Question

Answer the following

(1) State the different positions of source of light with respect to concave mirror−

(a) in torches

(b) projector lamps

(c) food lights.

(2) What are the rules used for drawing ray diagrams for the formation of image by spherical mirror?

(3) What is meant by power of accommodation of eye?

(4) State the function of iris and ciliary muscles in human eye.

(5) If you are given a part of hollow spherical glass, how will you convert it into concave mirror?

Answer 1

(1)

(a) In torches: The source of light is placed at the focus.

(b) Projector lamps: The object is placed at the focus.

(c) Flood lights: The source of light is placed just beyond the centre of curvature.

(2) Sign Conventions for spherical Mirrors:

I. Objects are always placed to the left of the mirror i.e., light must fall on the mirror from left to right.

II. All distances are measured from the pole of the mirror.

III. Distances along the direction of the incident ray (along positive x- axis) are taken as positive, while distances along the direction of the reflected ray (along negative x-axis) are taken as negative.

IV. Heights measured perpendicular to and above the principal axis (along positive y-axis) are taken as positive.

V. Heights measured perpendicular to and below the principal axis (along negative y-axis) are taken as negative.

These sign conventions can also be represented in the following diagram:

(3) When the ciliary muscles are relaxed, the eye lens becomes thin, the focal length increases, and the distant objects are clearly visible to the eyes. To see the nearby objects clearly, the ciliary muscles contract making the eye lens thicker. Thus, the focal length of the eye lens decreases and the nearby objects become visible to the eyes. Hence, the human eye lens is able to adjust its focal length to view both distant and nearby objects on the retina. This ability is called the power of accommodation of the eyes.

(4) The iris separates the anterior and the posterior chambers of the eyeball. It controls the light level similar to the aperture of a camera. It helps in controlling the amount of light entering the eye by regulating the size of pupil which enables us to see objects at various distances.

The ciliary muscles contract and relax in order to change the lens shape for focusing to see both far and near objects by changing the focal length of the eye lens.

In order to see nearby objects the focal length of the lens is reduced by contraction of the ciliary muscles and the lens becomes thick.

In order to see far objects the focal length of the lens is increased by relaxation of the ciliary muscles and the lens becomes thin.

(5) The inner side or concave side of the hollow spherical glass will be polished to get a highly reflecting bulged inward surface, thus a concave mirror can be made.