Viva Voce: Focal Length of Concave Mirror & Convex Lens | Lab Manuals for Class 10 PDF Download

Q.1. State the importance of pole of a mirror.

Pole is the mid-point of the mirror which makes the incident light to go at some angle on the other side of principal axis.

Q.2. All the rays of light, after reflection from a concave mirror meet at a point infront of the mirror. Name this point.

When parallel rays of light fall on a concave mirror along its axis, then the rays meet at a point infront of the mirror after reflection from it. This point is called the focal point.

Q.3. Which surface of a shining spoon should be polished, to use the spoon as a concave mirror?

The surface of the spoon which is bulged outward should be polished to use the spoon as a concave mirror.

Q.4. Why do we use a screen for obtaining an image from a concave mirror?

Since, the image formed by the mirror is real, it can be obtained on a screen.

Q.5. Why do we obtain blurred image from a concave mirror sometimes?

The reason behind the blurred image is that the mirror is away from the object.

Q.6. Give the condition to hold a mirror for finding the focal length of the concave mirror.

While holding the mirror for finding the focal length of the concave mirror, the aperture of the mirror must not be obstructed.

Q.7. What type of mirror is used in torch? Give reasons. 

In torch, concave spherical or parabolic mirror is used because when the bulb (source) is kept at the focus of a concave mirror, parallel beam of light is obtained which travels a large distance.

Q.8. What will be the nature of the image formed by a concave mirror for a distant object? 

The nature of the image formed by a concave mirror for a distant object is real and inverted.

Q.9. How will you distinguish between a concave and a convex mirror? 

A concave mirror is the spherical mirror with inward curved reflecting surface, whereas a convex mirror is the spherical mirror with outward curved reflecting surface. Concave mirror forms a sharp image, whereas a convex mirror cannot form a sharp image of the distant object.

Q.10. How does the size of the image vary as an object is moved from close to the pole to a large distance?

Size of the image decreases as the object is moved away from the pole to a large distance. The image will be virtual for the object between the pole and the focus.

Q.11. When the light of the Sun is directed on a sheet of paper with the help of a mirror, then the paper starts burning. Why?

The light from the Sun is converged at a point, as the sharp, bright spot by the mirror. Actually, this spot of light is the image of the Sun on the sheet of paper. The heat produced due to concentration of sunlight ignites the paper.

Q.12. Is the centre of curvature a part of a spherical mirror? Comment in support of your answer.

The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. So, it is not a part of the mirror as it lies outside its reflecting surface.

Q.13. How can we find the focal length of a concave mirror, when the image is obtained by using a concave mirror?

Focal length can be found out by measuring the distance between the mirror and the screen.

Q.14. Give the position of an object to obtain a virtual, erect and an image larger than the object using a concave mirror.

The object should be placed between the focus and pole of the mirror.

Q.15. What type of mirror is used as a shaving mirror or in vanity boxes? 

Concave mirror is used as a shaving mirror or in vanity boxes, because when the object is placed between its focus and pole, the magnified, erect and virtual images of the object will be formed.

Q.16. In reflector type solar cookers, special concave (parabolic) mirrors are used. In such cookers, what should be the preferable position of food vessel for cooking? 

In reflector type solar cookers, the preferable position of food vessel should be at focus of the concave mirror.

Q.17. To determine the focal length of a concave mirror, a student focuses a classroom window, a distant tree and the Sun on the screen with the help of a concave mirror. In which case will the student get more accurate value of focal length?

Student will get more accurate value of focal length in the case of Sun.