Introduction to Light. Light is an EM wave All EM waves travel at the same speed through a vacuum: c = 3 x 10 8 m/s - FASTEST! fast faster.

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Introduction to Light

Light is an EM wave



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All EM waves travel at the same speed

through a vacuum: c = 3 x 108 m/s - FASTEST!

fast

faster

4 Properties of Waves

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reflection

refraction

diffraction

interference

Let’s start with Reflection

• When a ray of light is incident upon a reflecting surface, the angle of incidence is equal to the angle of reflection.

• We measure both angles with respect to the normal.





Law of Reflection

Types of Reflection



Diffuse ReflectionAlthough each ray obeys the law of reflection, the normal is different everywhere so the light scatters in many directions.



Specular ReflectionOccurs when surface is very smooth. A surface is considered polishedif the distance between successive elevations is less than 1/8 the of light.

Reflect on these questions…

• When light hits the paper you are taking notes on, does it reflect specularly or diffusely? How do you know?

• Why is it more difficult to see when driving on a wet road?





Next we’ll discuss Refraction

• What happens to light What happens to light when it enters a new when it enters a new medium?medium?

• Why does it bend?Why does it bend?

Refraction is the bending of light as it enters a new medium at an angle.

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How much slower is the light in the water?

The speed is reduced by a factor of n, where n is the index of refraction of water.

v =cn



n is a measure of the optical density of the medium.

Snell’s Law• Just like the lifeguard,

light wants to take the fastest path possible.

• Light will take a more direct path when it enters a denser medium - it will bend towards the normal.

• Snell’s Law tells us the the exact angle at which light bends:

n1 sinθ1 =n2 sinθ2



Example:

At what angle with respect to the normal does the light ray emerge from the glass?



Determine how fast light travels through this slab of glass if the angle of incidence is 60 and the angle of refraction is 34.



What if the light was shined closer to the right so that the ray exited the right end as shown? Now what would be the exit angle?

1.94 x 108 m/s, 60, ???

Total Internal Reflection

• As the angle of incidence increases, the refracted angle increases until it reaches 90. The incident angle that corresponds with the ray skimming the surface is called the critical angle. When the angle of incidence exceeds the critical angle, the ray undergoes total internal reflection.



Examples of TIR








Finding the Critical Angle



In order to have TIR, light mustoriginate in the denser medium.

Use Snell’s Law and set θ2 = 90

n1 sinθc =n2

sinθc =n2

n1

What is the critical angle for a diamond? The index of refraction for a diamond is 2.42.

24.4º
