Light-Reflection and Refraction | Activities in Text Book with Solution
Activity 10.3
You have already learnt a way of determining the focal length of a concave mirror. In Activity 10.2, 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.
Note down and tabulate your observations.
✅ Answer:
|
Position of the object |
Position of the image |
Size of the image |
Nature of the image |
|
At
infinity |
At
the focus F |
Highly diminished, |
Real & inverted |
|
Beyond
C |
b/w
F & C |
Diminished |
Real & inverted |
|
At
C |
At
C |
Same
size |
Real & inverted |
|
b/w
C & F |
Beyond
C |
Enlarged |
Real & inverted |
|
At
F |
At
infinity |
Highly
enlarged |
Real & inverted |
|
b/w
P & F |
Behind
the mirror |
Enlarged |
Virtual & erect |