflection & refraction of light 9

demo Ø 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? 2=sin-1(n 1 sin 1/n 2) =sin-1(1 xsin 300/1. 33)= = 22. 080 Ø Now, consider the light-ray traveling from water to air at the same incident angle. What is the angle of refraction? n 1/n 2=sin 2/sin 1 2=sin-1(n 1 sin 1/n 2) =sin-1(1. 33 xsin 300/1. )= = 41. 60 reflection & refraction of light 10

question Ø 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 =300 Ø a) <300 Ø b) =300 Ø c) >300 at air-glass interface: 2=sin-1(n 1 sin 1/n 2)=sin-1(1 xsin 300/1. 5)= 19. 470 at glass-air interface 2=sin-1(n 1 sin 1/n 2)=sin-1(1. 5 xsin 19. 470/1. )= 300 =? ? ? http: //www. phys. pe. kr/bbs/view. php? id=wave&page=1&sn 1=&divpage=1&category=3&sn=off&ss=on&sc=on&select_arrange=headnum&desc=asc&no=11 reflection & refraction of light 11

question: harpoon fishing Ø 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? answer c) a b c reflection & refraction of light 12

total internal reflection Ø consider a ray of light traveling from water to air. Ø since nwater > nair a light ray is bent away from the normal nair sin 2 = nwater sin 1 Ø If nwater sin 1 is larger than 1. 00, this equation has no solution, Total Internal Reflection. (There is always some reflection – reflection gets stronger as the critical angle is approached. ) reflection & refraction of light 13

internal reflection II Ø when traveling from a medium 1 to a medium 2 with n 1>n 2: if sin I>n 2/n 1 refraction is not possible and all light will be reflected; =sin-1(n 2/n 1) is called the critical angle reflection & refraction of light 14

transport of light! demo reflection & refraction of light 15

dispersion Ø The index of refraction depends on the wavelength (I. e. color) of the light. Ø this can be used to separate white light into its component colors. demo reflection & refraction of light 16

rainbow Ø 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. Ø The angle (location) changes depending on where the drop is, so that one sees different colors at different locations in the sky… reflection & refraction of light 17