Medical Physics: Vision and the Eye

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Questions and Answers

What is the reason why the eye appears black?

Because it is not sensitive to light

Because all light is reflected back

Because all light that enters is absorbed inside the eye (correct)

Because it does not have a lens

What is the size of the opening in the eye under average light conditions?

5mm

4mm (correct)

3mm

6mm

What is the shape of the lens in the eye?

Circular

Constant

Rectangular

Variable (correct)

What is the function of the aqueous humor in the eye?

To maintain internal pressure Signup and view all the answers

What happens when the drain tubes that remove excess aqueous humor become blocked?

The eye pressure increases Signup and view all the answers

What is the function of the Vitreous humor in the eye?

To maintain the shape of the eye Signup and view all the answers

What is the purpose of the sclera in the eye?

To protect the eye from external damage Signup and view all the answers

What is the function of the Retina in the eye?

To convert light image into electrical nerve impulses Signup and view all the answers

What is the refractive index of the aqueous humor?

1.33 Signup and view all the answers

What is the shape of the lens at the back compared to the front?

More curved at the back Signup and view all the answers

What is the main function of the retina in the eye?

To convert light images into electrical nerve impulses Signup and view all the answers

What is the name of the small area in the yellow spot where detailed vision takes place?

Fovea Signup and view all the answers

How many photoreceptors are there in each eye?

120 million rods and 6.5 million cones Signup and view all the answers

What is the region where rods and cones are not distributed symmetrically?

Blind spot Signup and view all the answers

What is the wavelength of light that cones are most sensitive to?

550 nm Signup and view all the answers

What is the maximum density of rods found at an angle of about?

20° from the vision axis Signup and view all the answers

What is the purpose of rods in the retina?

To facilitate night vision and peripheral vision Signup and view all the answers

What is the term for the area where detailed vision takes place?

Fovea Signup and view all the answers

What is the purpose of cones in the retina?

To detect colors and fine details Signup and view all the answers

What happens when a light photon is absorbed by a photoreceptor?

It initiates a photochemical reaction Signup and view all the answers

What is the primary function of the cornea in the eye?

To focus light rays onto the retina Signup and view all the answers

What is the purpose of the iris in the eye?

To regulate the amount of light entering the eye Signup and view all the answers

What is the role of the pupil in the eye?

To allow light to enter the eye Signup and view all the answers

What is the function of the retina in the visual system?

To convert light into electrical signals Signup and view all the answers

What is the result if any one of the three major components of the visual system fails to function?

Blindness Signup and view all the answers

What is the function of the lens in the eye?

To focus light rays onto the retina Signup and view all the answers

What is the importance of the index of refraction in the cornea?

It is nearly constant for all corneas Signup and view all the answers

What is the role of the nerves in the visual system?

To transmit signals to the brain Signup and view all the answers

What is the result of the cornea's focusing ability?

About two-thirds of the eye's total focusing power Signup and view all the answers

What is the importance of the visual cortex in the visual system?

It interprets visual information Signup and view all the answers

What is the primary cause of myopia?

Too strong eye lens or too long eye ball Signup and view all the answers

What is the purpose of a diverging lens in correcting myopia?

To diverge parallel rays from a distant object Signup and view all the answers

What is the age at which presbyopia generally starts to appear?

45 years old Signup and view all the answers

What is the function of the concave lens in a bifocal lens?

To facilitate distant vision Signup and view all the answers

What is the term for the ability to see near objects clearly, but distant objects appear blurry?

Nearsightedness (Myopia) Signup and view all the answers

What is the correction for farsightedness?

Placing a converging lens in front of the eye Signup and view all the answers

What is the combination of the focal lengths of two lenses given by?

F = 1 / (1/F1 + 1/F2) Signup and view all the answers

What is the term for the loss of the eye's ability to change its focus to see objects that are near?

Presbyopia Signup and view all the answers

What is the purpose of a converging lens in correcting hyperopia?

To converge parallel rays from a distant object Signup and view all the answers

What is the duration for which the eyes continue to dark adapt?

30 to 60 minutes Signup and view all the answers

Study Notes

Visual System

The sense of vision consists of three major components: the eyes that focus an image from the outside world on the light-sensitive retina, the system of millions of nerves that carries the information deep into the brain, and the visual cortex where it is all put together.
Blindness results if any one of these parts does not function.

Vision Elements of the Eye

The cornea:
- A clear, transparent bump on the front of the eye that does about two-thirds of the focusing of light.
- Focuses by bending (refracting) light rays, with the amount of bending depending on the curvatures of its surfaces and the speed of light in the lens compared to the surrounding material.
The iris:
- The colored part of the front of the eye.
- Aids the eye by increasing or decreasing incident light on the retina until it adapts to new lighting conditions.
- Plays an important role in reducing lens defects under bright light conditions.
The pupil:
- A small opening in the center of the iris where light enters the lens.
- Appears black because essentially all of the light that enters is absorbed inside the eye.
- Can change from approximately 3mm in diameter in bright light to 8mm in diameter in dim light.
The lens:
- Variable in shape and has the ability to focus objects at various distances both at its front and back surfaces.
- More curved in the back than in the front.
- Focusing power is smaller than that of the cornea (1/3) because it is surrounded by substances that have indexes of refraction close to its own.

Focusing Elements of the Eye

The cornea and lens are the two major focusing components of the eye.

Aqueous Humor

Fills the space between the lens and the cornea.
Mostly water (n ≈ 1.33).
Continuously produced and surplus escapes through the drain tube ( Canal of Schlemm).
Blockage of the drain tubes results in increased pressure in the eye (Glaucoma).
Maintains the internal pressure of the eye at about 20mm Hg.
Contains many of the components of blood and provides nutrients to the non-vascularized cornea and lens.

Vitreous Humor

A clear, jelly-like substance that fills the large space between the lens and the retina.
Helps to keep the shape of the eye fixed and is essentially permanent.

Sclera

The tough, white, light-tight covering over all of the eye except the cornea.
Protected by a transparent coating called the conjunctiva.

Retina

The light-sensitive part of the eye that converts the light image into electrical nerve impulses that are sent to the brain.
Most vision is restricted to a small area called the macula lutea, or yellow spot.
All detailed vision takes place in a very small area in the yellow spot (0.3mm in diameter) called the fovea.

Eye Photoreceptors

There are two general types of photoreceptors in the retina: the cones and the rods.
Rods:
- 120 million in each eye.
- Used for night vision and peripheral vision.
- Have a maximum density at an angle of about 20° from the visual axis.
Cones:
- 6.5 million in each eye.
- Used for daylight, where we see fine details and recognize different colors.
- Primarily found in the fovea.

Image Formation on the Retina

The retina, the light-sensitive part of the eye, converts the light images into electrical nerve impulses that are sent to the brain.

Diopter Strength of the Eye

The focal length F of a combination of two lenses with focal lengths F1 and F2 is given by:

Vision Defects

Nearsightedness (Myopia):
- Ability to see near objects clearly, whereas distant objects are blurry.
- Due to the too strong eye lens or too long eyeball.
- Corrected by placing a diverging eyeglass (concave) lens in front of the eye.
Farsightedness (Hyperopia):
- Ability to see far objects clearly, whereas near objects are blurry.
- Due to the too weak eye lens or too short eyeball.
- Corrected by placing a converging eyeglass (convex) lens in front of the eye.
Presbyopia:
- Loss of the eye's ability to change its focus to see objects that are near.
- Part of the natural aging process of the eye.
- Corrected by using bifocal lenses.

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Description

This quiz covers the basics of vision and the eye, including the visual system, elements of the eye, focusing elements, sensitivity, image formation on the retina, and vision defects.