REFRACTION OF LIGHT
Skills assessed: MM/ORR
Aim:
To verify Snell’s law in a glass block
Pre-Lab Questions:
1. What is refraction?
2. State Snell’s law?
3. Draw a fully labelled diagram showing the boundary, normal, incident ray, refracted ray,
angle of incidence, angle refraction
4. Where are the angle of incidence and the angle of refraction measured from?
Apparatus
Glass block, ruler, letter sized paper, protractor, 4 optical pins, sharp pencil, optical board.
Procedure
1. Tack the letter sized paper onto the optical board.
2. Place the glass block in the centre of the paper and draw the outline of the glass block in pencil.
3. Use the protractor to draw in a normal on the middle of the long side of the block. Label the
intersection of the normal and the glass block as point O.
4. Use the protractor to measure out and draw in angles of incidence, i, of 10 0, 20 0 up to
60 0. Extend the lines to the edge of the paper.
5. On the 10 0 line, place two pins A and B upright, so that A is as close to the block as possible and
B is a close to the edge of the paper as possible.
6. Looking through the glass block at the images A and B, turn your head in the position where the
image of B is directly behind the image of A. Place a pin C close to the block so that it appears to
be in line with the images of A and B.
7. Place a fourth pin D close to the edge of the page, so that it is in line with C and the images of A
and B.
8. Remove the pins and place an X over the holes for pins C and D that have the correct alignment.
Remove the block as well.
9. Draw a line through the pinholes and extend it back to the glass block (point R). This represent
the emergent ray.
10. By connecting point O to the R, draw in the refracted ray.
11. Draw arrows on the incident and emergent rays indicating the direction in which the light travels.
12. Use the protractor to measure the angle of refraction, r, record r and its corresponding angle of
incidence, i.
13. Repeat the procedure for all the angles of incidence drawn.
Data Collected
Draw a table showing the values of i, r, sin i and sin r.
Data Analysis
1. How do your observations show that light is refracted?
2. Plot a graph of sin i (y-axis) against sin r (x-axis)
3. Does your graph show proportionality?
4. From your graph find the value of the angle of incidence when the angle of refraction is 25 o.
5. What is the connection between the gradient and the refractive index, n.
6. Use your graph to determine the refractive index of the glass.
7. Given that n = 1/sin c, where c is the critical angle of glass, find the value of c the critical angle
of glass.
8. Discuss any precautions and sources of error in this experiment.
9. The refractive index of glass is glass is approximately 1.5. Calculate the Percentage error
(accuracy) of your experimental result using the formula:
10. Comment on your percentage error.
Criteria (ORR) Marks
table titled [1] 1
accuracy of results [2] agreement with expected results 2
appropriate units in table headers [2] 2
experimental result appropriately titled [1] 1
experimental result appropriately labelled [2] refracted ray, incident ray, 2
angles of incidence and angles of refraction especially
appropriate plots of sin I vs sin r [2] 2
graph titled and axes labelled [2] 2
Total 12
