To Verify Snells Law
Materials:
Glass block, Ray box, Protractor
Lampbox
0 - 360°
Protractor
i
Glass
Bloc
k
A
B
r
C
Procedure:
 Set up apparatus as shown in the diagram.
 Shine a ray of light onto the block.
 With a pencil mark the incident and emergent ray
 Remove the glass block
 Draw the incident and emergent ray
 Then draw a line from where the incident ray meets the glass block to the
emergent ray
 Thus you have drawn the reflected ray within the block
 Use a protractor to measure these angles
 Record these values
 Repeat at least 5 times varying the angles each time.
Results:
Graph the results.
Conclusion:
The Graph shows a straight line through the origin hence Sin i/ Sin r is a constant.
Precautions:
 Repeat at least 5 times to prove in full to reduce percentage error.
 Draw around the glass block
 Make sure the glass block doesn’t move
 Measure to one tenth of a degree
Frequently Asked Questions!
Why would you use the larger part of the block instead of the smaller?
Using the smaller side would mean that there is a smaller path for the ray to refract and
this would increase percentage error when drawing the refracted ray.
The smallest angle of incidence chosen as 20˚. Why would smaller values lead to a
less accurate result?
The smaller values would lead to a less accurate result as it would increase the angle of
incidence/refracted percentage error.
How exactly are the refractive and incidence angles measured?
The refractive and incidence angles are measured by drawing the normal at 90˚ to the
point of contact and then measuring the angle between the normal and each ray.
(Ideally you would draw this and label)
Diagram!!
Question 7 - Mocks 2004
A ray of red light is directed at a glass prism of side 80 mm as shown.
Using this diagram, show that the refractive index of the glass for this light is 1.52. (6)
Calculate the critical angle for the red light in the prism.
(5)
Draw a ray diagram to show the path of the ray of red light through the prism. Indicate on
the diagram the values of all relevant angles.
(12)
Answers:
To prove that the refractive index of the glass for this Red Light is 1.52:
n
=
Sin i
Sin r
n
=
Sin 20
Sin 13
n
=
1.52
To find the Critical Angle:
n
=
1
Sin C
Sin C =
1
n
Sin C =
1
1.52
Sin C =
0.6579
C
=
Sin–1 0.6579
C
=
410
Demonstrate Total Internal Reflection:
Explaination:
1. We need to find the angle of incidence
2. Taking (600 + 770) from 1800 we find the third angle to be 430
3. This then tells us that the angle of incidence is 470
4. This is greater than the critical angle which is 410, so total internal reflection
occurs and the ray does not leave the prism
5. We next look at the small triangle formed in the left hand corner.
6. The top angle is 1800 – (430 + 470 + 470) = 430
7. As the other angle is 600, we now know that the third angle is 770
8. This then tells us that the angle of incidence is 130
9. Refraction takes place and using
n
=
Sin i
Sin r
1.52
=
Sin 130
Sin r
r
=
200
The following are very likely to be asked:
Critical Angle:
When light travels from a denser medium to a rarer medium, the angle of incidence
whose corresponding angle of refraction is 90 degrees is called the critical angle.
The relationship between the critical angle and the refractive index is:
n
=
1
Sin C
Where n is the refractive index and C is the critical angle.