0.1 Geometrical optics - grade 10  (Page 4/14)

Apparatus:

You will need:

1. a small ball (a tennis ball or a table tennis ball is perfect)
2. a smooth surface, like the floor inside the classroom
3. a very rough surface, like a rocky piece of ground

Method:

1. Bounce the ball on the smooth floor and observe what happens.
2. Bounce the ball on the rough ground floor and observe what happens.
3. What do you observe?
4. What is the difference between the two surfaces?

Conclusions:

You should have seen that the ball bounces (is reflected off the floor) in a predictable manner off the smooth floor, but bounces unpredictably on the rough ground.

The ball can be seen to be a ray of light and the floor or ground is the reflecting surface. For specular reflection (smooth surface), the ball bounces predictably. For diffuse reflection (rough surface), the ball bounces unpredictably.

Reflection

1. The diagram shows a curved surface. Draw normals to the surface at the marked points.

   

2. Which of the points, A–H, in the diagram, correspond to the following:
   1. normal
   2. angle of incidence
   3. angle of reflection
   4. incident ray
   5. reflected ray

   

3. State the Law of Reflection. Draw a diagram, label the appropriate angles and write a mathematical expression for the Law of Reflection.
   
4. The diagram shows an incident ray $I$ . Which of the other 5 rays (A, B, C, D, E) best represents the reflected ray of $I$ ?

   

5. A ray of light strikes a surface at 15 ${}^{\circ }$ to the surface normal. Draw a ray diagram showing the incident ray, reflected ray and surface normal. Calculate the angles of incidence and reflection and fill them in on your diagram.
   
6. A ray of light leaves a surface at 65 ${}^{\circ }$ to the surface. Draw a ray diagram showing the incident ray, reflected ray and surface normal. Calculate the angles of incidence and reflection and fill them in on your diagram.
   
7. Explain the difference between specular and diffuse reflection.
   
8. We see an object when the light that is reflected by the object enters our eyes. Do you think the reflection by most objects is specular reflection or diffuse reflection? Explain.
   
9. A beam of light (for example from a torch) is generally not visible at night, as it travels through air. Try this for yourself. However, if you shine the torch through dust, the beam is visible. Explain why this happens.
   

Refraction

In the previous sections we studied light reflecting off various surfaces. What happens when light passes through a medium? The speed of light, like that of all waves, is dependent on the medium in which it is travelling. When light moves from one medium into another (for example, from air to glass), the speed of light changes. The effect is that the light ray passing into a new medium is refracted , or bent. Refraction is therefore the bending of light as it moves from one optical medium to another.

Refraction

Refraction is the bending of light that occurs because light travels at different speeds in different materials.