1. How do we see objects?

2. The Eye and the Brain
Stare at the centre of the rotating spiral for about 20 seconds, then look at your teacher
The Spiral Illusion
Show that the brain and the eye are related and work together

3. Understanding Sight
The tip of the candle flame emits light in all directions
But we only see the light that enters our Eye-Brain
Demonstrate that we can’t see the light that does not enter our eyes by using a laser pointer (or flashlight). Direct the laser beam across the classroom. Make the beam visible by misting water over the beam with a hand-held atomizer. As the tiny drops of water fall through the beam, they scatter light into the students’ eyes. This is called the Tyndall Effect. The Tyndall Effect is caused by reflection of light by very small particles in suspension in a transparent medium. It is often seen from the dust in the air when sunlight comes in through a window, or comes down through holes in clouds.

4. Understanding Sight
We don’t see light that does not enter our eyes

5. Understanding Sight
We don’t see light directed away from our eyes unless it is reflected into our Eye-Brain by something

6. Understanding Sight
On diagrams in this unit, we will tend to ignore all light that does not enter our Eye- Brains

7. Light Rays The path light takes is called a ray
Diagrams that show how light moves from the object to the eye are called ray diagrams

8. Light Rays
Can you see the Eye-Brain’s feet?

9. Light Rays Light travels in straight lines
Since no ray of light can go straight from the Eye-Brain’s feet into your eyes, you can’t see them

10. Light Rays

11. Ray Diagrams
Luminous objects emit light rays in all directions

12. Ray Diagrams
Non-Luminous objects scatter the light rays from luminous objects in all directions

13. Ray Diagrams

14. Ray Diagrams
To make things simpler, we only show the rays coming from the top of the object
We always only show the light rays that enter the observer’s eye

15. Opaque Medium An opaque medium absorbs or scatters all the rays
A textbook is an opaque medium