Refraction of Light

• • The flower looks small when viewed through the water droplet. The light from the flower is bent because of the shape of the water droplet and the change in material as the light passes through the water.

22.4 Measuring the Speed of Light The speed of light was known to be very large, although careful studies of the orbits of Jupiter’s moons showed that it is finite.

One important measurement, by Michelson, used a rotating mirror:

22.4 Measuring the Speed of Light Over the years, measurements have become more and more precise; now the speed of light is defined to be: This is then used to define the meter.

Which usually round off to

c

 3 .

00  10 8

m

/

s

23.4 Index of Refraction In general, light slows somewhat when traveling through a medium. The index of refraction of the medium is the ratio of the speed of light in vacuum to the speed of light in the medium: (23-4) n is always greater than 1

Example

Calculate speed of light in diamond.

n



c v v



c n

 3 .

00  10 8 2 .

42  1 .

24  10 8

m

/

s

23.5 Refraction: Snell’s Law Light changes direction when crossing a boundary from one medium to another. This is called refraction, and the angle the outgoing ray makes with the normal is called the angle of refraction.

Refraction

• Refraction occurs when light velocity changes.

• The presence of material (glass, water, ice, diamond, etc.) slows down the speed of light.

Refraction

What happens if the ray travels along the normal?

It does not change direction.

When the angle enters more dense medium (air -> water), the angle bends _______the normal.

Towards.

When the light ray travels from water to air – Away.

23.5 Refraction: Snell’s Law Refraction is what makes objects half submerged in water look odd.

23.5 Refraction: Snell’s Law The angle of refraction depends on the indices of refraction, and is given by Snell’s law in 1621: (23-5)

(

Index of refraction of first medium

) x (

sine of the angle of incidence

) = (

index of refraction of second medium)x (sine of the angle of refraction

)

Snell’s Law

Willebrord Snell (1591-1626) Dutch astronomer and mathematician.

it is now known that this law was first discovered by Ibn Sahl

Ibn Sahl

(c. 940-1000) Muslim Persian mathematician, physicist and optics engineer

Atmospheric Refraction

• • Mirages are the result of refraction.

Wave fronts of light travel faster in the hot air near the ground, thereby bending the ray of light upward. The image appears upside down.

23.6 Total Internal Reflection; Fiber Optics If light passes into a medium with a smaller index of refraction, the angle of refraction is larger. There is an angle of incidence for which the angle of refraction will be 90 °; this is called the critical angle: (23-5)

23.6 Total Internal Reflection; Fiber Optics If the angle of incidence is larger than this, no transmission occurs. This is called total internal reflection.

23.6 Total Internal Reflection; Fiber Optics Binoculars often use total internal reflection; this gives true 100% reflection, which even the best mirror cannot do.

23.6 Total Internal Reflection; Fiber Optics Total internal reflection is also the principle behind fiber optics. Light will be transmitted along the fiber even if it is not straight. An image can be formed using multiple small fibers.

Internal Reflection

• Diamonds achieve their brilliance partially from total internal reflection. Because diamonds have a high index of refraction (about 2.3), the critical angle for the total internal reflection is only about 25 degrees. Incident light therefore strikes many of the internal surfaces before it strikes one less than 25 degrees and emerges. After many such reflections, the colors in the light are separated, and seen individually.