In the fig.-3 we have stuck two pins on a thermocole block.

The pins are parallel to each other. As we see in the fig., when a source of light is kept very near, we see the shadows diverging (from the base of the pins). As we move the source away from the pins, the angle of divergence gets reduced. If we move the source far away we will get parallel shadows. But as we move the candle away, the light intensity becomes low. That means to get a beam of parallel rays the source should be at a long distance and it must be of sufficient intensity.

Where do we find one such source?

Yes, we have one easily available source, you probably have guessed it: The Sun.

Let us do an experiment with sun rays and a concave mirror.

Activity 2

Hold a concave mirror such that sunlight falls on it. Take a small paper and slowly move it in front of the mirror and find out the point where you get the smallest and brightest spot, which will be the image of the sun. (See to it that your paper is small so that it does not obstruct the incoming sun rays.)

The rays coming from the sun parallel to the principal axis of concave mirror converge to a point (see fig.-4).

This point is called Focus or focal point (F) of the concave mirror. Measure the distance of this spot from the pole of the mirror. This distance is the focal length (f) of the mirror. The radius of curvature will be twice this distance (R=2f).

Does this help you to verify the conclusions you arrived at, with your drawing?

What happens if you hold the paper at a distance shorter than the focal length from the mirror and move it away?
• Does the image of the sun become smaller or bigger?

You will notice that the image of the sun first keeps on becoming small, beyond the focal point it keeps on becoming enlarged.

Note: while drawing a ray diagram sometimes it is not clear which is the reflecting side of the mirror. Hence we follow a convention of showing lines on the non-reflecting side(coated side).

click here to know about "Focus and Focal length"
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