The Human Eye and Vision Defects

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Questions and Answers

How does the contraction of the ciliary muscle affect the lens and its focal length?

  • It tenses the zonules, thickens the lens, and decreases the focal length.
  • It relaxes the zonules, thins the lens, and increases the focal length.
  • It tenses the zonules, thins the lens, and increases the focal length.
  • It relaxes the zonules, thickens the lens, and decreases the focal length. (correct)

An individual can clearly see objects at 30 cm, but struggles with objects at a distance. What condition might they have, and what type of lens would correct it?

  • Myopia, corrected with a converging lens.
  • Myopia, corrected with a diverging lens. (correct)
  • Hyperopia, corrected with a diverging lens.
  • Hyperopia, corrected with a converging lens.

In the eye model experiment, what happens to the image formed on the retina when the retina screen is moved to the 'far' position in a hyperopic eye model?

  • The image remains in focus, but its size decreases.
  • The image becomes clearer and more focused.
  • The image goes out of focus. (correct)
  • The image becomes inverted.

A person has a near point of 50 cm. What type of refractive error do they likely have, and what does this mean about their ability to see near and far objects?

<p>Hyperopia; they struggle with near objects but see far objects clearly. (B)</p> Signup and view all the answers

If the focal length of a lens is doubled, what happens to the power of the lens?

<p>The power is halved. (D)</p> Signup and view all the answers

If an object is placed 40 cm from a lens and the image is formed 10 cm from the lens on the opposite side, what is the magnification?

<p>-0.25 (A)</p> Signup and view all the answers

What adjustments does the eye make to focus on an object that is moving closer?

<p>The ciliary muscles contract, increasing lens curvature and shortening focal length. (A)</p> Signup and view all the answers

Which of the following best explains how a corrective lens works for a myopic eye?

<p>It diverges light rays before they enter the eye, so the image focuses further back on the retina. (D)</p> Signup and view all the answers

How does the shape of the crystalline lens change when transitioning from viewing a distant object to viewing a near object?

<p>It becomes rounder, increasing its refractive power. (D)</p> Signup and view all the answers

Consider a lens with a focal length of 0.5 meters. What is its power, and what type of lens is it?

<p>2 diopters, converging lens (A)</p> Signup and view all the answers

If an object's height is 5 cm and its image height is 1 cm, what is the magnification, and what does this indicate about the image?

<p>Magnification is -0.2; the image is reduced and inverted. (A)</p> Signup and view all the answers

What is the primary function of the cornea in the eye's lens system?

<p>To perform the majority of the eye's focusing. (C)</p> Signup and view all the answers

How does a diverging lens correct myopia?

<p>By decreasing the convergence of light rays, allowing them to focus directly on the retina. (B)</p> Signup and view all the answers

What typically causes hyperopia?

<p>An eyeball that is shorter than normal. (B)</p> Signup and view all the answers

What effect does relaxing the ciliary muscle have on the tension of the zonules and the shape of the lens?

<p>Increases zonular tension, flattening the lens. (B)</p> Signup and view all the answers

Which of the following is true about the image formed on the retina of a normal eye?

<p>It is real, inverted, and diminished. (B)</p> Signup and view all the answers

An object is placed at a distance equal to twice the focal length (2f) from a converging lens. Where is the image formed?

<p>At twice the focal length (2f) on the opposite side of the lens. (B)</p> Signup and view all the answers

A person with myopia has a far point closer than infinity. What does this mean?

<p>They can only see objects closer than a certain distance clearly. (D)</p> Signup and view all the answers

If a corrective lens creates a virtual image at the patient's near point, what is the purpose of this virtual image?

<p>To enable the eye's lens to focus on closer objects. (A)</p> Signup and view all the answers

How does the image formed by the +120 mm lens differ from one formed by a lens to correct myopia?

<p>The +120 mm lens produces both real and virtual images depending on object distance; myopic correction only produces virtual images. (D)</p> Signup and view all the answers

If an optical bench is set up with a light source, a lens, and a screen, and the sharpest image is formed when the screen is closer to the lens than expected, what might this indicate about the lens?

<p>The lens has a shorter focal length than expected. (C)</p> Signup and view all the answers

In an eye model experiment designed to simulate hyperopia, how is the condition typically modeled?

<p>By positioning the retina screen closer to the lens. (B)</p> Signup and view all the answers

What is the relationship between the image position (vi), object position (vo), and focal length (f) in the thin lens equation, and how would you use this to calculate the focal length?

<p>$\frac{1}{v_o} + \frac{1}{v_i} = \frac{1}{f}$; $f = ( rac{1}{v_o} + \frac{1}{v_i})^{-1}$ (B)</p> Signup and view all the answers

If the image width is smaller compared to the object width, what does this mean about the magnification, and how will the image appear?

<p>Magnification is less than 1 and the image is reduced. (A)</p> Signup and view all the answers

What happens to the light rays in a hyperopic eye which causes the image to form behind the retina when viewing near objects?

<p>The light rays do not converge enough. (D)</p> Signup and view all the answers

Flashcards

Accommodation

The process where the ciliary muscle adjusts the lens's focal length to focus on objects at different distances.

Near Point

The point nearest to the eye at which an object can be seen clearly.

Far Point

The farthest point from the eye at which an object can be seen clearly.

Myopia (Nearsightedness)

A condition where distant objects appear blurred because the image is focused in front of the retina.

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Diverging Lens

Corrective lenses that diverge light rays, used to treat myopia.

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Hyperopia (Farsightedness)

A condition where near objects appear blurred because the image is focused behind the retina.

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Converging Lens

Corrective lenses that converge light rays, used to treat hyperopia.

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Power (P)

A measure of the degree to which a lens converges or diverges light.

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Magnification (M)

Ratio of image height to object height; indicates size and orientation.

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Cornea

The transparent front part of the eye that refracts light.

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Crystalline Lens

The eye's lens changes shape to focus on near or far objects.

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Ciliary Muscle

Muscle that controls the shape of the lens for accommodation.

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Zonules

Fibers that suspend the lens and transmit tension from the ciliary muscle.

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Retina

The light-sensitive tissue lining the inner surface of the eye.

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Pupil

The opening in the iris through which light passes.

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Study Notes

  • Light enters the eye via the pupil.
  • The lens system, comprised of the cornea and crystalline lens, focuses light on the retina.
  • Accommodation is the process where the ciliary muscle changes the lens' focal length to focus on objects at varying distances.
  • When the ciliary muscle is relaxed, the zonules are at maximum tension, the lens is thinnest, and the focal length is longest.
  • When the ciliary muscle is contracted, the zonules are at minimum tension, the lens is thickest, and the focal length is shortest.
  • A person with normal vision can clearly see objects at infinity (far point) and 25 cm (near point).

Vision Defects

  • If the eye cannot focus correctly, the image forms either in front of or behind the retina, causing blurred vision.
  • Myopia (nearsightedness) occurs when the image forms in front of the retina when viewing distant objects.
  • Myopia results from an overly converging lens system (short focal length) or an elongated eyeball.
  • Myopia is corrected with diverging lenses, allowing myopic individuals to see near objects clearly but not distant ones.
  • Hyperopia (farsightedness) occurs when the image forms behind the retina when viewing close objects.
  • Hyperopia results from an insufficiently converging lens system (long focal length) or a short eyeball.
  • Hyperopia is corrected with converging lenses, enabling hyperopic individuals to see distant objects clearly but not near ones.
  • The thin lens equation is: 1/vo + 1/vi = 1/f , where vo is the object distance, vi is the image distance, and f is the focal length.
  • Power (P) is the inverse of focal length: P = 1/f (measured in m-1 or diopters).
  • Magnification (M) is the ratio of image height (Hi) to object height (Ho), and also the negative ratio of image distance (vi) to object distance (vo) : M = -vi/vo = Hi/Ho

Examining Images on the Retina (Experiment Results)

  • The image produced on the retina is real, inverted, and diminished.

Examining Accommodation (Experiment Results)

  • The lens was at its thickest when the eye model was 25 cm from the light source.
  • The lens had the shortest focal length when the eye model was 25 cm from the light source.
  • Object position: 37 cm
  • Object width: 4 cm
  • Image position: 12 cm
  • Image width: 1 cm
  • Effective focal length of the cornea and lens (calculated): 9.06 cm
  • Magnification of the image (calculated): -0.25 cm
  • The lens to be placed in the septum when viewing objects at infinity is 0.12 mm

Examining Hyperopia (Experiment Results)

  • When the retina screen is moved to the "far" slot, the image goes out of focus.
  • The distance between the lens and retina in a hyperopic eye is shorter compared to a normal eye.
  • The power of the corrective lens used for the hyperopic eye model: 2.5 Diopters (m-1).

The Role of the Lens in Producing a Clear Retinal Image

  • The lens helps focus the image on the retina.
  • In hyperopia, a convex lens converges light rays directly onto the retina.
  • The image formed on the retina is inverted, smaller, and real.
  • The image formed by the lens is upright, larger, and virtual at the patient's near point.
  • This virtual image is then used to create the image on the retina.

Hyperopia and Clear Vision of Far Objects

  • A hyperopic eye can see far objects clearly.
  • In hyperopia, near object light rays diverge, forming an image behind the retina due to a short eyeball or weak lens.
  • For far objects, light rays converge by the time they reach the eye, reducing the lens' effort to converge them.

Examining Myopia (Experiment Results)

  • When the retina screen is moved to the "near" slot, the image goes out of focus.
  • The distance between the lens and the retina of a myopic eye is greater than a normal eye's.
  • Power of the corrective lens used: 1 Diopter (m-1).
  • The distance is shorter than normal

Understanding Concepts of Vision

  • The +120 mm lens produces images that are only inverted.
  • A myopic eye has an eyeball longer than normal.
  • In corrective lens diagrams, object position 3 corresponds to image position A.
  • In the hyperopic eye model, a real and inverted image is produced on the retinal screen when a corrective lens corrects vision.

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