Focusing the Eye

Pupil Reflex

Pupil reflex is a reflex action, carried out by the iris, which controls the amount of light that enters the eye. This involuntary response happens because too much light can damage the retina.

The iris is a coloured ring of muscle with a hole in the middle, which is called the pupil. To get to the lens, light must pass through the pupil.

The size of the pupil will change depending on the two sets of muscles in the iris: radial muscles and circular muscles.

  • In bright light – Circular muscles contract and radial muscles relax, so the pupil becomes smaller, which allows less light into the eye.
  • In dim light – Radial muscles contract and circular muscles relax, so the pupil becomes larger, allowing more light into the eye.

So remember:

Radial musclesCircular musclesSize of the pupilAmount of light entering the eye
Bright lightRelaxContractSmallerLess
Dim lightContractRelaxLargerMore

Accommodation of the Eye

Accommodation is the process of changing the shape of the lens to focus on near or distant objects. This occurs through the contraction or relaxation of a circular ring of muscle called the ciliary muscle, which is connected to suspensory ligaments.

Near objects

To focus on near objects, the ciliary muscles contract, so they become smaller in diameter. Also, the suspensory ligaments loosen so they don’t pull on the lens.

  • The lens becomes larger and more spherical, so it refracts light more strongly onto the retina

Distant objects

To focus on distant objects, the ciliary muscles relax, so they become bigger in diameter. Also, the suspensory ligaments are pulled tight so they pull on the lens.

  • The lens becomes thinner and flatter, resulting in only slight refraction of light onto the retina

So remember:

Distance of objectCiliary musclesSuspensory ligamentsLens shapeRefraction
NearContractLoosenBigger/sphericalLight is refracted strongly onto the retina
DistantRelaxTightenFlatterLight is refracted slightly onto the retina