GCSE Physics – P1.5.2 – Reflection
Learning Objectives
Objectives:(you are required to know):
a)
The normal
is a construction
Distinguish
between
vector an line perpendicular to
the
reflecting
surface
at the point of incidence.
quantities, and give examples
b) The angle of incidence is equal to the angle of
reflection.
c) The image produced in a plane mirror is virtual,
upright and laterally inverted.
Comment:
Name: .......................................................
In this assignment we will look at reflection. We are also going to learn how to ray trace, which is a practical skill we
need to make accurate optical measurements.
Part A: Angles of Incidence and Reflection - Light
In the first section you will carry out a procedure to find the relationship between the angle of incidence and the
angle of reflection for a beam of light.
The first thing to do is to make sure we have the kit required for this experiment. Check that you have the following:
-A power supply.
-A light box.
-A small piece of metal with lines cut out, which slots into the light box.
-A plane mirror.
-A protractor.
Plug in the power supply and switch it on. Connect the light box to the black and red
terminals of the supply. Place the metal into the end of the box so that a single beam of
light is created.
Be careful, as the box
will get hot!
You will need to work on the practical worksheet –section 1. This will be given to you with this assignment. As you
complete each step of the method, tick it off in the check box provided.
1. On diagram on the first practical sheet, use a ruler to draw a line from point O that is perpendicular to
line AB, and extends downwards for about 10cm. Label this line as the 'normal'.
2. Place your plane mirror on the line AB, and position the light box such that the beam of light shines
along the arrow NO. A beam of light should be reflecting off the mirror. Mark two points on this beam,
and label them P and Q.
Complete
Complete
3. Take the light box away and join these two points together with a straight line. Add an arrow to the
line to show the direction in which the light is travelling. Label this line as the 'reflected beam'
4.
a) Label the angle between the normal and the incident beam as the 'angle of incidence'.
Complete
Complete
b) Use your protractor to measure the angle of incidence. Record this in the table below in the first row.
Repeat
Angle of
incidence (i)
Angle of
reflection (R)
1
2
3
4
5. Label the angle made between the normal and the reflected beam as the 'reflected angle'. Measure
the reflected angle, and record this in the table above in the first row.
6. Now position your light box so that the angle of incidence is now larger. Ray trace the path of this ray
by adding points, and then taking away the apparatus and joining these points together. Make sure to
label the direction the light travels.
7. Measure the angle of incidence and angle of reflection again for this second line. Record your results
in the table.
8. Repeat this process twice more with two different angles of incidence, and record your results in the
table.
9. On a piece of graph paper plot a graph of i (x-axis) against R (y-axis). Add a line of best fit to your
graph.
Complete
Complete
Complete
Complete
Complete
10. Calculate the gradient of your line of best fit.
........................................................................................................................................................
Complete
11. Write a conclusion to this experiment.
........................................................................................................................................................
Complete
........................................................................................................................................................
12. Make a comment about your results. Can you account for any errors in this experiment? (Explain
why your points do not fall exactly on the line of best fit, if this is the case.)
........................................................................................................................................................
........................................................................................................................................................
Complete
Part B: Exercises
Now try the following exercises to test your knowledge of reflection and refraction. Make sure to use a pencil and a
ruler when asked to draw diagrams, and label the direction that the light is travelling in each case.
1. A periscope uses the reflection of light to allow a submarine captain to see above the surface of the sea when the
sub is submerged. A simple diagram of a periscope is shown below.
a) On the diagram, continue the path of the light beam as it approaches the periscope, right through to where it
leaves the periscope at the bottom.
The diagram below shows a maze created by a group of IGCSE students for a laser beam. The beam is seen entering
the maze.
b) The students need to place five mirrors in the maze such that the laser beam emerges out of the exit on the right
hand side. Carefully draw the location and orientation of these mirrors on the diagram, and draw the path of the
laser beam as it passes through the maze.
2. A group of GCSE students are investigating reflection. In a very dark room they shine a laser through a piece of
glass and record the intensity of the beam of using an intensity meter. They then add another layer of glass and
record the intensity again. The students carry out this procedure for six further different thicknesses of glass.
Laser
a) What is the independent variable in this experiment?
........................................................................................................................................................
b) What is the dependent variable?
........................................................................................................................................................
c) What are two controlled variables in the experiment?
........................................................................................................................................................
........................................................................................................................................................
d) How could this group make their results more reliable?
........................................................................................................................................................
The student’s data is shown below:
Glass thickness /
0
6
12
18
24
30
36
42
mm
Intensity /
26.4
24.1
21.5
19.3
17.1
18.2
12.9
10.4
W/m2
e) Plot a graph of the student’s data. Don’t forget axes labels, and a line of best fit.
f) Calculate the gradient of the line and include a unit.
........................................................................................................................................................
........................................................................................................................................................
g) When carrying out the experiment the students were interrupted by a teacher who opened the door to the room
the students were working in. Is it possible to determine when this occurred, looking at the experimental data?
Explain your answer.
........................................................................................................................................................
........................................................................................................................................................
Part C: Reflected Images
You will need to work on the practical worksheet again for this set of procedures.
1. Look at the 4 pictures on practical worksheet section 2. You have to place a plane mirror on the
dotted line in each case, and then carefully draw the image that you see in the space above the line.
Complete
2. Are the reflected images upright, or vertically inverted?
........................................................................................................................................................
3. Are the reflected images laterally inverted? How could you describe this in simple terms?
........................................................................................................................................................
........................................................................................................................................................
4. Are the reflected images real or virtual? (Here’s a clue on this. A real image is formed in front of a mirror, like in an
overhead projector. A virtual image is formed behind a mirror – it’s not really there).
........................................................................................................................................................
5. The diagram shows how light rays can be used to determine the position of an image in a mirror. Watch carefully
how I construct the diagram, and then use the same method to construct diagrams to determine the position of the
image for the two examples on practical sheet - section 3.
Part D: Angles of Incidence and Reflection – Other Types of EM Waves
In section P1.5.2 we learned that light is a type of electromagnetic radiation, and that there are lots of different
other types of electromagnetic radiation. These other types are organised on the electromagnetic spectrum.
Which type of electromagnetic wave has the longest wavelength (λ)?
...................................................................................
What type of electromagnetic wave has the highest frequency?
...................................................................................
Which type of electromagnetic wave is the most penetrating (which can pass through matter easily)?
...................................................................................
Which type of electromagnetic wave causes you to get a tan?
...................................................................................
Which type of electromagnetic wave is detected by our skin as heat?
...................................................................................
What type of electromagnetic radiation is used to cook food?
...................................................................................
What type of electromagnetic wave can pass through living tissue, but not bone?
...................................................................................
Which type of electromagnetic radiation is dangerous? Why is it dangerous?
........................................................................................................................................................
........................................................................................................................................................
In the first part of this section we looked at how the angle of incidence equals the angle of reflection for light, like in
the diagram below:
i
R
i=R
The aim of this practical is to see if this is also the case for other types of electromagnetic radiation, like microwaves
and infrared radiation.
Infrared Waves
i
Does i = R ?
Microwaves
i
The problem is that we cannot see infrared and microwaves. We need different detectors. What could we use to
detect these types of radiation?
Infrared Experiment
Set up the apparatus as shown in the diagram. You have to make sure the remote control is pointing at the
intersection of the normal and the mirror.
Mirror
1. Move your camera phone until the white
dot of infrared radiation is in the direct
centre of the screen.
2. Ray-trace the path of the infrared from
the remote control to the camera on your
phone.
3. Measure the angles of incidence and
reflection, and record them in the table on
the next page.
4. Repeat for two/three different values of
incident angle.
Sheet of paper
i/o
R/o
What do you conclude from this experiment? Is there enough evidence to support the prediction that i = R for
infrared waves?
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
Microwaves Demonstration
Examine the apparatus. How is this different from the infrared experiment? How is it similar?
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
Fill in the table of results as we complete the demonstration together.
i/o
R/o
What do you conclude from this experiment? Is there enough evidence to support the prediction that i = R for
microwaves?
........................................................................................................................................................
........................................................................................................................................................
.......................................................................................................................................................
Part E: Extension Exercises
The diagram shows a ray of white light entering a prism.
a) Explain this effect. Use words like refract and wavelength if possible in
your answer.
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
b) We occasionally notice a rainbow when it is a sunny, but
damp day. Can you explain this effect? (Hint, look at the
shape of the raindrop.)
…………………………………………………………………………………………
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
c) Red light has a wavelength of 700nm (700 x 10-9m), and violet light has a wavelength of 400nm (400 x 10-9m).
Calculate the frequencies of these two colours of light. The speed of electromagnetic waves in a vacuum is 3 x 108
m/s. (300,000,000m/s).
........................................................................................................................................................
........................................................................................................................................................
.......................................................................................................................................................
d) Estimate the frequency of typical infrared waves and typical ultraviolet waves using your answers from c) as a
guide.
........................................................................................................................................................
........................................................................................................................................................
.......................................................................................................................................................