Solutions of Reflection of Light (Page No- 190) - Physics By Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Lakhmir Singh Physics Class 10 Solutions Page No:190

Question 18: Which mirror is used as a torch reflector ? Draw a labelled diagram to show how a torch reflector can be used to produce a parallel beam of light. Where is the bulb placed in relation to the torch reflector ?

Solution : Concave mirror is used for a torch reflector.
Diagram:
Bulb is placed at the focus of the torch reflector.

Question 19: State where an object must be placed so that the image formed by a concave mirror is :
 (a) erect and virtual.
 (b) at infinity.
 (c) the same size as the object.

Solution : (a) Between pole and focus of the mirror.
(b) At the focus of the mirror.
(c) At the centre of curvature of the mirror.

Question 20: With the help of a labelled ray diagram, describe how a converging mirror can be used to give an enlarged upright image of an object.

Solution :

For obtaining an enlarged upright image of an object, the object is placed between focus (F) and pole (P) of the concave mirror.
A ray AD parallel to the principal axis, gets reflected at D and then passes through the focus F. A second ray AE passing through the centre of curvatue C strikes the mirror normally at point E and gets reflected back along the same path.

Now, two reflected rays DF and EC are diverging rays and, therefore, do not intersect each other on the left side. The reflected rays DF and EC are produced backwards (as shown by dotted lines). On producing backwards, they appear to intersect at point A’ behind the concave mirror. Thus, A’ is the virtual image of point A of the object. To get the complete image of the object we draw A’B’ perpendicular to the axis. Thus, A’B’ is the image of the object AB formed by the concave mirror. The image is bigger in size than the object, hence it is an enlarged image.

Question 21: Make labelled ray diagrams to illustrate the formation of :
 (a) a real image by a converging mirror.
 (b) a virtual image by a converging mirror.
 Mark clearly the pole, focus, centre of curvature and position of object in each case. 

Solution : (a) A real image by a converging mirror 

(b) A virtual image by a converging mirror

Question 22: Briefly describe how you would find the focal length of a concave mirror quickly but approximately.

Solution : When the object is at a considerable distance (or at infinity) from a concave mirror, then its image is formed at the focus. This fact can be used to find out the focal length of a concave mirror quickly but approximately.
We focus a distant object (several metres away) like a window or tree on a screen by using a concave mirror whose focal length is to be determined. The sharp image of window or tree will be formed at the focus of the concave mirror. That is, the distance of image (or screen) from the concave mirror will be equal to the focal length of concave mirror. This distance can be measured with a scale. It will give us the approximate focal length of the concave mirror.

Question 23: Which type of mirror is used in a solar furnace ? Support your answer with reason.

Solution : Concave mirror is used in a solar furnace.
The solar furnace is placed at the focus of a large concave reflector. When parallel rays of light from the sun fall on the surface of the concave mirror, rays gets reflected and meet at the focus of the mirror due to the converging nature of concave mirror. Thus, the furnace kept at the focus becomes very hot. Even steel can be melted in this furnace.

Question 24: Name the type of mirror used by dentists. How does it help ?

Solution : Concave mirror is used by dentists.
The dentist holds a small concave mirror in such a way that the tooth lies within its focus. A magnified image of the tooth is then seen by the dentist in the concave mirror. Since the tooth looks much bigger, it becomes easy to examine the defect in the tooth.

Question 25: Explain why, concave mirrors are used as shaving mirrors.

Solution : Concave mirrors are used as shaving mirrors. This is because when the face is placed close to a concave mirror (so that the face is within its focus) the concave mirror produces a magnified and erect image of the face. Since a large image of the face is seen in the concave mirror, it becomes easier to make a smooth shave.

Question 26: Give two uses of concave mirrors. Explain why you would choose concave mirrors for these uses.

Solution : Uses of concave mirror
(i) Concave mirrors are used as shaving mirrors. This is because when the face is placed close to a concave mirror (so that the face is within its focus) the concave mirror produces a magnified and erect image of the face. Since a large image of the face is seen in the concave mirror, it becomes easier to make a smooth shave.
(ii) Concave mirrors are used by dentists to see the large images of the teeth of patients. This is because when a tooth is within the focus of a concave mirror, then an enlarged image of the tooth is seen in the mirror. Due to this, it becomes easier to locate the defect in the tooth.

Question 27: (a) Draw ray-diagrams to show the formation of images when the object is placed in front of a concave
 mirror (converging mirror) :
 (i) between its pole and focus
 (ii) between its centre of curvature and focus Describe the nature, size and position of the image formed in each case.
 (b) State one use of concave mirror based on the formation of image as in case (z) above.

Solution :

fig. formation of Image by the concave mirror when the object

Is pelted between Its pole and focus.

When object is placed between the pole and focus of a concave mirror, the image is formed behind the mirror, it is virtual, erect and larger than the object. 

When object is placed between the centre of curvature and focus of a concave mirror. the image is formed beyond the centre of curvature. it is real. inverted and larger than the object.

(b) One use of concave mirror based on the formation of image as in case (i) above: As a shaving mirror. 

Question 28: (a) Give two circumstances in which a concave mirror can form a magnified image of an object placed in
 front of it. Illustrate your answer by drawing labelled ray diagrams for both.
 (b) Which one of these circumstances enables a concave mirror to be used as a shaving mirror ?

Solution : (a) (i) When the object is placed between the pole and focus of a concave mirror, a magnified image is formed.

fig. formation of image by the concave mirror when the object is palced between its pole and focus. 

(ii) When the object is placed between the focus and the centre of curvature of a concave mirror, a magnified image is formed.

(b) A concave mirror can be used as a shaving mirror when the object is placed between the pole and focus of a concave mirror.