How we see

Question 1

Explain refraction in regards to the eye

Answer 1

• Refraction: Light changing speeds when travelling through different media

- This means that the light bends
- The parts of the eye that cause refraction are: the cornea, the aqueous humor, the lens and the vitreous humor 
- It is mainly the cornea and the lens that cause the biggest refraction however.
- The cornea causes the most refraction and the lens can alter its shape to change how much the light will be refracted
- When objects move closer the eye needs more bending power and so the lens thickens

Question 2

Explain accomodation

Answer 2

• Accommodation: our eyes have the power to change focus on distant objects (infinity) to close objects

- The lens changes shape
- The pupil constricts (sharpens the focus) 
- The eyes converge (we use our medial rectus muscle to converge)

Question 3

How does the ciliary muscle control the shape of the lens?

Answer 3

• The shape of the lens is controlled by the ciliary muscle contracting
- Ciliary muscle contracts
- Spaces in the middle decreases
- Suspensory ligaments become lax
- Lens is no longer under stretch
- Lens becomes thicker
• A thicker lens is more powerful and can focus on close objects

Question 4

What is myopia?

Answer 4

• Far away objects appear blurry and close up objects are in focus
  
  • The most common cause is the eyeball being too long and so the focus point of the light is in front of the retina causing far away objects to appear blurry
  • The cornea and the lens bend the light too much for the eyeball
  • When the object comes closer then this bending power comes to use. The image focuses on the retina without having to rely on increasing the curvature of the lens

Question 5

What are the symptoms of myopia?

Answer 5

• Headaches
  - Complaining of not seeing thing on the blackboard
  - Divergent squint (infants and preverbal children)
  - Loss of interest in sports/ people and more interest in books and pictures
  - Loss of interest in class

Question 6

How do you correct myopia?

Answer 6

To correct myopia you need to decrease the bending power (spectacles, contact lenses, laser eye surgery

Question 7

What is hyperopia?

Answer 7

• Close objects appear blurry and far away objects are in focus
  
  • Eyeball too short or cornea and lens too flat
  • The image of an object formed behind the retina
  • Person automatically starts to use accommodative power and makes the lens thicken which causes the image to be formed on the retina
  • They use more power up close until it is all used up

Question 8

What are the symptoms of hyperopia?

Answer 8

• Eyestrain after reading/ working on the computer

- Convergent squint (children/ toddlers)

Question 9

How do you correct hyperopia?

Answer 9

• The convergent squint needs immediate correction with glasses/ lenses to preserve vision in both eyes and to prevent lazy eye
  
  • Biconvex glasses
  • Contact lenses
  • Laser eye surgery

Question 10

Describe the properties of astigmatic eye and explain how this vision defect can be corrected

Answer 10

• Astigmatism (not being able to see far away or close up objects)

- The surface of the eye is more like a rugby ball than a football
- The bending of light rays along one axis is never the same as along the other axis
- Image is always hazy
- Needs special glasses called cylindrical glasses (curve in only one axis)
- Laser eye surgery can also be used to correct
- Need special contact lenses called toric lenses

Question 11

Describe the properties of presbyopic eye and explain how the vision defect can be corrected

Answer 11

• Presbyopia (long-sightedness in old age)

- With age the lens gets less mobile/ elastic
- So when the ciliary muscle contracts it is not as capable as before to change shape
- Seeing near objects/ reading becomes more difficult
- Need glasses to read
- Usually starts in the 5th decade 
- Correction by using biconvex reading glasses

Question 12

Describe the photo transduction mechanism whereby light energy is converted into neural signal and the role of vitamin A in photo transduction

Answer 12

• Before light hits the eye 11-cis retinal is connected to opsin which makes rhodopsin
• Once light hits 11-cis retinal it isomerises to all-trans retinal
• All-trans retinal cannot split into the opsin and so rhodopsin splits into opsin and all-trans retinol. This results in the bleaching of the visual pigment purple
• Once this has happened the all-trans retinol travels to the pigment epithelial cells, where it is converted back to 11-cis retinal, this is done by:
- All-trans retinol and vitamin A (from the liver) combine to make 11-cis retinol which is converted to 11-cis retinal
• Vitamin A is also important for healthy epithelium

Question 13

How is colourblindness caused?

Answer 13

• Cones have a specificity to the type of light they can see: red, green and blue
  
  • If there is something wrong with them then this causes you to not be able to see this colour as well