Physics
Cambridge IGCSE

TOPIC 3: WAVES

# 3.2a Reflection

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

1. The ray that hits the mirror is called the 'incident ray'.
2. The ray that bounces off the mirror is the 'reflected ray'.
3. The mirror is drawn as a straight line with small diagonal lines to show the back of the mirror.
4. The angle of incidence (labelled 'i') and the angle of reflection (labelled 'r') are equal. This is the law of reflection.
5. Both angles i and r are measured from a dotted line that is perpendicular to the mirror. This line is called the normal line.
6. 6. A flat mirror is called a plane mirror.

So to recap, the law of reflection states that:

The angle of incidence = the angle of reflection

angle i = angle r

Learn this rule!

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:

• be straight (drawn with a ruler)
• be reflected off the glass so that i = r (The reflection should occur directly below the word 'glass' on the diagram, from between 'g' to the last 's').
• have arrows pointing right to left. The light ray enters Nadia's eyes. There are no laser beams shooting out of her eyes!
Example solution:

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:

• a) Draw a normal line to the mirror at the point where ray hits the mirror.
• b) Draw the reflected ray.
• c) Measure or calculate the angle between the incident and reflected ray.

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!

## Mirror Images

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.

### Mirror Image Properties

Plane mirror optical images always have the same properties:

1. Images are the same size as the object.
2. Images are the same distance from the mirror as the object.
3. The image is virtual. This means it is imaginary - that no real light rays came from the image, and it cannot be projected onto a screen.
4. A line joining object and image crosses the mirror at right angles. (this makes the object appear 'directly behind' the mirror).
5. The image is 'laterally inverted' - this means flipped left to right, not top to bottom. (Try waving your right hand in front of a mirror - your image waves the left hand).

There are no Grade Gorilla IGCSE quizzes on this topic, which should be revision from pre-IGCSE work.

Tier

## Please choose a tier of entry

 Extended Tier (Core and Supplementary content, Grades A* to G) Core Tier (Core content only, grades C to G) Remember my choice