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Aim
To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result.
Theory
Refraction of Light: When light passes from one medium to another it deviates/changes its path, this property of light is called refraction of light.
Normal Ray: A ray of light that forms an angle of 90° with the refracting surface is said to be normal. When a ray of light travels along the normal, it does not suffer any refraction.
Incident Ray: A ray of light that travels towards the refracting surface is called an incident ray.
Refracted Ray: A ray of light that changes its path when passes through a refracting surface is said to be a refracted ray.
Emergent Ray: A ray of light that emerges out into the original medium after refraction is said to be an emergent ray.
Lateral Displacement: The perpendicular shift in the path of light, seen when it emerges out from the refracting medium is called lateral displacement.
Angle of Incidence (i): The angle formed between the normal and incident ray is called the angle of incidence.
Angle of Refraction (r): The angle formed between the refracted and normal ray is called the angle of refraction.
Angle of Emergence (e): The angle formed between the normal and emergent ray is called the angle of emergence.
DRAN: When a ray of light travels from a denser medium to a rarer medium it bends away from the normal.
RDTN: When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal.
During Refraction:
(i) Angle of incidence = Angle of emergence.
(ii) Incident ray and emergent ray are parallel.Laws of Refraction:
(i) The incident ray, the normal ray and the refracted ray, all lie in the same plane.
(ii) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant quantity for the two given media. This law is also known as Snell’s law. (sin i)/(sin r) = constant (this formulae on the white side)
This constant value is called the refractive index of the second medium with respect to the first.
Materials Required
A drawing board, 4-6 all pins, a white sheet of paper, a rectangular glass slab, a protractor, a scale, a pencil and thumb pins.
Procedure
Take a soft drawing board. Fix a white sheet on it with the help of thumb pins.
Place the rectangular glass slab in the centre of the white paper and draw its outline boundary with the pencil.
Mark this rectangular figure obtained as ABCD.
On one side of this figure, i.e., AB take one point E, draw a perpendicular EN and label it as a normal ray.
With the help of a protractor draw one angle of 30° with the EN. Fix two pins P and Q on the ray of this angle, the distance between the pins should be more than 4-5 cm.
Put the glass slab on the rectangular figure ABCD.
See through the glass slab from the side CD and fix pins R and S such that when seen through the glass slab all the pins lie in a straight line, [i.e., Pins P, Q, R and S should lie in a straight line when seen through the glass slab].
Now, remove the pins P, Q, R and S one by one and draw small circles around the pinpoints.
Remove the glass slab.
Join points R and S such that it meets CD at point F. Draw perpendicular to CD at point F as N’M’.
Join points E and F with the pencil.
Measure the angles formed at AB and CD, i.e., the incident angle, refracted angle and angle of emergence.
Extend ray PQ with scale and pencil in a dotted line. It will be parallel to ray FRS. The distance between these two parallel rays is called lateral displacement (d).
Measure the lateral displacement.
Repeat the above procedure for angles 45° and 60°.
ABCD = Glass slab
NM and N'M' = Normal rays
P, Q, R, S = All pins
∠PEN = ∠i = incident angle = 30°
∠MEF = ∠r = refracted angle
∠SFM' = ∠e = emergent angle = 30°
d = lateral displacement
(Note: You should even stick the lab activity sheet on a white page in your Lab Manual)
Observation Table
During the experiment performance, ∠i – ∠e may not be zero at times as shown above due to human error.
Conclusion
The angle of incidence is nearly equal to the angle of emergence.
The angle of refraction is less than the angle of incidence because the light is travelling from rarer to denser optical medium.
The lateral displacement remains the same for different angles of incidence.
When the light ray travels from an optically rarer medium (air) to an optically denser medium (glass) the light bends towards the normal.
Precautions
The glass slab should be perfectly rectangular with all its faces smooth.
The drawing board should be soft so that pins can be easily fixed on it.
The angle of incidence should lie between 30° and 60°.
All pins bases should lie in a straight line.
While fixing the pins P and Q or the pins R and S, care should be taken to maintain a distance of about 5 cm between the two pins.
Draw thin lines using a sharp pencil.
Use a good quality protractor having clear markings.
Place the protractor correctly to measure the angles.
Perpendiculars should be drawn correctly.
Sources of Error
The glass slab should not have any air bubbles.
Measurement of all the angles using a protractor should be done accurately.
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