The eye and vision.

Step 1 of 7

Focusing

The eye is a sense organ: it turns light into nerve signals. Light enters through the cornea, passes the pupil and the lens, and lands on the retina at the back, where rods and cones turn it into impulses that leave along the optic nerve to the brain. For the image to be sharp, the light from each point of an object has to come to one point on the retina.

The cornea does most of the work, but only the lens can change its power. A lens’s power is measured in dioptres: one over the distance in metres at which it focuses. To focus on an object at a distance $d$⁠, the lens has to add $\dfrac{1}{d}$⁠ dioptres.

Step 2 of 7

The near point

When you look far away, the lens is relaxed and flat. When you look close up, a ring of muscle around it contracts and it rounds up. But there is a limit to how far it can round up, so there is a distance closer than which nothing looks sharp: the near point. Move Card distance and watch the range of sharp sight on the chart.

Step 3 of 7

The ageing eye

Zoom to the retina is at Image on the retina. Over the years the lens stiffens and loses its spring, and it no longer rounds up as it used to.

Step 4 of 7

Short sight

Some eyes are a little too long for their power. A millimetre is enough: the light from a distant object comes to a focus in front of the retina, and by the time it reaches the retina it has spread into a blur again. Eye is fixed at Short-sighted.

Step 5 of 7

Near or far

The name “short sight” confuses many people. Think about what you found in the last Step: at what distance did this eye see sharply?

Step 6 of 7

Glasses

Glasses are an extra lens in front of the eye that changes the total power. Card distance is fixed at 6 m. Switch Glasses on and off and compare the image on the retina.

Step 7 of 7

Reading glasses

The ageing eye lacks power only up close. Reading Glasses add a fixed power to it, and the card at 30 cm is sharp again.