Theory of glass slab

1. Refraction through rectangular glass slab with parallel faces:
   The refraction takes place at both air-glass interface and glass-air interface has the following characteristics:

    

   • When a light ray travels from air to glass, the angle of incidence is greater than angle of refraction as ray bends towards normal.
   • When a light ray travels from glass to air, the angle of refraction (also called angle of emergent in case of glass) is greater than the angle of incidence of glass-air interface as ray of light bends away from the normal.
   • If the angle of incidence is zero, i.e. the incident ray is normal to the interface, the ray of light continues to travel in the same direction after refraction.
   • The angle of emergence and angle of incidence will be equal.
     Emergent ray is parallel to the incident ray along with original direction but it will be laterally displaced to the left of the incident ray.
   • For the same angle of incidence, lateral displacement is proportional to the thickness of the glass slab.
   • For the same thickness of glass slab, the lateral displacement is proportional to the angle of incidence.

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, and sharp pointed pins.

THEORY
The refraction takes place at both air-glass interface and glass-air interface of a rectangular glass slab.

• When the light ray incident on air-glass interface (DC) obliquely, it bends towards the normal.
• The refracted ray is incident obliquely on the second parallel surface inside the rectangular glass slab, i. e. glass-air interface (AB) and after refraction, it moves away from the normal.
  These refractions at both the surfaces obey the laws of refraction (Refer to basic building concept).
  

PROCEDURE

1. 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.
2. 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.
   
3. Draw a normal (perpendicular) MN on the side A₁B₁ at a point O₁, slightly away from the centre towards A₁
4. Draw an oblique line P₁Q₁ (incident ray) such that ∠P₁O₁M = 30° (Angle of incidence). Fix two sharp pointed pin P₁ and Q₁ vertically erected on the line P₁O₁ at a distance of 4 to 6 cm apart.
5. Place the glass slab again within its boundary. Look at the feet of pins (not their heads) P₁ and Q₁from 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 Q₁ lie on a same straight line.
6. Remove the glass slab and all the four pins. Encircle all the prick of the four pins. Join the points R₁and S₁ within the encircle and produce upto the edge C₁D₁. Let R₁S₁ meet C₁D₁ at O₁. This will act as an emergent ray.
7. 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.
8. Measure the angle of emergence, i.e. ∠e = ∠N₁O₁S₁ and angle of refraction, i.e. ∠r = ∠NO₁O₂
9. 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

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.