Question

Which part of the human eye controls the focus of the eye by varying its thickness?
A.Ciliary muscles
B.Iris
C.Pupil
D.Retina

Answer 1

Hint: Focus of the eye is because of the lens that captures the light. But this focus is created by adjusting its thickness with the help of muscles.

Complete step by step answer:
-Human eye is one of the complicated sense organs of the body.
-The human eye contains a convex lens that focuses all the incoming light rays from the object, through cornea, by refracting them and forming a real, inverted image on the retina.
-Lens thickness is responsible for the focus of the eye. This change of thickness or shape of the lens is governed by the ciliary muscles.
-Ciliary muscles help in focusing the vision of the eye. This adjustment of the eye is called accommodation of the eye.
-Power of accommodation is the changing of the focal length of the lens to focus near or distant objects.
- To focus the near objects, ciliary muscles contracts cause thickening of the lens and to focus distant objects, ciliary muscles relax and cause thinning of the lens.

Figure: focusing light rays on the retina by distant object and near object respectively.
Hence option A is correct.

Note:
 The other parts,
Iris is a dark muscular tissue that determines the colour of the eye. It lies just behind the cornea.
The small opening of the iris is called a pupil. Its size is controlled by the iris which is also controlled by the ciliary muscles. Pupil controls the amount of light entering the eye.
Retina is the light sensitive screen on which image is formed. It helps to convert the formed image in the electrical impulse that is transmitted to the brain through optic nerves.
Hence, they are accessory parts but do not control the size of the lens.