Eye Spy

Eye Spy

Your eye is a living camera. It bends light, adjusts to focus, and turns the world into signals your brain can read. Learn its parts, how it focuses on near and far, and the simple lenses that fix blurry vision.

The parts of the eye

The eye is a sense organ full of light receptors. Follow the light from the front of the eye to the brain, and note that the focusing is shared between two parts, not done by the lens alone:

Label the eye

Label these parts of the eye (light enters at the front, on the left).

Follow the light

Light entering the eye first passes through the transparent _____, which does most of the bending. How much gets in is controlled by the coloured _____, which changes the size of the pupil. The light is then focused by the _____, which can change shape, onto the light-sensitive _____ at the back, and from there the optic nerve carries the impulses to the brain.

Focusing: accommodation

To focus on objects at different distances the eye changes the **shape of the lens**, which is called **accommodation**. The chain runs from the **ciliary muscles** through the **suspensory ligaments** to the lens, and it is worth learning as a chain because the exam asks for all three links:

The focusing chain

To focus on something near, the ciliary muscles _____, which lets the suspensory ligaments _____. Freed from that pull, the lens becomes _____ and refracts the light more strongly. For a distant object every link reverses: the muscles relax, the ligaments pull tight, and the lens is pulled _____ and flat.

When focus goes wrong

If light does not focus exactly ON the retina, vision is blurred. Two defects, and each answer needs BOTH where the image forms and which lens fixes it: - **Myopia** (short-sightedness): distant objects look blurred, because the image is focused **in front of** the retina. A **concave** (diverging) lens spreads the light so the focus moves back. - **Hyperopia** (long-sightedness): near objects look blurred, because the image would focus **behind** the retina. A **convex** (converging) lens bends the light more so the focus moves forward. Spectacles are not the only fix. **Contact lenses** do exactly the same job while sitting on the eye, and **laser surgery** reshapes the **cornea** so it refracts by the right amount in the first place.

The long-sighted patient

A patient reads road signs at a distance easily, but has to hold a book at arm's length to make out the print. What is the defect, where does the image form, and which lens corrects it?

• Hyperopia: the image would form behind the retina, and a convex lens corrects it
• Myopia: the image forms in front of the retina, and a concave lens corrects it
• Hyperopia: the image forms in front of the retina, and a concave lens corrects it
• Myopia: the image forms behind the retina, and a convex lens corrects it

The optician

A patient comes in with blurry vision. Diagnose and correct it.

• They can read a book clearly, but distant road signs are blurred. What is the defect?
• In this eye, where does the image of a distant object come into focus?
• Which spectacle lens will move the focus back onto the retina?

Mark the claims

Five statements about the eye. Tap the TWO that are WRONG.

• The cornea does most of the focusing, and the lens fine-tunes it.
• To focus on a distant object the ciliary muscles contract and the lens becomes thicker.
• A concave lens spreads the light out, so the image forms further back, on the retina.
• Laser surgery corrects vision by reshaping the retina.
• Contact lenses do the same job as spectacle lenses, but sit on the eye itself.

Explain the focusing

Explain how the eye changes focus from a distant object to a nearby one.

• Name the muscles involved, and say what they do
• Say what happens to the suspensory ligaments as a result, and why
• Say what happens to the shape of the lens
• Say what that does to the light, and where the image has to end up

In the exam

Light is refracted first by the _____, which does most of the focusing, and then fine-focused by the lens onto the _____. In accommodation the lens becomes thicker for a near object and thinner for a distant one. In _____, distant objects blur because the image forms in front of the retina, and a concave lens corrects it; in hyperopia the image would form behind the retina, and a _____ lens corrects it.