Tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence

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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.

Materials Required

• Drawing board
• Drawing pins
• Three plane sheets of white paper
• A rectangular glass slab
• Geometry instruments
• Sharp pointed pins

Theory

When the light ray enters the rectangular glass slab from air, it bends towards the normal.

The refracted ray is incident obliquely on the second parallel surface inside the rectangular glass slab and after refraction, it moves away from the normal.

Procedure

• Fix a white paper on a drawing board with the help of drawing pins and divide the sheet in two parts by a vertical line.
• Place a rectangular glass slab in the first part. Draw its boundary. Remove the glass slab and label the boundary as A₁, B₁, C₁, D₁ as shown below.

• Draw a normar MN on side A₁B₁ at a point O₁, slightly away from the centre towards A₁.
• Draw an oblique line P₁O₁ (incident ray) such that ∠P₁O₁M = 30° (Angle of incidence). Fix two sharp pointed pin P₁ and O₁ vertically erected on the line P₁O₁ at a distance of 4 to 6 cm apart.
• Place the glass slab again within its boundary. Look at the feet of pins (not their heads) P₁ and O₁ from the other parallel opposite face of the slab, i.e. from C₁D₁ along the plane of paper. Fix other two pins R₁ and S₁ in such a way that R₁, S₁ and the image of P₁ and O₁ lie on a same straight line.
• Remove the glass slab and all the four pins. Encircle all the prick of the four pins. Join the points R₁ and S₁ within the encircle and produce upto the edge C₁D₁. Let R₁S₁ meet C₁D₁ at O₂. This will act as an emergent ray.
• Draw a normal M₁N₁ at O₂. Join O₁ and O₂. It will represent the path of ray inside the glass slab, i.e. refracted ray.
• Measure the angle of emergence, i.e. ∠e = ∠N₁O₂S₁ and angle of refraction, i.e. ∠r = ∠NO₁O₂.
• Repeat the experiment by taking the different angles of incidence such as 45° and 60° on the other part of paper and measure the angle of refraction and emergence accordingly and tabulate them.

Observation Table

S.No.	Angle of incidence (∠i)	Angle of refraction (∠r)	Angle of emergence (∠e)
1.	30°	18°	30°
2.	45°	26°	45°
3.	60°	33°	60°

*These values may be different for different glass slab.

Result

1. At the point of incidence the incident ray, refracted ray and the normal to the air-glass interface, all lie in the plane of paper.
2. Within experimental limits, the angle of emergence and angle of incidence are equal.
3. The emergent ray is parallel to the incident ray.
4. Emergent ray is laterally displaced.
5. When the light ray travels from optically rarer medium (air) to optically denser medium (glass), the angle of refraction is less than the angle of incidence.
6. The refracted angle at the air-glass interface and the incident angle at the glass-air interface are found to be equal.
7. From the observation table, it is clear that with the increase in angle of incidence, angle of refraction also increases.

Precautions

• Drawing board should be made of soft wood.
• Glass slab must be clean and should be free from air bubble.
• The pins should be fixed in vertical position.
• The base of all the pins (on the paper) should be placed in a straight line.
• Keep your eyes along the plane of paper and in line while observing the image and fixing the pins.
• For incident ray, the angle of incidence must lie between 30° and 60°.

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