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Verification of the Law of Reflexion of Light

In order to prove the law, that the angle which a reflected ray makes with the normal to a plane surface is equal to the angle made by the incident ray with the normal, and that the two rays are in the same plane with the normal, two methods may be adopted:

(1) The direct method, in which the angles of incidence and reflexion are measured and compared, and the positions of the rays determined.

(2) An indirect method, in which some result is verified which may be theoretically deduced on the assumption that the law holds.

The following experiment is an example of the second method.

It may be proved, by assuming the law of reflexion, that an image of a luminous point is formed by a plane mirror at a point on the normal to the plane surface drawn through the luminous point, and at a distance behind the mirror equal to the distance of the luminous point from the front of the mirror. This we can verify experimentally.

Take as the luminous point the intersection of cross-wires mounted on a ring, which can be placed in any position in a clip.

We can place another similar cross in the exact position occupied by the image in the mirror of the first, in the following manner.

Scrape a horizontal strip of the silvering off the back of the mirror and place the one cross in front, so that on setting the eye on a level with the cross, half of the image is seen coming just to the edge of the silvering.

Then place the other cross behind, so that it can be seen through that part of the glass from which the silvering has been scraped. Place this second cross so that the upper half of it can be seen through the gap, and so that the intersection of the second appears to coincide with the image of the intersection of the first. In order to determine whether or not this is really the case, move your eye from side to side across the first cross-wire, then if the second cross and the image are coincident, the two will appear to move together as the eye moves, and will remain coincident wherever the eye is placed. If, however, the actual cross is nearer to the mirror than the image, then on moving the eye to the right the two will appear to separate, the further, viz. the image, going to the right hand, the real cross to the left

Place, then, the second cross so that on moving the eye from side to side no separation between the cross and the image occurs. It is then in exactly the same position as that occupied by the image of the first cross in the mirror.

Let the first cross be placed at a distance of 1 foot (about) from the reflecting surface of the mirror. Measure the distance by means of a pair of compasses and a scale, and measure, also, the distance between the same surface of the mirror and the second cross, which has been accurately placed to coincide with the image of the first in the mirror. Then displace the second cross from coincidence with the image and replace it and read the distance again in order to ascertain the limit of accuracy to which your observation can be carried. Repeat three times.

The experiment may be very conveniently made with a piece of unsilvered plate glass instead of the mirror. The image of the first cross formed by reflexion at the surface of the glass is generally sufficiently bright to permit of the second cross being accurately placed to coincide with it. If the glass is very thick, allowance must be made for the displacement of the image of the second cross as seen through the glass. A corresponding allowance may, of course, also be necessary in the case of the mirror whose thickness will alter the apparent position of the reflected image of the first cross.

Two vertical pins in stands may be used instead of cross-wires, and the upper part of the second one may be viewed directly over the top of the mirror, while the lower part of the image of the first is seen in the mirror.

In order to verify that the image and object are on the same normal to the mirror, place the eye so that the image and object are in the same straight line with it, and notice that the image of the eye is in the same line too, no matter how far from or how near to the mirror the eye be placed; this can only be the case if the line is a normal.

In case the result obtained does not apparently confirm the law of reflexion, the discrepancy may be due to the fact that the mirror is, cylindrical or spherical and not truly plane. To distinguish between the cases, repeat the experiment, moving the eye vertically up and down instead of horizontally.

Experiment. - Verify the truth of the law of reflexion of light.

Enter results thus:

Distance of objects     Distance of image
      75 cm                  75 cm
      65                     63
      80.5                   78
      71.5                   71.5
      61                     59

Last Update: 2011-03-27