1. UNIT 3 ~ PHYSICS Lesson P6 Part 1 ~ Human Vision

2. Human eyes can focus on both objects far in the distance and close up.
They can also adjust from a bright environment to a very dim environment.

3. How Light Enters the Eye
Light enters the eye through the pupil.
The pupil is an opening that appears dark because light passes through it without reflecting back.

4. How Light Enters the Eye
The iris (colored portion of eye) is a muscle that controls the amount of light entering the eye by changing the size of the pupil.
In dim light, the iris dilates (expands) the pupil to allow more light to enter.
In bright light, the iris contracts the pupil to allow less light to enter.

5. Focusing Light Rays
The cornea is a transparent (light can pass through it) tissue that covers the iris and pupil
 holds the eye together
 does the majority of the focusing of light that enters the eye
 refracts incoming light rays so they converge toward the back of the eye

6. Internal Structures of the Eye
Surrounding the cornea is an opaque tissue called the sclera.
 The white region of our eye.
Behind the pupil is a flexible convex lens  as light rays enter, the lens changes shape to converge light rays to a point at the back of our eye
 This is to fine-tune our focus
The retina is like a “screen” at the back of the eye where light rays fall to form an image

7. Forming An Image
Special light-sensitive cells in the retina detect the image.
Other cells in the retina convert the light rays into electrical signals that are sent to the brain through the optic nerve.
Image is inverted
(upsided down) but
our brain “flips” it
to see upright!

8. Blind spot
The area where the optic nerve enters the retina does not have any light- sensing cells. This area is known as the blind spot.
 When light rays strike the blind spot, no image is formed, as no signal is sent to the brain (see textbook page 205)

9. Rod and Cone Cells The retina consist of 2 types of cells.
Rod cells  Absorb any colour of light but only detect shades of light and dark
 Allow us see shapes and movement in dim light.
Cone cells allow us to detect colour.
 can detect red, green and blue.
 are activated in bright light

11. OPTICAL ILLUSIONS
What are they?  Visually perceived images that differ from reality
Types of illusions:
images can remain after overstimulation of cells in the eyes (see text page 202)
Receptors in the eyes compete to be active, and when one is over stimulated, they become “tired”
Our brain interprets shades and lines and tries to make sense of the signals sent to it from the optic nerve (eyes)

12. OPTICAL ILLUSIONS
What is the FIRST thing that you see when you look at this picture?

14. Duck or Rabbit?

16. Which square is darker, A or B?

17. Which center circle is bigger?
Neither, they are the same size!!!