**Concave Mirror Experiment Viva Questions**

Ans: Rays proceeding from an object after
reflection or refraction converge to a point. The image formed is real. If the
rays appear to diverge from a point, the image is virtual. Real images can he
caught on a screen. Virtual images cannot be obtained on a screen

*(ii) Define principal focus of a concave mirror*

Ans: Paraxial rays parallel to the principal axis after
refraction converge to a point. That point is the principal focus.

*(iii) What is the position of the object for which we get enlarged virtual image?*

Ans: Object must be between focus and pole of the mirror

*(iv) At what distance from a concave mirror should an object be placed so that its image is formed at the same place?*

Ans: At the centre of curvature of the concave mirror

*(v) What is the shape of u.v. graph?*

Ans: It is a part of a hyperbola

*(vi) Define centre of curvature of a concave mirror*

Ans: It is the centre of the sphere of which the mirror
forms a part.

*(vii) The aperture of the mirror is small. Why?*

Ans: To reduce spherical aberration

*(viii) If the radius of curvature of a concave mirror is 20 cm. What is its focal length?*

Ans: Focal length = 10 cm

*(ix) What type of mirror do we use to avoid spherical aberration?*

Ans: Paraboloidal mirror

*(x) For real images formed by a concave mirror, are the positions of the object and image interchangeable*

Ans: Yes

*(xi) Aim of Concave mirror Experiment*

Ans: To find the focal length of the given concave mirror.

*(xii) Apparatus of Concave mirror Experiment*

Ans: Concave mirror, screen, illuminated wire gauze, etc.

*(xiii) Theory of Concave mirror Experiment***The u and v are the object distance and the image distance from the mirror,**

f = uv/(u + v)

*(xiv) Procedure of Concave mirror Experiment*

The focal length of the concave mirror can be found out by
different methods.

**1. Distant object method**

The concave mirror is faced to a distant scenery. The
position of the screen is adjusted so that a clear image of the scenery is
obtained on it. The distance between the mirror and the screen is the focal
length. The experiment is repeated three times and the mean value is found out.

**2. Normal incidence method**

A piece of wire gauze fitted in a hole on the side of a
wooden box and illuminated by an electric lamp serves as the object. The
concave mirror is placed in front of the object and its position is adjusted so
that a clear image of the wire gauze is formed by the side of the object. The
distance between the object and the mirror is the radius of curvature r of the
mirror. Half the radius of curvature is calculated as focal length. The
experiment is repeated and the mean value of the focal length f of the mirror
is found out.

**3. u-v method**

The illuminated wire gauze, the object, is placed in front
of the concave mirror at a distance more than the focal length (f) of the
mirror. A screen is placed in front of the mirror and its position is so adjusted
that a clear image of the wire gauze is formed on it. The distances of the
object and image to the mirror are measured as u and v. Then the focal length
of the mirror can be found out using the formula, f = uv/(u + v). The
experiment is repeated for different distances. Readings are taken for both
magnified and diminished images.

**4. u-v graph**

A graph is drawn with u along the X axis and v along the Y
axis taking a common origin and same scale for both axes. A bisector to the
angle XOY is drawn which meets the graph at P. The coordinates of P is the
radius of curvature. Thus OA = OB = 2f . From this the focal length f is found
out.

**5. 1/u - 1/v graph**

A graph is drawn with 1/u along the X-axis and 1/v along the
Y-axis taking 0-0 as origin and same scale for both axes. The graph is a
straight line intercepting the axes at A and B. Then OA = OB = 1/f. Hence f
can be calculated.

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