reflection & refraction when waves are incident on the boundary between to media, some of the...
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Reflection & Refraction
When waves are incident on the boundary between to media, some of the wave will be reflected back into the first medium, some of the wave will be transmitted into the second medium.
Answer Me!!!
Can you always see yourself in a mirror? Why or why not?
Representation of Waves
A ray is used to represent one wavelet and show the direction of the wave along that ray.
Barrier
Wav
e fro
nts
ray
Wave fronts
ray
Law of Reflection
If an incident ray strikes a reflective surface, the ray will be reflected at the same angle as measured from the normal.
Angle of incidence (I) = Angle of reflection (R)
R
Nor
mal
Incident rayRef
lecte
d ra
y
Diffuse Reflection
regular reflection. diffuse reflection.
Virtual images
If an object is placed in front of a mirror. An image of that object is created behind the plane of the mirror.
The location of this image can be found by tracing reflected rays back until they intersect.
object image
Practice Problem 1
Which of the following is the appropriate reflected ray?
Incident ray
AB
C
D
Answer Me!!!
What do you notice about the straw in the glass on Mr. Scholz’s desk?
Why do you think the straw appears this way?
Refraction of Light
Light travels through different media at different speeds.
As these wavefronts travel slower, rays of light bend toward the normal.
The bending of light as it travels through different media is called refraction.
Refraction Example 1
AIR: less dense,Light travels faster
WATER: more dense, light travels slower
Boundary
Bends toward normal
Refraction Example 2
AIR: less dense,Light travels faster
WATER: more dense, light travels slower
Boundary
Bends away from normal
Refraction Example 3
AIR: less dense,Light travels faster
WATER: more dense, light travels slower
Boundary
Ray is not refracted when parallel to normal
Practice Problem
A man is trying to catch a fish using a spear. The water is clear so he can se the fish swimming by. As one get close, he sends the spear exactly where he sees the fish. Does he catch his fish? Why or Why not?
Where is the Sun?As the light from the sun travels through our atmosphere, it is refracted. So, where we see the sun is actually an optical illusion
Actual sun
Where wesee the sun
Index of Refraction
An object’s index of refraction (n) is a measure of how fast light will travel through that object.
speed of light in a vacuum (c) speed of light in the given medium (v).
The Physics Reference Tables have a list of common indices of refraction.
v
cn
Practice Problem 2
What is the speed of light through Lucite?
Practice Problem 3
What material allows light to pass through it at a speed of 2.25 x 108 m/s?
Practice Problem 4
The speed of light in a transparent material is 0.75 times its speed in air. What is the index of refraction of the material?
Index of Refraction
Since the frequency of light does not change as it passes through materials:
n11 = n22
OR
n1v1 = n2v2
1
2
1
2
2
1
2
1
22
11
So,
and
and
v
v
fcfc
n
n
f
cn
f
cn
fvv
cn
Practice Problem 5
As a monochromatic beam of light passes obliquely from flint glass into water, how does the characteristics of the light beam change?Wavelength will _____________Frequency will ______________Speed will _______________Angle will _____________
Snell’s Law
Snell’s Law describes the relationship between index of refraction and the angle light will travel at.
2211 sinsin nn
1
2
Index: n1
Index: n2
Practice Problem 7
A beam of light is incident on a Lucite block in air at an angle of 33o. Calculate the angle of refraction.
Practice Problem 8
A ray of light passes from air into a block of transparent material. The angle between the incident ray and normal is 45o. Then angle between the refracted ray and normal is 26o. Calculate the index of refraction of the material.
Practice Problem 9
Find the angle of the light beam in the Lucite.
What material is the light traveling slowest in?
Air
Lucite
Medium Y
Air
Practice Problem 10
A beam of light passes through parallel sections of glycerol, medium X and medium Y. What could medium X and Y be made from?
Glycerol
Medium X
Medium Y
40o
35o
40o
Practice Problem 11
A monochromatic beam of yellow light is incident upon a Lucite block in air at an angle of 33o. Calculate the angle of refraction and draw the incident and refracted rays.