Question

A student has two concave mirrors A and B of focal lengths 20 cm and 15 cm respectively. For each concave mirror he performs the experiment of image formation for three values of object distance of 30 cm, 10 cm and 20 cm. (a) For the three object distances, identify the mirror which will form an image equal in size to that of object. Give reason for your answer. (b) Draw a ray diagram to show the image formation in the above case. (c) Why is the focal length of a concave mirror taken negative?​

Answer 1

Answer:

Given fA=20cm fA=20cm, fB=15cm fB=15cm, fC=10cm fC=10cm, u1=30cm u1=30cm, u2=10cm u2=10cm, u3=20cm u3=20cm

a.) For same size image, object should be placed at Center of Curvature

So for Mirror A, Image will not of same size for these positions

For Mirror B,Image will be of same size for position u1=30cmu1=30cm

For mirror C,Image will be of same size for position u3=20cmu3=20cm

b.)We need enlarged and erect image for shaving. For erect and enlarged image, Object should be placed between Pole and Focus. Face would be generally kept at more than 10 cm distance, so mirrors A and B are suitable for shaving

Answer 2

Answer:

Given:

Three concave mirrors A, B,C

Focal length of mirror

A= 20cm;

B= 15cm

C= 10cm

Object distances for these mirrors are 30cm, 10cm and 20cms respectively.

1) Mirror c will produce image of same size as object because object is placed on c.

2) Mirror B can used as shaving mirror because when object is kept inbetween P and F we get virtual , erect and magnified image.

3) please refer attachment for Ray diagrams.