10th Class Science Light - Reflection and Refraction Image formation by a convex mirror

          Image formation by a convex mirror

When the object is placed between P and F

When the object is placed in front of convex mirror, the image formed has following properties:

• The image is virtual.
• The image is erect.
• The image is enlarged in size to that of the object.
• The image is formed behind the mirror.

The Summary of the above Observation is given in the Table below:

           Uses of Concave Mirror

• It is used as a reflector in torches, vehicle head light and search lights. It is used in search light because it produces powerful beam of parallel light, which helps us to see the objects at a considerable distance in the darkness of night.
• It is used as a shaving mirror to see the large image of the face.
• It is used by the dentist to see the large images of the teeth and to find out the problem, the person is suffering from.
• It is also used in solar furnace to focus the solar radiation for the heating purpose.

            Uses of Convex Mirror

• It is used as a rear view mirror in vehicles to see other vehicles coming from behind, as it produces an erect image of the object, which is easy to identify. It also gives the driver the wider view of the traffic behind him.

             Sign Convention for Spherical Mirrors

Concave Mirror

• For Object in front of the mirror U will be negative.
• For real image formed in front of mirror V will be negative.
• Focus (F) is negative.
• For virtual image formed behind the mirror V is positive.

Convex Mirror

• For object in front of mirror U is negative.
• Image is formed behind the mirror V is positive.
• Focus is behind the mirror F is positive.

              Mirror Formula

The formula which gives the relation between the object distance (U), image distance (V) and focal length (F) is called the mirror formula. The distance between the pole and the object is called the object distance and is denoted by U. The distance between the pole and image is called the image distance and is denoted by V and the distance between the pole and focus is called focal length and is denoted by F. Hence the mirror formula is given by

\[\frac{\text{1}}{\text{Focal}\,\text{Length}}\text{=}\frac{\text{1}}{\text{Object}\,\text{Distance}}\text{+}\frac{\text{1}}{\text{Image}\,\text{Distance}}\,\,\frac{\text{1}}{\text{F}}\text{=}\frac{\text{1}}{\text{U}}\text{+}\frac{\text{1}}{\text{V}}\]

While finding one of the three value in the relation we must take care of sign convention of the mirror. Out of three parameter in the relation we can find any one of them provided two of them is given.

             Magnification

It is defined as the extent to which an image can be enlarged or diminished by a mirror. For the spherical mirror the linear magnification is defined as the ratio of height of image to the height of object. If the magnification has positive sign then the image is virtual and erect. On the other hand, if the sign is negative

the image is real and inverted. The linear magnification is also defined as the ratio of image distance to the object distance. It is expressed as:

\[M=\frac{{{h}_{2}}}{{{h}_{1}}}=-\frac{V}{U}\]

Where,

\[{{h}_{1}}\]is the height of object,

\[{{h}_{2}}\]is the height of image,

U is the object distance

V is the image distance

         What is the position of image when the object is placed at the infinity?

(a) Focus

(b) Between F and C

(c) At C

(d) Beyond C

(e) None of these

Answer: (a)

           When an object is placed in front of the plane mirror the left appears right and the right appears left. This phenomenon is called

(a) Reflection

(b) Mirage

(c) Lateral Inversion

(d) Looming

(e) None of these

Answer: (C)

          The point on the principal axis at which the ray of light converges after reflection is called

(a) Centre of Curvature

(b) Focus

(c) Pole

(d) Radii

(e) None of these

Answer: (b)

         What is the nature of image when it is reflected from convex mirror?

(a) Real

(b) Inverted

(c) Highly Enlarged

(d) Virtual

(e) None of these

Answer: (d)

         An object at 5 cm high forms an image at a distance of 6 cm and focal length of 15 cm. Find the position of image.

(a) 5cm

(b) 10cm

(c) 15cm

(d) 20cm

(e) None of these

Answer: (B)