Activity . You have already learnt a way of determining the focal length of a concave mirror. In Activity . , you have seen that the sharp bright spot of light you got on the paper is, in fact, the image of the Sun.
It was a tiny, real, inverted image. You got the approximate focal length of the concave mirror by measuring the distance of the image from the mirror. Take a concave mirror. Find out its approximate focal length in the way described above.
Note down the value of focal length. (You can also find it out by obtaining image of a distant object on a sheet of paper.) Mark a line on a Table with a chalk. Place the concave mirror on a stand. Place the stand over the line such that its pole lies over the line.
Draw with a chalk two more lines parallel to the previous line such that the distance between any two successive lines is equal to the focal length of the mirror. These lines will now correspond to the positions of the points P, F and C, respectively. Remember – For a spherical mirror of small aperture, the principal focus F lies mid-way between the pole P and the centre of curvature C . Keep a bright object, say a burning candle, at a position far beyond C.
Place a paper screen and move it in front of the mirror till you obtain a sharp bright image of the candle flame on it. Observe the image carefully. Note down its nature, position and relative size with respect to the object size. Repeat the activity by placing the candle – (a) just beyond C, (b) at C, (c) between F and C, (d) at F, and (e) between P and F.
In one of the cases, you may not get the image on the screen. Identify the position of the object in such a case. Then, look for its virtual image in the mirror itself. n Note down and tabulate your observations.
You will see in the above Activity that the nature, position and size of the image formed by a concave mirror depends on the position of the object in relation to points P, F and C. The image formed is real for some positions of the object. It is found to be a virtual image for a certain other position. The image is either magnified, reduced or has the same size, depending on the position of the object.
A summary of these observations is given for your reference in Table . . Table . Image formation by a concave mirror for different positions of the object Position of the Position of the Size of the Nature of the object image image image At infinity At the focus F Highly diminished, point-sized Beyond C Between F and C Diminished At C At C Same size Between C and F Beyond C Enlarged At F At infinity Highly enlarged Between P and F Behind the mirror Enlarged Virtual and erect