NCERT Book Class 10 Science Lab Manual The Natural Phenomenon

The Natural Phenomenon

AIM

To verify the laws of reflection of light using a plane mirror.

THEORY

When light falls on a smooth polished surface, it gets reflected in a definite direction. Fig. 34.1 shows a ray of light PO, incident on a plane polished surface (plane mirror) SS’. Line OQ shows the changed path of the incident ray after reflection at the point O. The ray PO is called incident ray and ray OQ is called reflected ray. The point O where the incident ray strikes the polished surface is called point of incidence. If ON be the normal to the polished surface SS′ at point O, then the angle PON and the angle NOQ are called the angle of incidence (i) and the angle of reflection (r) respectively. The plane containing the incident ray and normal is called plane of incidence.

The laws of reflection as deduced from the experiments states that the reflected ray lies in the plane of incidence along with the normal at the point of incidence, and ∠i = ∠r.

MATERIALS REQUIRED

A plane mirror with a support to hold it vertical, a drawing board, sheet of white paper, protractor, measuring scale, pins, drawing pins or adhesive tape.

PROCEDURE

1. Fix a white sheet of paper on the drawing board using either adhesive tape or drawing pins.

2. Draw a thin line SS′ in the middle of the paper. Also draw a normal ON to the line SS′ at point O as shown in Fig. 34.2.

3. Draw a thin line PO at any angle to the line SS′. Place the mirror vertically on line SS′ with the help of a support so that its polished surface faces line PO.

4. Vertically fix two pins P1 and P2 with their tips, separated by a suitable distance of about 5 to 6 cm at two points on line PO. Look at theimages P’1 and P’2 of pins P1 and P2 respectively from the same side of the plane mirror.

5. Fix two pins P3 and P4, vertically such that their feet appear to be in the same straight line as that of images P’1 and P’2. Look through the feet of pins P1 and P2, whether the feet of images (not shown in the Fig. 34.2 of pins P3 and P4, as seen in the mirror appear to be on the same straight line. If it is so, you have correctly fixed the pins P3 and P4.

6. Remove all the pins and the mirror. Mark the positions of feet of pins P3 and P4. Draw a thin line OQ joining the points that mark the position of feet of pins P3 and P4. Also extend this line till it meets the line SS¢. This extended line should meet the surface SS¢ at the point O. The line OQ shows the path of the reflected ray corresponding to the incident ray along the line PO, at the point of incidence.

7. Measure angles PON (∠i) and NOQ (∠r) and record the values in observation table.

8. Repeat the experiment for two more angles of incidence.

QUESTIONS

• Why do we prefer a thin mirror strip to verify the laws of reflection?

• Can you obtain the image of a lighted candle placed in front of a plane mirror on a screen? Justify your answer.

• If the incident ray is perpendicular to the plane mirror, what will be the angle of reflection?

• An incident ray is reflected backwards along the same path, from a plane mirror. What is the angle of incidence?

• A pin is fixed at a distance of 5 cm in front of a plane mirror. Where and at what distance will the image be formed?

• Sometimes the image formed by a concave mirror of an object placed at C is not of the same size and at location C. What could be the possible reason(s) for such a situation?

• In what way will the position and size of the image affected if the pencil used for drawing ray diagrams is not sharp and thin?

• What is the advantage of joining the point C with the point of incidence D, while drawing ray diagrams for the image formation by a concave mirror?

• Why does a ray of light bend towards the normal when it enters from air in a glass slab and bends away from the normal when it emerges out into air?

• Draw the path of a ray of light when it enters perpendicular to the surface of a glass slab.

• If the incident and emergent rays are not parallel to each other in case of a recatangular glass slab, what could be the reason?

• While tracing the path of ray of light through a glass slab, the angle of incidence is generally taken between 30º and 60º. Explain the reason on the basis of your performing this experiment for different angles of incidence.

• How does the lateral displacement of emergent ray depend on the width of the glass slab and angle of incidence?

• On what factors does the refractive index of a medium depend?

• Is Snell’s law obeyed in case of normal incidence of light on a rectangular glass slab?

• Define angle of deviation.

• List the factors on which the angle of deviation through a prism depend?

• Why does a ray of light bend towards the base when it passes through a glass prism?

• Why does white light split into different colours when passes through a glass prism?

• Why does the white light not split into different colours when it passes through a glass slab?

Please refer to attached file for NCERT Class 10 Science Lab Manual The Natural Phenomenon