Accommodation of the Eye

What is accommodation?

A reflex that changes the refractive power of the lens.

What do suspensory ligaments do?

Control the shape of the lens.

What happens when light hits the eye?

It has to be focused on the fovea on the retina.

To do this, it has to be refracted so it changes direction.

What does the cornea do?

Always refracts light by the same amount.

What is the role of the lens?

To fine tune the refraction so that the light rays always converge exactly on the fovea.

What does it mean if the light ray is coming far away from the eye?

The light rays are likely to be more parallel.

What happens if light rays are coming from closer to the eye?

Rays will hit the eye at a wide angle.

Light ray has to be refracted a lot. After being refracted by the cornea, it must still be refracted some more.

What does the lens have to do to refract light more (it is closer)?

The lens must be short and fat, and more curved, so it can refract more strongly.

What happens when you look at a nearby object?

• The ciliary muscle contracts inwards towards the lens.

• Causes suspensory ligaments to slacken. Because they are no longer pulling tight on the lens, the lens is free to return to its natural, fatter shape. Can now refract light more strongly.

What happens when you look at a distant object?

• Light does not need to be refracted as greatly.

• Lens doesn’t need to do as much as cornea has already refracted the light most of the way.

• Ciliary muscle relaxes, moving away from the lens and pulling the suspensory ligaments taut. This stretches the lens out.

What is important to remember?

The suspensory ligaments are not muscles - they can only be pulled taut or slacken.

Ciliary muscle is a muscle so can contract and relax.

What does it mean if someone is long sighted?

Their eyes cannot refract enough. Unable to focus on nearby objects.

What happens when the lens can’t refract enough?

• Light still isn’t focused by the time it hits the retina - the image appears behind the retina.

• Object appears all blurry

How do we fix being long sighted?

• Use glasses with convex lenses which provide some extra refraction.

What does it mean if people are short sighted?

The lens refracts too much.

Light from distant objects ends up being over refracted. Image is formed before the retina. Not focused on retina so object appears blurry.

How do we fix being short sighted?

• Use concave lenses as they refract light outwards, so counteract the over refraction of the lens.

What is it called when you are long sighted?

Hyperopia

What is it called when you are short sighted?

Myopia.