The Human Eye and Vision

• The structure of the eye– Iris– Cornea– Lens

• Focusing– Cornea– Accommodation

• The Retina– Photoreceptors– Processing time– Sensitivity

Rods and Cones• Because of their different functions, rods and cones are

present in varying densities in the retina. The blind spot is due to the connection of the optic nerve

Light Sensitivity• Remember we talked about rods and cones• Cones:

– Sensitive to bright light, photopic conditions– Densely packed in the fovea– Only a few cones per nerve fiber

• Rods:– Sensitive to low light, scotopic conditions– Widely distributed across the retina– Up to 1000s of rods per nerve fiber (think of this as

many many drops falling into the same pipe, one drop can’t be detected, but many drops generate some water flow that can be measured)

Dark Adaptation

Object must be very bright to be seen

Dim objects can be seen

Concept Question

The most important means by which you can see light intensities varying by over 13 orders of magnitude is

a) the variable opening of your iris which acts like a diaphragm

b) your retina's ability to change its sensitivity to lightc) your optic nerve d) your cornea letting in more lighte) your photoreceptors turning on and off faster or

slower

Color Effects• The rods and cones are sensitive to different

wavelengths (colors) of light. Recall our discussions of resonance.

Rods

Cones

Color Effects

• Rods are only sensitive to green and blue light, and not sensitive at all to red and yellow light.

• In low light conditions, red objects will appear very dim, because the rods are not sensitive to the light from those objects

• This causes the relative brightness of different colored objects to change when viewed in different lighting conditions

Chapter 6: Optical Instruments

• Eyeglasses– nearsightedness– farsightedness– contact lenses

• Magnifying Glasses

Near and Far Points

• The eyelens has two extreme points, fully relaxed and fully “bulged”, called fully accommodated.

When the lens is fully accommodated, which object is in focus on the retina?

A. A distant starB. A tree outside in your yardC. Your cell phone screen when texting

Far Points

• When the lens is fully relaxed, a normal eye cornea and lens will focus distant objects (at infinity) on the retina

• This is known as the “far point” of the eye

Near Points

• When the lens is fully accommodated (bulged), the eye will focus an object at about 25 cm (10 inches) away onto the retina

• This is known as the “near point” of the eye

25 cm (10 inches)

Imperfect Vision

• Let’s consider light coming into the eye from a distant object, approximated as parallel rays. In a normal eye, these rays focus on the retina when the eyelens is fully relaxed

• If the cornea is not properly shaped, these rays will not focus on the retina

parallel rays focus past the retina parallel rays focus in front of the retina

Myopia (Nearsightedness)

Myopia• Myopia occurs when the cornea is too powerful.

• When the eyelens is fully relaxed, the far point is not at infinity, but closer

• This results in distant objects appearing blurry

far point is less than infinity

Hyperopia (Farsightedness)• Hyperopia is the opposite problem, when the

cornea is not powerful enough, and parallel rays are not focused by the time they reach the retina.

• The eyelens can partially accommodate to increase the power of the cornea-lens system, and focus these rays on the retina

eyelens partially accommodated to increase lens power

Hyperopia• Because the eyelens has to partially accommodate to

focus rays from distant objects, its range will not be sufficient to focus near objects on the retina

25 cm

more than 25 cm

This results in a near point that is more distant than the standard 25 cm

Power of a Lens• It’s going to be easier to think about corrective lenses

using lens power rather than focal length, so let’s review what this means

• Remember:

• The more a lens bulges, the shorter its focal length, and the larger its ray-bending power

Power of a Lens

When the eyelens is fully relaxed, the power of the cornea plus the eyelens is 60 diopters in a normal eye.

If the eyelens then fully accommodates, does the power of the cornea plus eyelensA.increaseB.stay the sameC.decrease

Power of a Lens• When fully accommodated, the power of the

cornea plus eyelens increases by about 4 diopters.

• Your eyeglass or contact lens prescription is given in diopters, the power of the lens needed to correct the imperfect curvature of your cornea

• Converging lenses have a positive power (positive focal length) and diverging lenses have a negative power (negative focal length)

Corrective Lenses

• Myopic (nearsighted) eyes have a cornea plus lens that is too powerful – They will require a negative (diverging) lens to

compensate

• Hyperopic (farsighted) eyes have a cornea plus lens that is not powerful enough– They will require a positive (converging) lens to

compensate

Multiple Lenses: Review

Multiple Lenses

f1 f1f2 f2

If we add a second lens, we can find the image produced by the combination of lenses by using the image from Lens 1 as an effective image for Lens 2

Image from Lens 1 only, effective image for Lens 2

Lens 1 Lens 2

Multiple Lenses

f1 f1f2 f2

• We know where rays from the original object have to hit Lens 2 because we know where the image is. We can use this to find the special rays for Lens 2, and the final image.

Lens 1 Lens 2

Corrective Lenses

f1 f1f2 f2

Lens 1 Lens 2 Retina

• Here the eye lens system produces an image behind the retina.

• If we add a lens in front, like glasses or contacts, the combination of the two lenses will produce an image correctly located at your retina

Multiple Lenses: Power

Lens Power

A myopic eye is too powerful, say it has a power of 63 diopters. What power of lens should we put next to it to get a combined power of 60 diopters (normal eye)

A. -2 dioptersB. -3 dioptersC. 2 dioptersD. 3 diopters

Lens Power

If we have a hyperopic eye of power 58 diopters wearing corrective lenses of power 2 diopters, what is the focal length of the combined set of lenses?

A. 1.5 cm (0.015 m)B. 1.7 cm (0.017 m)C. 2 cm (0.02 m)

Corrective Lenses: Myopia

To correct myopia (nearsightedness), a diverging lens creates an intermediate image of a distant star at your far point so that your eye can see it even though the star is beyond your far point.

Corrective Lenses: Myopia

To correct myopia (nearsightedness), a diverging lens creates an intermediate image of a distant star at your far point so that your eye can see it even though the star is beyond your far point.

far point

image of distant object

Corrective Lenses: Hyperopia

To correct farsightedness your contact lens creates an (intermediate) image of a book 25 cm away at your near point so that your farsighted eye can see it even though the book is closer than your near point

25 cmnear point

Corrective Lenses: Hyperopia

To correct farsightedness your contact lens creates an (intermediate) image of a book 25 cm away at your near point so that your farsighted eye can see it even though the book is closer than your near point

near point 25 cm

focal point of corrective lens

Determining Prescription

Determining Prescription

Determining Prescription

You are near sighted and your far point is 1 meter away. What is your prescription?

A. +1 diopterB. -1 diopterC. +2 dioptersD. -2 dioptersE. +3 diopters

Determining Prescription

You are far sighted and your near point is 1 meter away instead of 25 cm. What is your prescription?

A. +1 diopterB. -1 diopterC. +2 dioptersD. -2 dioptersE. +3 diopters

Presbyopia: Bifocals

• It is possible to have both a near point that is more distant than 25 cm and a far point that is closer than infinity.

• In this case, you need bifocals, which have two lenses in them, one to correct each imperfection

The top part of the lens (the picture shows a pair of bifocals upside down) corrects the far pointThe bottom part of the lens corrects the near point

Contact Lenses

Contact lenses are just a thinner and smaller version of glasses that rest directly on the cornea, with a thin layer of fluid in between.

Magnifying Glasses

FF

Recall this configuration (also on the exam) that produces an upright, magnified image.

Magnifying Glasses

FF

Where should we put the lens to get the biggest image on our retina? If we move the object closer to the magnifying glass, the image gets smaller.

Magnifying Glasses

FF

You would think we would want to put the object close to the focal point of the lens, which would make the biggest image. But we want the biggest image on our retina

Image Size on the Retina

• The size of an object on your retina is related to the angle between the axis and the ray passing through the center of the lens

• A large angle means a large image on the retina.

• Here we see an example that you all know intuitively: that objects look smaller when they are farther away

this angle is large so the object is large

this angle is small so the object is small

Image Size on the Retina• So to make the image as large as possible on the

retina, we want the object to be as close as possible to the eye

• But we can only focus objects as closer as our near point (25 cm in a normal eye, closer for nearsighted people)

• A magnifying glass allows you to bring objects closer to your eye and still keep them in focus

Magnifying GlassesPlacing an object at the focal point of the magnifying glass will produce an image at infinity, which your eye can focus on (its far point) with the eyelens in its fully relaxed state

Magnifying Glasses

You get a slightly larger image on the retina if you move the object such that the magnifying glass produces an image at your near point.

Magnifying Power

Magnification

Magnification

A magnifying glass has a focal length of 10cm. What is its magnification?

A. 1XB. 1.5XC. 2XD. 2.5XE. 3X