Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 3: WAVES
When you look at a mirror and see an image of yourself, what is happening to the light rays that hit the mirror? The answer is that they are reflected, which basically means that the light rays 'bounce' off the mirror and change direction.
As long as the mirror is completely flat - called a 'plane' mirror - then the change in direction of the light ray follows a simple, basic rule. This rule is called the law of reflection.
Figure 1 shows how this law works:
Figure 1. The law of reflection
There are a few things to note about this diagram:
So to recap, the law of reflection states that:
The angle of incidence = the angle of reflection
angle i = angle r
Questions on this topic are often diagram-based, requiring drawing. Remember to use a pencil and a ruler, so that your diagram looks neat and you can change it if needed! Question 1 below is an example of an easy diagram question. You will always need a protractor for questions on this topic.
Questions:
1. Nadia is on her bicycle, waiting to pull out from a road junction. A row of shops was built opposite the junction. The shops have glass windows which act as a mirror. Nadia could see Joan’s motorbike reflected in the glass window.
a) On the diagram above, draw a ray of light to show how Nadia can see Joan’s motorbike reflected in the glass window. Add arrows to the ray. Use a ruler.
b) How does the glass window help to reduce the number of accidents?
(Note: As this is an e-book, you could either print out the image and try yourself, or simply sketch out what you would have drawn on a piece of paper, and see if you are correct).
UK QCA SAT tests / National Archive - licence here
a) The ray should:
b) Accidents are prevented because without the glass, Nadia would not be able to see the motorbike until it is much closer to her. (She can see the bike when it is behind the houses).
2. The diagram below shows a plane mirror angled at 300 to a flat table. A ray of light is directed vertically down onto the mirror as shown here:
On the diagram shown above:
See the example solution here:
a) The normal line should be (approximately) perpendicular to the mirror.
b) The reflected ray should be above the horizontal. It should be drawn so that the normal line is (approximately) in the centre of incident and reflected ray.
c) The angle between the incident and reflected rays should be 600. This can be measured, or you could calculate it by extending the normal line and incident lines and using your maths skills with similar triangles!
Figure 2. Mirror Image of an apple
martinophuc
When we look in a mirror, we can see mirror images - a copy of the object that looks like it is 'behind' the mirror. How does this work and why ?
The answer is that the rays of light reflected by the mirror give the appearance of another object- we are tricked into assuming that the light came from a second source behind the mirror. Here is how it works.
Light rays from the apple (the 'object') travel to the mirror, shown in figure 3, light rays a and b. They are reflected by the flat mirror (light rays c and d). However when we see rays c and d our brain tells us that the light ray travelled in a straight line, and must have come from behind the mirror. We see the image at the location formed by imaginary light rays e and f.
Figure 3. Mirror Image of an apple with construction lines
You should be able to draw this diagram, preferably with 2 light rays, but try it first with just a single ray reflected to show the principle of how the image is formed.
Plane mirror optical images always have the same properties:
There are no Grade Gorilla IGCSE quizzes on this topic, which should be revision from pre-IGCSE work.
