NCERT Solutions for Class 8 Science Chapter 16
Important NCERT Questions
8th Chapter 16 Light
NCERT Books for Session 2022-2023
CBSE Board and State Board
Questions No: 5
Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
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Fix a white sheet of stiff paper on a drawing board or a table. Take a comb and close all its openings except one in the middle. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side. With slight adjustment of the torch and the comb we will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of the light ray. Let the sheet project a little beyond the edge of the Table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflectedray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls. Bring the paper back to the original position.
When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When we bend the paper you create a plane different from the plane in which the incident ray and the normal lie. Then we do not see the reflected ray. It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
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To demonstrate the alignment of the incident ray, reflected ray, and the normal in the same plane as per the laws of reflection, follow these steps using a plane mirror, a protractor, and a light source:
1. Setup: Position the plane mirror vertically on a flat surface.
2. Mark the Normal: Use a protractor to draw a line perpendicular to the mirror’s surface, indicating the normal. This line represents the direction perpendicular to the mirror.
3. Incident Ray: Shine a light source (like a flashlight or a laser pointer) onto the mirror surface, directing it at an angle relative to the normal line. This ray of light signifies the incident ray.
4. Observation of Reflection: Observe the reflected ray. Adjust the angle of incidence to ensure the reflected ray is visible after bouncing off the mirror’s surface.
5. Alignment Confirmation: Verify that the incident ray, reflected ray, and the line representing the normal all maintain alignment. Rotate the incident ray while keeping the angle relative to the normal constant. Notice that throughout the rotation, these elements remain within the same flat plane.
This simple experiment showcases that the incident ray, reflected ray, and the normal line indeed exist within a single plane, precisely as described by the laws of reflection. This alignment persists regardless of variations in the angle of incidence, emphasizing the fundamental principle that these elements lie in a common two-dimensional plane.