# Concave Mirror Viva Questions

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.