The document summarizes how vision and hearing work. It describes how light enters the eye through the cornea and pupil, is focused by the lens, and stimulates light-sensitive cells in the retina which send signals through the optic nerve to the brain. Stereoscopic vision from both eyes allows depth perception. Hearing is described as sound waves causing the eardrum and middle ear bones to vibrate, stimulating sensory neurons in the inner ear which also help with balance through fluid-filled semicircular canals that detect movement.

1. Chapter 15, section 3:
Sight and Hearing
How do your eyes enable you to see?
How do you hear and maintain your sense of
balance?

2. Standards
• 7.5.g Students know how to relate the
structures of the eye and ear to their
functions.
• 7.6.b Students know that for an object to be
seen, light emitted by or scattered from it
must be detected by the eye.

3. Vision
• Vision: your eyes
respond to the
stimulus of light.
• They convert that
stimulus into
impulses that your
brain interprets,
enabling you to see.

4. How Light Enters Your Eye
• Cornea- a clear tissue
that covers the front of
the eye
• Pupil-the opening
through which light
enters the eye
• Iris-A circular struture
that surrounds the
pupil and regulates the
amount of light
entering the eye.

5. How Light is Focused
• Lens-Flexible structure that focuses light
• The image it produces is upside down and
reversed.
• Muscles attached to the lens adjust its shape,
focusing the image.

6. How You See an Image
• Retina-After light enters the lens, it goes to the retina, a
sheet of light-sensitive cells that lines the back of the
eye.
• Retina has 130 million cells
• Rod cells- dim light and enable you to see black, white
and shades of gray.
• Cone cells-allows you to see colors
• This is why you see colors best in bright light, but you see
only shadowy gray images in dim light.
• Optic Nerve-rods and cones send electrical impulses to
the brain through the nerve.
• First the brain turns it right-side up then it combines both
images.

7. Seeing in Depth
• Humans have both eyes in front of their head
– BUT they see slightly different images.
• This comparison, interpreted by your brain,
gives you three-dimensional vision.

8. Hearing and Balance
• Your ears are the sense organs that respond to the
stimulus of sound.
• The ears convert sound to nerve impulses that your
brain interprets.

9. How Sound Is Produced
• Sound is produced by vibrations that travel as
waves.
• The vibrations move outward from the source
of the sound, like ripples moving out from a
drop of water.
• The sound waves cause air particles to
vibrate and this is how sound is carried.
• Sound waves can also travel through liquids,
such as water and solids, such as wood.

10. (1) The External Ear
• Shaped like a funnel.
• This enables the external ear to collect sound
waves, which then travel down the ear canal.

11. (2) The Middle Ear
• Sound waves
strike the
eardrum.
• Eardrum
• Hammer, anvil
and stirrup.

12. (3)The Inner Ear
• Stirrup vibrates
against thin
membrane, labyrinth
and cochlea
• Sensory neurons
send nerve impulses
to the cerebrum.

13. The Inner Ear and Balance
• Structures in your inner ear control your sense of
balance.
• Semicircular canals
• Filled with fluid
• Lined with tiny cells that have hair-like extensions
• Movement of the fluid causes hair-like extensions
to bend
• This produces nerve impulses that travel to the brain
• Dizziness