Refraction of Light When can light “bend”?. Review Light travels in a straight line…. most of the time. The speed of light? 3.0 x10 8 m/s This is only.

Refraction of Light

When can light “bend”?

Review

• Light travels in a straight line….

• most of the time.

• The speed of light? 3.0 x108 m/s

• This is only true in a vacuum.

Concept Check

• You run faster in air than you do in water….

• Some things, you can’t run through if they are dense enough (like walls)

• Light is the same way!

How “c” changes

• The more dense a medium is, the slower light travels.

• Note: Some stuff you can’t run through, light can still go through, like really thick glass.

• The “ocular density” of an object we will call n (index of refraction).

To find n…

• You can calculate the value of n by the equation:

• n will always be bigger than 1.• A table of values of n can be found on pg 490 of

your book.

Why the car crashes

• When a speeding car hits water with the tires on one side, that side slows down, causing the car to spin out.

What happens when light slows down?

• When light switches mediums, it takes a turn as well….

Water tank

AirӨi

Өr

Normal Line

Өi - Angle of incidence

Өr -Angle of refraction

Key Points

• The air and the water each have their own density, so the values of “n” (index of refraction) are different too.

• You can see the light bends, so the angle of incidence and the angle of refraction ARE NOT EQUAL.

How the light bends

• If the light slows down, it bends toward the normal line (glass < air).

• If it’s going the other way, it speeds up and bends away from the normal (air > glass ).

Refraction

• Where does the cat see the fish?

• Where does the fish see the cat?

Snell’s Law

• Def: Snell’s Law relates the indices of refraction and the two angles as:

• Make sure your calculator is in “degrees” mode, or things will be much harder…..

How come water bends light and reflects it too???

• You can see reflections in water and glass, but you can also see through them.

• If the angle of incidence is big enough, light will mostly reflect, and you can see images.

• If the angle of incidence if small, most of the light will go into the new medium, and you can see through it.

Water tank

Air

Өi

Өr

Normal LineThe angle when light switches from bouncing off water to going into it is called the critical angle

Why is the sky blue?• Each color of light turns a different amount when it

goes from the medium of space to the medium of the atmosphere.

Example

• A light ray of wavelength 589 nm traveling through air strikes a smooth, flat slab of crown glass at an angle of 30.0˚ to the normal. Find the angle of refraction, θr.

Practice

• Complete Page 493 Practice A in your textbooks using Snell’s Law.