The human eye, defects of vision and the camera: WASSCE Physics

Your eye is a living camera: a lens that focuses light onto a light-sensitive screen, with an automatic shutter and focus. Understanding it explains why some people need glasses — and how a camera copies its design.

Learning objectives

By the end of this lesson you should be able to:

describe the structure of the human eye and the function of each part
explain accommodation, the near point and the far point
explain short sight and long sight and how they are corrected
compare the eye with the camera

The structure of the eye

Part	Function
Cornea	transparent front window; does most of the bending (refraction) of light
Iris	coloured ring of muscle that controls the size of the pupil
Pupil	the hole through which light enters; smaller in bright light, larger in dim light
Eye lens	converging lens of variable thickness; fine-focuses the image
Ciliary muscles	change the thickness (focal length) of the lens
Suspensory ligaments	hold the lens in place
Retina	light-sensitive layer where the image forms; contains rods (dim light) and cones (colour)
Fovea (yellow spot)	most sensitive part of the retina, where vision is sharpest
Blind spot	where the optic nerve leaves; no light-sensitive cells
Optic nerve	carries signals from the retina to the brain
Aqueous and vitreous humour	transparent fluids that keep the eye's shape

The image on the retina is real, inverted and diminished. The brain interprets it as upright.

Accommodation

Definition

Accommodation is the ability of the eye to change the focal length of its lens so that objects at different distances are focused clearly on the retina.

Distant object: ciliary muscles relax, the lens becomes thinner (longer focal length).
Near object: ciliary muscles contract, the lens becomes thicker (shorter focal length).

Definition

The near point is the closest point at which an object can be seen clearly. For a normal adult eye it is about 25 cm (the least distance of distinct vision).
The far point is the furthest point that can be seen clearly. For a normal eye it is at infinity.

Defects of vision

Short sight: rays from a distant object focus in front of the retina and a diverging lens corrects it. Long sight: rays from a near object focus behind the retina and a converging lens corrects it

Figure 1. Short sight and long sight, and their correction. WASSCE Tutor diagram.

Short sight (myopia)

Symptom: near objects are clear; distant objects are blurred. The far point is closer than infinity.
Cause: the eyeball is too long, or the lens is too strong (converges too much).
Effect: rays from a distant object focus in front of the retina.
Correction: a diverging (concave) lens, which spreads the rays slightly before they enter the eye.

Long sight (hypermetropia)

Symptom: distant objects are clear; near objects are blurred. The near point is further than 25 cm.
Cause: the eyeball is too short, or the lens is too weak.
Effect: rays from a near object would focus behind the retina.
Correction: a converging (convex) lens, which converges the rays a little before they enter the eye.

Other defects

Presbyopia: with age the lens stiffens and cannot accommodate well, so older people struggle to read; corrected with converging (reading) glasses or bifocals.
Astigmatism: the cornea is unevenly curved, so lines in some directions are blurred; corrected with cylindrical lenses.
Cataract: the lens becomes cloudy; treated by surgically replacing the lens.

In Sierra Leone

Free eye-screening camps run by health NGOs in Sierra Leone often find students who cannot read the board from the back of the class — short sight, quickly fixed with diverging spectacles. Cataract surgery restores sight to many older people.

Worked example

A short-sighted student cannot see clearly beyond 2.0 m. What lens does she need, and what is its power?

She needs a diverging lens that forms a virtual image, at 2.0 m, of very distant objects ( $u = \infty$ ). So $v = -2.0$ m:

\dfrac{1}{f} = \dfrac{1}{\infty} + \dfrac{1}{-2.0} \Rightarrow f = -2.0\ \text{m}, \quad P = -0.5\ \text{D}

\begin{gathered}\begin{aligned}\dfrac{1}{f} &= \dfrac{1}{\infty} + \dfrac{1}{-2.0}\end{aligned} \\[6pt] \Rightarrow f = -2.0\ \text{m}, \\[6pt] P = -0.5\ \text{D}\end{gathered}

The eye and the camera compared

Eye	Camera
eye lens and cornea	camera lens (converging)
retina	film or digital sensor
iris and pupil	diaphragm and aperture
eyelid	shutter
focusing by changing the thickness of the lens (accommodation)	focusing by changing the distance between lens and film
image real, inverted, diminished	image real, inverted, diminished
dark choroid layer absorbs stray light	inside of the camera painted black

Key points

The cornea and lens focus a real, inverted, diminished image on the retina.
Accommodation: ciliary muscles change the lens thickness; near point ≈ 25 cm, far point at infinity.
Short sight: focus in front of the retina; diverging lens. Long sight: focus behind the retina; converging lens.
The camera focuses by moving the lens; the eye by changing its shape.

WASSCE examination focus

WAEC tests this

Label a diagram of the eye and give functions of its parts, explain accommodation, explain short and long sight with ray diagrams showing the defect and its correction, and compare the eye and the camera. A calculation of the power of a correcting lens may follow.

Quick check

Which part of the eye controls how much light enters?
Which type of lens corrects short sight?
How does a camera focus objects at different distances?

Answers: 1. The iris (by changing the size of the pupil). 2. A diverging (concave) lens. 3. By moving the lens towards or away from the film or sensor.

The human eye, defects of vision and the camera