CHAPTER 12 LIGHT AND REFLECTION Uses of plane mirrors
Because plane mirrors are convenient for bouncing virtual images from one place to another, they are commonly used in: • periscopes (Fig. 12.12) • binoculars • viewing yourself (Fig. 12.13).
In order to see with a periscope (Fig. 12.12), the light has to bounce off the object, travel to the top mirror, reflect down to the bottom mirror and then reflect to your eyes. In this way wherever you point the top mirror, the periscope will show you that image in the bottom mirror. This allows us to view objects over an obstacle or a wall.
The image in a plane mirror is ‘laterally inverted’. For example, the dancer in Fig. 12.13 is holding up her left arm but the reflection shows her right arm being held up. It is an important developmental stage in children when they begin to predict how their laterally inverted image behaves in a mirror.
ray of light
mirror
mirror
FIG. 12.12 A periscope works using plane mirrors
FIG. 12.13 Images in plane mirrors are ‘laterally inverted’ You may now complete Exercise 12A.
CHAPTER 12 EXERCISE 12A
1. State the laws of refl ection. 2. What is the diff erence between a real image and a virtual image?
3. A girl approaches a plane mirror at a velocity of 3 m s–1. At what speed is she approaching her image?
4. Proxima Centauri is our closest star after the Sun. It is 3.8 × 1016 m from Earth. How long will it take light to travel from Proxima Centauri to Earth?
5. Draw a ray of light striking a plane mirror at an angle of incidence of 30°. Show the normal and refl ected ray as well as the angle of refl ection.
