experimental procedure to prove laws of reflection of light

Describe an experiment to verify the laws of reflection of light.

Laws of reflection. (i) the incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane. (ii) the angle of incidence and angle of reflection are equal i.e. $$\angle i = \angle r $$. verification take a wooden drawing board and fix a white sheet of paper on it. in the middle of paper draw a straight line kk\ mark a point b on it. draw a perpendicular bn. place a mirror xx' on line kk' such that the polished side of the mirror is along the line. hold the mirror in the mirror holder. fix two steel pins p and q on the straight line ab at least 10 cm apart. look for the images of the pins p and q and fix two pins p a such that p', q', and images of p and q are all in the same straight line. remove the pins and draw small circles around the pinpricks. remove the mirror also. join p'q' and produce the straight line to meet at b. measure $$\angle abn$$ = i and $$\angle cbn$$ = r. it is found that $$\angle i = \angle r$$. this proves that the angle of incidence is equal to the angle of reflection. as the incident ray, the reflected ray and the normal lie in the plane of the paper, therefore, they lie in the same plane..

Describe an experiment to verify laws of reflection.

State the laws of reflection and describe an experiment to verify them.

1.2 The Law of Reflection

Learning objectives.

By the end of this section, you will be able to:

• Explain the reflection of light from polished and rough surfaces
• Describe the principle and applications of corner reflectors

Whenever we look into a mirror, or squint at sunlight glinting from a lake, we are seeing a reflection. When you look at a piece of white paper, you are seeing light scattered from it. Large telescopes use reflection to form an image of stars and other astronomical objects.

The law of reflection states that the angle of reflection equals the angle of incidence, or

The law of reflection is illustrated in Figure 1.5 , which also shows how the angle of incidence and angle of reflection are measured relative to the perpendicular to the surface at the point where the light ray strikes.

We expect to see reflections from smooth surfaces, but Figure 1.6 illustrates how a rough surface reflects light. Since the light strikes different parts of the surface at different angles, it is reflected in many different directions, or diffused. Diffused light is what allows us to see a sheet of paper from any angle, as shown in Figure 1.7 (a). People, clothing, leaves, and walls all have rough surfaces and can be seen from all sides. A mirror, on the other hand, has a smooth surface (compared with the wavelength of light) and reflects light at specific angles, as illustrated in Figure 1.7 (b). When the Moon reflects from a lake, as shown in Figure 1.7 (c), a combination of these effects takes place.

When you see yourself in a mirror, it appears that the image is actually behind the mirror ( Figure 1.8 ). We see the light coming from a direction determined by the law of reflection. The angles are such that the image is exactly the same distance behind the mirror as you stand in front of the mirror. If the mirror is on the wall of a room, the images in it are all behind the mirror, which can make the room seem bigger. Although these mirror images make objects appear to be where they cannot be (like behind a solid wall), the images are not figments of your imagination. Mirror images can be photographed and videotaped by instruments and look just as they do with our eyes (which are optical instruments themselves). The precise manner in which images are formed by mirrors and lenses is discussed in an upcoming chapter on Geometric Optics and Image Formation .

Corner Reflectors (Retroreflectors)

A light ray that strikes an object consisting of two mutually perpendicular reflecting surfaces is reflected back exactly parallel to the direction from which it came ( Figure 1.9 ). This is true whenever the reflecting surfaces are perpendicular, and it is independent of the angle of incidence. (For proof, see [link] at the end of this section.) Such an object is called a corner reflector , since the light bounces from its inside corner. Corner reflectors are a subclass of retroreflectors, which all reflect rays back in the directions from which they came. Although the geometry of the proof is much more complex, corner reflectors can also be built with three mutually perpendicular reflecting surfaces and are useful in three-dimensional applications.

Many inexpensive reflector buttons on bicycles, cars, and warning signs have corner reflectors designed to return light in the direction from which it originated. Rather than simply reflecting light over a wide angle, retroreflection ensures high visibility if the observer and the light source are located together, such as a car’s driver and headlights. The Apollo astronauts placed a true corner reflector on the Moon ( Figure 1.10 ). Laser signals from Earth can be bounced from that corner reflector to measure the gradually increasing distance to the Moon of a few centimeters per year.

Working on the same principle as these optical reflectors, corner reflectors are routinely used as radar reflectors ( Figure 1.11 ) for radio-frequency applications. Under most circumstances, small boats made of fiberglass or wood do not strongly reflect radio waves emitted by radar systems. To make these boats visible to radar (to avoid collisions, for example), radar reflectors are attached to boats, usually in high places.

As a counterexample, if you are interested in building a stealth airplane, radar reflections should be minimized to evade detection. One of the design considerations would then be to avoid building 90 ° 90 ° corners into the airframe.

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• Authors: Samuel J. Ling, Jeff Sanny, William Moebs
• Publisher/website: OpenStax
• Book title: University Physics Volume 3
• Publication date: Sep 29, 2016
• Location: Houston, Texas
• Book URL: https://openstax.org/books/university-physics-volume-3/pages/1-introduction
• Section URL: https://openstax.org/books/university-physics-volume-3/pages/1-2-the-law-of-reflection

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Verification of the Laws of Reflection

The laws of reflection are,

• The incident ray, reflected ray, and normal to the surface lies in the same plane.
• The angle of incidence \(i\) is equal to the angle of reflection \(r\).

Students find it difficult to comprehend these laws because they involve three dimensional geometry. The experiments given here will help in complete understanding of these laws.

You need a plane mirror, scale, protractor, thermocol sheet, white paper, and pins for this experiment.

• Fix the white paper on the thermocol sheet with the help of pins.
• Place the mirror near to one edge of the paper and mark its position.
• Place two pins vertically on one side of the normal so that line joining two pins make an angle of approximately 30 degree with the normal.
• See image of two pins from other side of the normal and place a pin vertically so that it is in line with images of other two pins. Similarly, place one more pin at some distance from this pin.
• Extend the lines. Measure angle of incidence and angle of reflection.
• Repeat for two more incident angles (say 45 degree and 60 degree).
• Now, see the head of the pins in step (3). Raise one of the pin and press another pin. Now repeat step (4) such that images of the pin-head are aligned. Visualise the plane containing incident ray, reflected ray and normal. You may place a paper on this plane. Raise another pin and visualize again.

Place the plane mirror vertically on the table. Place a laser torch at some height \(h_1\) in front of the mirror so that light falls on the mirror traversing a path in horizontal plane. Locate the spot made by the reflected beam and measure its height \(h_2\).

Check that \(h_1=h_2\). Since the source is at height \(h_1\) and incident ray is in horizontal plane, the point of reflection should be at that same height. As the mirror is vertical, the normal lies in the horizontal plane at height \(h_1\). This shows that incident ray, reflected ray and normal lies in same plane (horizontal plane in this case).

One more variant: Take a transparent plastic box of approximate size approx 12 in by 18 in by 6 in. Paste a plane mirror on one of the vertical face (from inside the box) with the help of adhesive (quickfix etc). Make a small hole to push a incense stick (dhoop, agarbatti). A big protector placed on the top can be used to measure angle. You can mark horizontal lines on the mirror so that height of the spot where incident ray strikes is known. This setup can be used to show both laws of reflection.

• Multiple images with plane mirror
• Reflection from a Plane Surface
• Reflection from Spherical Mirrors