CBSE Class 8 Science Chapter 16: Light NCERT Solutions

In a completely dark room, where there is no light source, you cannot see any objects. This is because for us to see an object, light must reflect off that object and then reach our eyes. Without any light, there is nothing to reflect. However, if there is light outside the room, and the room is not completely sealed, you might be able to see objects outside the room if light from outside can enter the room and reflect off those objects towards your eyes.

Here's a comparison between regular and diffused reflection:

• Regular Reflection: Occurs when light rays strike a smooth, polished surface (like a mirror). The reflected rays travel in a specific, predictable direction, remaining parallel to each other.
• Diffused Reflection: Occurs when light rays strike a rough or uneven surface (like a wall or paper). The reflected rays scatter in many random directions, even if the incident rays are parallel.

Does diffused reflection mean the failure of the laws of reflection?

No, diffused reflection does not mean the failure of the laws of reflection. The laws of reflection still hold true for every individual ray of light. The reason for the scattering is that the surface is microscopically uneven. At each point on the rough surface, the normal (a line perpendicular to the surface) is oriented differently. Therefore, while the angle of incidence equals the angle of reflection at each point, the scattered directions of the reflected rays lead to the overall appearance of diffused reflection.

1. Polished wooden table: Regular reflection will take place. A polished surface is smooth, causing incident parallel rays of light to reflect as parallel rays.
2. Cardboard surface: Diffused reflection will take place. A cardboard surface, though it may appear smooth to the eye, has microscopic irregularities that scatter the light rays in different directions.
3. Chalk powder: Diffused reflection will take place. Chalk powder consists of many small, irregular particles, creating a very rough surface that scatters light.
4. Mirror: Regular reflection will take place. Mirrors have a very smooth and polished surface, ensuring that light reflects predictably and parallel rays remain parallel.
5. Marble floor with water spread over it: Regular reflection will take place. The layer of water spread over the marble floor creates a smooth, flat surface, similar to a mirror, leading to regular reflection.
6. Piece of paper: Diffused reflection will take place. Even a clean sheet of paper has tiny bumps and ridges on its surface, causing light to reflect in various directions.

The two laws of reflection are:

1. The angle of incidence is always equal to the angle of reflection.
2. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

Here is an activity to demonstrate that the incident ray, reflected ray, and normal lie in the same plane:

1. Take a sheet of white paper and fix it on a drawing board or a flat table.
2. Use a comb with one narrow slit in the middle. Close all other slits with black paper.
3. Hold the comb vertically on the paper and direct a beam of light from a torch through the slit so that it falls on the paper, forming a narrow ray of light.
4. Place a strip of a plane mirror vertically on the paper, in the path of the light ray. Adjust the mirror so that the light ray strikes it. You will observe a reflected ray on the paper.
5. Mark points along the path of the incident ray, the reflected ray, and the point where the mirror is placed. Draw the incident ray and the reflected ray.
6. Draw a line perpendicular to the mirror surface at the point where the light ray strikes. This line represents the normal.
7. Now, carefully lift the paper off the board, keeping the comb and torch steady. You will see that the incident ray, the reflected ray, and the normal are all visible on the paper, indicating they lie in the same plane. If you were to fold the paper along the line of the incident ray, the reflected ray would not be visible on the folded part, further confirming they are coplanar.