Name:
Waves Workbook 3
Year 12 Science
Ruawai College 2014
When light travels from one medium to another which is optically less dense (i.e. has a
smaller refractive index than the first), the angle of incidence is less than the angle of
refraction (i.e. the light bends away from the normal).
1.
Draw a diagram that demonstrates this.
When the angle of refraction is 90o, the angle of incidence is called the critical angle, ϴc .
2.
Draw a diagram that demonstrates this.
When the angle of incidence is greater than the critical angle, all the light reaching the
boundary is reflected, and total internal reflection occurs.
3.
Draw a diagram that demonstrates this.
Example:
The refractive index of glass – nglass = 1.52
The refractive index of air – nair = 1.00
For light passing from glass to air, the refractive index is given by Snell’s Law:
n1.sinϴ1 = n2.sinϴ2
If ϴc is the critical angle for glass then:
n1.sinϴc = n2.sinϴ2
Rearranging:
sinϴc = n2.sinϴ2
n1
=
1 x sin 90o
=
1.52
1.00
=
1.52
ϴc = 41.1o
4.
What is the formula for Snell’s Law?
n1.sinϴ1 = n2.sinϴ2
5.
What is ϴc an abbreviation for?
critical angle
6.
What is n an abbreviation for?
refractive index
7.
What is the refractive index of glass?
1.52
8.
What is the refractive index of air?
1.00
9.
What is the sine of 90o?
1
10.
Where did 0.658 come from?
1.00 ÷ 1.52
11.
Where did 41.1o come from?
inverse sign 0.658
0.658
Common Refractive Indices:
Air or vacuum: 1.00
Water: 1.33
Crown Glass: 1.523
Barium glass: 1.60
Flint glass: 1.70
Diamond: 2.45
Snell’s Law
n1.sinϴ1 = n2.sinϴ2
Exercises:
12.
A ray of light travelling from air into crown glass strikes the surface at an angle of
o
30 . What will the angle of refraction be?
19.2o
13.
Calculate the angle of refraction when a light wave travels from air (n = 1) into water
(n = 1.33), at an angle of 35o.
25.55o
14.
A ray of light travelling from air into water strikes the surface at an angle of 60o.
What will the angle of refraction be?
40.63o
Snell’s Law Practical
Aim: To test Snell’s Law of Refraction
n1.sinϴ1 = n2.sinϴ2
Method:
1.
The acrylic block is placed in front of a single ray from the ray box.
2.
A single ray is directed at the centre and no refraction should occur.
3.
The ray box is directed at 10o intervals and the angle of refraction is recorded.
Results:
Angle of
incidence ϴi
Angle of
refraction ϴr
sin ϴi
sin ϴr
Sin ϴi
Sin ϴr
What is the refractive index of the acrylic block?
What is the critical angle for the block?
Extension:
Try the same procedure again this time using different glass blocks, and a tray containing
water.
Find the refractive indices and the critical angles for the materials you use. Remember the
refractive index of air is 1.
15.
Calculate the critical angle for light travelling between air and water.
49o
16.
In which direction – water into air or air into water – must the light be travelling for
total internal reflection to occur?
Water to air
17.
Calculate the critical angle for light travelling between glass and water.
61o
18.
In which direction – water into glass or glass into water – must the light be travelling
for total internal reflection to occur?
Glass to water
Dispersion
Different frequencies of light slow down by slightly different amounts when they enter
optically more dense media.
19.
What is happening to the blue light as it passes through the glass?
Being refracted at an angle of ϴblue
20.
What is happening to the red light as it passes through the glass?
Being refracted at an angle of ϴred
21.
Which light refracts more as it passes through the glass?
Blue light ϴblue > ϴred
22.
Why do optical fibres use monochromatic light (one colour) to carry digital signals?
If light of many frequencies was used the differing colours would separate, with faster
parts of the signal overlapping with slower parts of the previous pulse. Confusions and
loss of signal would result.