Tracing Path of a Ray of Light Passing Through a Glass Slab

Tracing the path of a ray of light passing through a glass slab involves the study of the phenomenon of refraction. In general terms, the bending of light while passing from one medium to another is called refraction. The refraction and the angle of refraction are measured along with the angle of incidence and sometimes the angle of emergence is also considered.

According to the law of refraction, the “Ratio of the sine of the angle of incidence to the angle of refraction is equal to constant”. Tracing the path of a ray of light lies in relation to the study of the speed, wavelength and frequency of the light. 

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Aim of Experiment

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To trace the path of a ray of light that is passing through a glass slab. To measure the angle of refraction, angle of incidence and the angle of emergence.

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Theories in Experiment

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The concepts covered in the experiment include:

Refraction of Light

After a ray of light falls obliquely on the surface of the two optical media, the direction of its path suddenly changes while entering into the other medium. This phenomenon of change in the nature of rays of light is termed as refraction of light. Mainly, refraction takes place due to the modifications in the speed of light as a result of the nature of the mediums it passes through.

• Moreover, during refraction, the wavelength and the speed of the beam of light keep on changing while transmitting from one medium to another. However, the frequency remains unchanged.
• The ray of light bends towards the normal when the light travels from rarer to the denser medium as its speed also gets slowed down (∠i > ∠r).
• The ray of light bends away from the normal, when the light travels from denser to the rarer medium as its speed increases (∠i < ∠r).

Refraction of Light

Laws of Refraction

According to the law of refraction or Snell’s law of refraction, at the point of incidence, the incident ray, the refracted ray and the normal to the interface lie in the same plane.

• Another rule states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a beam of light and for the same pair of transparent media. Therefore,

sin i / sin r = constant = n₂₁

Here,

n₂₁ represents the refractive index of the second medium in relation with the first medium.

Refraction Through Glass Slab with Parallel Faces

The refraction is known to occur at both air-glass interface as well as glass-air interface. Some of the important characteristics of the following are as follows:

• The angle of incidence becomes greater than the angle of refraction, when a ray of light travels from air to glass, that is, from the rarer medium to the denser one, as the ray bends towards normal.
• The angle of refraction becomes greater than the angle of incidence, when a ray of light travels from glass to air, that is, from the denser medium to the rarer one, as the ray bends away from the normal.
• When the incident ray is normal to the interface, that is, when the angle of incidence is zero, the ray of light follows the same path even after undergoing refraction.
• The angle of emergence is always known to lie equal to the angle of incidence.
• Lateral displacement lies in proportionality with the thickness of the glass slab when the angle of incidence is the same.
• For the same thickness of the glass slab, the lateral displacement is proportional to the angle of incidence.

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Materials Required

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Here are some important materials that are required for performing the given experiment: 

1. A drawing - board
2. 4 - 6 all pins
3. A plain sheet of paper
4. A glass - slab
5. A protractor
6. A scale
7. A pencil
8. Thumb pins

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Procedure

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The entire process of creating a ray diagram involves several steps:

• Place a plain sheet of paper on the soft board using thumb pins.
• Draw an outline of the rectangle, using a sharp pencil and place it in the middle of the white paper.
• Now, let PQRS be the obtained figure of the glass slab.
• Further, construct a perpendicular EN on some point E on PQ and mark it as a normal ray.
• Along EN, draw an angle of 30° with the help of the protractor.
• Take two points at 4 - 5 cm on the ray that is obtained by the angle and mark them as A and B and fix pins at each of them.
• Now, place the glass slab on the rectangular figure PQRS.
• For fixing C and D, see through the side RS of the glass slab, such that when seen through the glass slab, all the pins A, B, C, and D lie in a straight line.
• Now, make a small circle using a pencil around the pins A, B, C, and D and remove the pins. Simultaneously, remove the glass slab.
• The points C and D shall be joined in such a way that they meet RS at point F.
• At RS, draw a perpendicular N1’N2’ at point F. Now, join the points E and F.
• Finally, measure the angles that are formed at PQ and RS, as they form the incident angle, refracted angle, and the emergent angle.
• The lateral displacement shall be obtained by stretching the ray AB in a dotted line that is a parallel ray to EFD. And finally, measure the lateral displacement.
• The same procedure shall be repeated for angles 45° and 60°.

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Ray Diagram

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Ray Diagram

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Observation Table

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Note – Under certain errors, the value of ∠i – ∠e may not be equal to zero.

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Conclusion

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1. The angle of incidence is equal to the angle of emergence.
2. The angle of refraction is less than the angle of incidence when the light travels from the rarer to the denser optical medium.
3. The lateral displacement remains the same for different angles of incidence.
4. When the light travels from an optically rarer medium to an optically denser medium, it bends towards the normal.

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Precautions

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1. The rectangular glass slab must have perfectly smooth faces.
2. The soft drawing board shall be used so that pins can be easily fixed on it.
3. It should be taken care that the angle of incidence lies in between 30° and 60°.
4. A minimum of 5 cm of distance shall be maintained between the pins P and Q or the pins R and S.
5. The quality of the protractor and the pencil used should be taken care of.
6. The protractor shall be placed correctly to get the correct measurements.

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Sources of Errors

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1. The glass slab shouldn’t contain any air bubbles in it.
2. All the required measurements should be done aptly and accurately.

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Things to Remember

• Refraction denotes a concept wherein the ray of light bends while transmitting from one medium to another, mainly depending upon the density of the medium.
• According to the law of refraction, the angle of incidence is always equal to the angle of refraction at the point of incidence.
• The perpendicular shift in the path of light after it emerges out from the refracting medium is termed the lateral displacement.
• The experiment also denotes that the angle of incidence lies equally to the angle of emergence.
• The experiment also shows that the ray of light moves towards the normal when it travels from the rarer medium to the denser one and it moves away from the normal when it travels from the denser medium to the lighter one.