1. Light & Mirrors 4

2. Learning Objectives
To investigate angles of reflection.

3. A Quick Recap!
A mysterious friend is worried that his rival is spying on his plans.
We’ve discovered how people see objects.
But our friend has walls and fences so we need to work out how his rival is seeing over and around those.
We think mirrors may have been used but need to investigate mirrors further to know exactly how.

4. Light Diagrams with mirrors
The mirror reflects the light.
Light is emitted from the light source.
Light enters the eye and and the ball is seen!
Light is reflected by the opaque object.

5. Does the angle of the mirror matter?
Line up the light beam along the black horizontal line on your whiteboards.
Place the mirror at an angle on the black line.
Measure the angle that the light hits the mirror at.
Measure the angle the light leaves the mirror at.

6. Record your results Angle light hits mirror
Angle reflected light leaves mirror
Angle between the beams of light (180 – both answers)
Light reflects in a very predictable manner and is covered by the Law of Reflection. This law states that the angle of incidence is equal to the angle of reflection. Put simply this means the pupils should find that the angle that the light hits their mirror is the same angle that it is reflected from the mirror.
The third column in the table is simply working on the law that all angles on a straight line total up to 180˚. This third column is actually the total of the angle of incidence and the angle of reflection. This fact will become important when we consider periscopes late in the presentation.

7. Our Findings What is our rule? Angle light hits mirror
Angle reflected light leaves mirror
Angle between the beams of light (180 – both answers)
20˚
140˚
67˚
46˚
32˚
116˚
What is our rule?

8. Periscopes
How can the submarine see what is happening on the island while under water?
They use a periscope!
Lets have a look at how they work…

9. Periscopes Light is emitted by a light source.
Light is reflected by the objects and into the periscope.
Light hits the first mirror and is reflected at 90˚ down the periscope.
Light hits the second mirror and is reflected at 90˚ and out of the periscope and into the eye.

10. Periscopes What angle must the mirror be at?
The mirror would need to be at 45˚. This is so the angle of incidence is 45˚, meaning the angle of reflection is also 45˚ giving a total of 90˚. Put simply, the light hits it at 45˚, leaves at 45˚, therefore leaving 90˚ between the two beams.

11. What Have We Discovered?
The rival is using a periscope to see over or around the walls and fences.
Periscopes use two mirrors at 45˚ angles.
Light enters the periscope, hits the first mirror, travels down the periscope, hits the second mirror and then leaves the periscope and enters the eye.
Tomorrow we shall let our mysterious friend know and help him win the trophy!