Question:
For an object placed at a distance of 20 cm from the pole of the mirror, and image formed, 40 cm farther from the object on the same side
a) what is the nature of the mirror? give reasons
b) Is the image formed real or virtual. give reasons
c) Draw a ray diagram to show the image formed
d) Calculate the focal length of the mirror used
Answer:
Distance of the object from the pole of the mirror = u = -20 cm
Distance of the image from the pole of the mirror = v = -20-40 = -60 cm
The negative sign indicates the object/image is on the left side of the mirror.
- Nature of the mirror and image formed:
Consider the nature of the image formed. It is on the same side of the object.
As the image is formed on the same side, it is a real image. We get only virtual image, which is image formed on the other side of the mirror and it cannot be captured on a screen, in case off convex mirror. Hence, since image is formed on the same side, which is real image, it is a concave mirror.
- Focal length is given by the formula is 1/v + 1/u = 1/f
1/f =- 1/20 - 1/60 = -4/60 = -1/15
f = -15 cm
As R = 2f, R = -30 cms
The value of -20 is between -30 and -15. Hence, the object is between Centre of curvature and Focal length (Between C and F). The image is formed beyond C which is beyond -30 cms. It will be a enlarged, real and inverted image
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