Reflection, Refraction, Snell's Law, Critical Angle Notes

Welcome Back!

You guys said you like starting chapters with doing notes this way, so round 2 I guess. Today's notes are all about how light bends. Let's get into it.

Reflection

● Law of reflection -

when a light ray
strikes a reflecting
surface, the angle of
reflection equals the
angle of incidence

● Both angles are

measured from the
normal

Regular vs Diffuse Reflection

● If the beam of light falls on a smooth surface the rays

undergo regular reflection (parallel rays reflect parallel)

● If the beam of light falls on a rough surface, the rays

undergo diffuse reflection (parallel rays do not reflect
parallel)

Why is it harder to see when driving at

night after it has rained?

● Answer:

● Because the wet road is a smooth surface

and very little light is reflected back to our eyes

Are the pages of your book smooth

or rough?

● Answer:

● They are rough

● If they were smooth, there would be a lot

more glare and it would be harder to read.

Refraction

●
Refraction- the change in
speed and direction of light at
the boundary between 2 media.

●
Snells Law: ni sin øi = nr sin ør

●
When light travels from a less
optically dense medium into a
more optically dense medium,
the speed is reduced and the
angle of refraction is smaller
than the angle of incidence.

●
V= c/ nr

●
V = velocity

●
c = speed of light (3 E 8 m/s)

●
nr = index of refraction

Multiple Choice

What is the speed of light as it passes through diamond (n=2.42) ?

• 1.24 x 10^8 m/s
• 1.50 x 10^8 m/s
• 2.00 x 10^8 m/s
• 3.00 x 10^8 m/s

Draw

Your turn to draw! Draw 2 light rays, 1 showing light reflecting off the surface, the other refracting through it.

​Snells Law: ni sin øi = nr sin ør

ni= index of refraction for incidence
nr= index of refraction for refraction
øi= angle of incidence
ør= angle of refraction

Snell's Law

Multiple Choice

A ray of light passes through air (n=1) into water (n=1.33) with an angle of incidence of 32 degrees. What is the angle of refraction?

• 45 degrees
• 23.48 degrees

• 15 degrees
• 30 degrees

Multiple Choice

A ray of light passes through air (n=1) and into an unknown substance. If the angle of incidence is 27 degrees and the angle of refraction is 14 degrees, what is the index of refraction for the unknown substance?

• 1.40
• 1.14
• 1.88

• 1.27

Dispersion - separation of light into

the spectrum of colors

● Red light travels

fastest; it bends the
least; has the smallest index of refraction

● Violet light travels the

slowest, it bends the
most; has the largest index of refraction

Rainbows

● A rainbow is formed when sunlight is dispersed

by water droplets.

● This is a result of both reflection and refraction

Total internal reflection occurs when light passes from a more optically dense medium to a
less optically dense medium at such an angle that the angle of
refraction is 90 degrees.


Critical angle - angle of incidence that causes total internal reflection or the refracted angle to be 90 degrees

Critical Angle = sin^-1(1/n)

​Total Internal Reflection

Multiple Choice

A material has an index of 2.1. What is its critical angle?

• 45.2 degrees
• 32.1 degrees
• 20.5 degrees
• 28.4 degrees

All Finished!

​That's the notes for the chapter.

We will practice some Snell's Law and the other math together soon. I hope you have a good day and that you learned something in class.
Catch you on the flipside
-Coach Bush