Name:_________________
Physics 11
Date: ________________
Lesson 4 - The Speed of Light
The speed limit for the universe appears to be the speed of light in a vacuum, which is
__________________________________________________________________
In air, the speed of light is slightly less, but the difference does not appear until the fourth digit
past the decimal point. For most purposes, the speed of light in a vacuum or in air can be taken to
be _____________________________
Just how fast is the speed of light? If a plane travels at the speed of sound, it is considered to be moving
very fast. But the speed of sound is _____________. To get a rough idea of how this compares with the
speed of light, round it off to 3 x 102 m/s. The ratio of speed of light to the speed of sound is
approximately
_______________________
The speed of light is about one million times the speed of sound! To travel the length of a typical
classroom, light would take only about 10-7 s, or 0.000 000 1 s. Light from the moon takes 1.3 s to reach
us, and light from the sun takes about 8.3 min to travel to earth!
Exercises
1. If light travels a million times as fast as sound, why does the sound from your television set
appear to be 'synchronized' with the picture you see?
______________________________________________________________________________
______________________________________________________________________________
____________________________________________________________
2. If a beam of light could be made to ‘circle’ the earth (using a series of mirrors), how long would it
take the light to circle the globe if the earth's circumference is 4.0 x 107 m? Also, how many times
could the beam of light circle the earth in one second?
3. The distance light travels in one year is called a light year. How many kilometres are there in one
light year?
4. What is the wave length of a 32 MHz cell phone signal?
Snell’s Law of Refraction
Refraction is the __________________________ that takes place at __________________________ between two
materials having different indices of refraction. Refraction is due to
________________________________________________________________ as it passes from one medium to another.
The boundary ________________________________________________________________________________
At a boundary, an incident ray can undergo _______________________________ or, in certain situations,
____________________________________________
No bending of the incident ray occurs if it strikes the boundary along the normal.
The ____________________________ is the ray approaching the boundary. It strikes the boundary at the
point of incidence. The _________________________ is the ray leaving the boundary through the second
medium.
The reflected ray is the ray undergoing partial (or total) reflection at the boundary.
The normal is a construction line drawn perpendicular to the boundary at the point of incidence.
The angle of incidence ( i ) is ______________________________________________________________________________
The angle of reflection ( r ) is ______________________________________________________________________________
The angle of refraction ( R ) is _______________________________________________________________________________
Both Reflection and Refraction occur when the light is incident on a more refractive medium.
Whenever light (or any wave) passes from one medium to another medium it may be
______________________________ from its original path. A Dutch mathematician Willebrord Snell (15911626) discovered that there is an exact relationship between the angle of incidence and the angle
of refraction for light traveling from one medium into another. This relationship is:
sin i
n
sin r
Where n is a constant for a particular medium and is called the ___________________________________.
The index of refraction is measured relative to a vacuum, which is assigned 1.
Examples of indices of refraction:
Water (at 20˚C)
Diamond
Glass
Air
Ethanol
Quartz Crystal
1.333
2.42
1.5-1.9 (depends upon composition)
1.00029
1.36
1.54
Note: the index of refraction depends upon the color of light used and on the purity, temperature
and composition of the material into which the light is traveling.
Snell’s Law can be expanded for any situation as follows:
ni sin i  nr sin r or
sin i nr

sin r ni
Example 1: Light rays hit a quartz crystal at an angle of incidence of 25˚ from air.
the angle of refraction?
Determine
Example 2: A ray of light traveling through air is incident upon a sheet of crown glass at an angle
of 30.00. What is the angle of reflection?
Example 3: A light wave traveling in air (refractive index of 1.0) passes into the water in a
swimming pool at an angle of 35° to the normal. Calculate the direction of the light in the
water, given that the refractive index of water is 1.4.
Applying Concepts: Pg. 363 #5, 9
Problems: Pg. 364 #2, 3, 4, 7, 10, 13
Applications Pg. 365 #20, 23
Name:_________________
Physics 11
Date: ________________
Lesson 4 - The Speed of Light
Snell’s Law and Critical Angles Assignment
1. Light entering a block of glass at an angle of incidence of 18.5˚ leaves the boundary between the
air and the glass at an angle of 12.0˚. What is the index of refraction of this type of glass?
2. Light is incident on diamond at an angle of 10.0˚. At what angle will it refract?
3. A beam of light is incident on a sheet of glass in a window at an angle of 30˚. Describe exactly
what path the light beam will take (a) as it enters the glass and (b) as it leaves the other side of
the glass. Assume n=1.500.
4. Light traveling in air has an angle of incidence of 40˚ as in passes into diamond. What is the
refracted angle in the diamond?
5. A transparent material has a refractive index of 1.27. What is the angle of incidence in air when
the angle of refraction in the substance is 43˚?
6. What is the index of refraction of a material if the angle of incidence in air is 50˚ and the angle of
refraction in the material is 40˚?
7. A ray of light passes from water into carbon disulphide (n=1.63) with an angle of incidence of
30˚. What is the angle of refraction in the carbon disulphide?
8. Using Snell’s Law with n = 1.33 for water and n = 2.42 for diamond determine the angle of
refraction in the diamond for the following situation.
53˚
water
Diamond
9. Using Snell’s Law with n = 1.33 for water and n = 2.42 for diamond determine the angle of
refraction in the water for the following situation.
63˚
diamond
Water
10. a) Green light traveling in air has an angle of incidence of 50˚ as it passes into a certain glass.
The refracted angle in the glass is 33˚. What is the refractive index for this type of glass?
b) If the frequency of green light is 5.36 x 1014 Hz, what is the wavelength of green light in space?
c) What is then the wavelength of this green light in the glass?
11. Calculate the critical angle for diamond.
12. What is the critical angle for a glass that has an index of refraction of 1.500?
13. A certain material has a critical angle of 52.0˚. What is its index of refraction?
14. Red light having a wavelength of 7.0 x 10-7 m travels at 3.00 x 108 m/s in space. What is the
frequency of this color?
15. a) A certain blue light has a frequency of 6.8 x 1014 s-1. Its wavelength in water is 3.3 x 10-7 m.
What is the speed of this blue light in water? b) What will be the wavelength of the same blue
light in space?