Reflection and Refraction of Light

Dispersion: Different wavelengths have different velocities and therefore different indices of refraction. This leads to differentrefractive angles for different wavelengths. Thus the light is dispersed.The frequency does not change when n changes.

v f changes when medium changesf does not change when medium changes

Snells Law

n2=1.509

Red

n1=1.000 θ1

θ2

n1 sin1 n2 sin2

sin2 1.00sin1

1.509

sin2 1.00sin30

1.509

0.51.509

0.331

2 19.34deg

Example: If the incident angle of red light striking glass in air is 30 degrees,what is the angle of refraction?

Snell’s Law at Work

Fiber Cable

Why is light totally reflected inside a fiber optics cable? Internal reflection

n1 sin1 n2 sin2

(1.509)sin1 (1.00)sin 90 1.001 sin1 1

1.509 41.505 deg

7

n increases with frequencyBlue light bends more than red

Equilateral prismdispersing sunlight late afternoon.

What causes a Mirage

1.06

1.091.08

1.07 1.071.08

1.09

sky eye

Hot road causes gradient in the index of refraction that increasesas you increase the distance from the road

Index of refraction

t (1v1

h12 y2

1v2

h22 (w y)2 )

dtdy

0

1v1

sin I 1v2

sinR

n1 sin I n2 sinR

Proof of Snell’s Law using the Principle of Least Time

t ( 1v1

h12 y2

1v2

h22 (w y)2 )

dtdy

0

0 1v1

yh1

2 y2

1v2

(w y)h1

2 y2

0 = cv1

sin I cv2

sinR

cv1

sin I cv2

sinR

n1 sin I n2 sinR

Diagram for Last Activity in Lab07