NCERT Solutions for Class 8 Science Chapter 16 Crop Production and Management

Ans : If we are in a dark room, then it is not possible for us to see objects in the room. However, objects outside the room are visible to us. An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the objects inside the room cannot reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then we can see the objects outside the room. 

Ans : Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays.
Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays.
This happens because of the presence of irregular microscopic surfaces. Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated in diffused or irregular reflections. 

Ans : (a) Polished wooden table → Regular reflection
A polished surface is an example of a smooth surface. A polished wooden table has a smooth surface. Hence, reflections from the polished table will be regular.
(b) Chalk powder → Diffused reflection
Chalk power spread on a surface is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place from chalk powder.
(c) Cardboard surface → Diffused reflection
Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.
(d) Marble floor with water spread over it → Regular reflection
Marble floor with water spread over it is an example of a regular surface. This is because water makes the marble surface smooth. Hence, regular reflection will take place from this surface.
(e) Mirror → Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular reflection.
(f) Piece of paper → Diffused reflection
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused reflection. 

Ans : Laws of reflection:
(i) The angle of reflection is always equal to the angle of incidence.
(ii) The incident ray, the reflected ray and the normal to the reflective surface
at the point of incidence all lie in the same plane. 

Ans : Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Make sure that the light in the room is not bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a normal line on the second sheet from the mirror. Now, light a torch on the mirror through the small hole such that the ray of light falls on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction. You can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the sheet placed on the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.