Refraction Of Light

MATERIALS REQUIRED

Drawing board, drawing pins, three plane sheets of white paper a rectangular glass slab, geometry instruments and pins.

THEORY

When a light ray incident on air to glass interface through a glass slab obliquely has the following characteristics:
1. When a light ray travels from air to glass, the ray bends towards the normal at the surface of the air-glass boundary.

2. When a light ray travels from glass to air, the angle of refraction is greater than the angle of incidence of glass-air interfaces as the ray of light bends away from the normal

3. 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.
4. The angle of emergence and the angle of incidence will be equal.

PROCEDURE

1. Fix a white paper sheet on a drawing board using pins or cello tape.

2. Place a rectangular glass slab in the middle of the paper part and marks its boundary ABCD with a pencil.

3. Remove the rectangular glass slab and label the boundary as A₁, B₁, C₁, D₁ as shown below.

4. Draw normal (perpendicular) MN on the side AB at a point O, slightly away from the centre towards A1.

5. Draw an oblique line P₁, Q₁ (incident ray) such that ∠P₁Q₁M = 30 (Angle of incidence). Fix two sharp point pin P₁ and Q₁ vertically erected on the line P₁Q₁ at a distance of 5-7 cm apart.

6. Place the glass slab again within the marked position. Observe the image of Look of pins (not their heads) P₁ and Q₁ from the other side. Fix other two pins R₁ and S₁ in such a way that R₁, S₁, and the image of P₁, and Q₁ lie on a straight line.

7. Now, remove the glass slab and pins. Mark all the prick of the pins. Join the points R₁ and S₁ and produce a line upto the edge C₁D₁. Let R₁S₁ meet C₁D₁ at O₂. This will act as an emergent ray.

8. Draw a normal M₁, N₁, at O₁. Join O₁ and O. It will represent the path of ray inside the glass slab, i.e. refracted ray.

9. Measure the angle of emergence, i.e. ∠e = ∠N₁O₂S₁ and angle of refraction, i.e. ∠r = N₁O₁O₂.

10. Repeat the experiment for two more values of angles of incidence such as 45 and 60 on the other part of the paper and measure the angle of refraction and emergence accordingly and tabulate them.

OBSERVATION TABLE

RESULT

1. law of refraction is verified, i.e. at the point of incidence the incident ray, the emergent ray and the normal to the air-glass interface, all lie in the plane of the paper.

2. The relation between the angle of incidence and the angle of emergence are obtained within the experimental limits.

3. From the observation, the emergent ray emerging out of the rectangular glass slab is parallel to the incident ray.

4. Emergent ray undergoes lateral displacement.

PRECAUTIONS

1. The glass slab should be perfectly rectangular with all its faces smooth.

2. Glass slab must be clean and free from scratches.

3. Thin lines should be drawn to obtain good accuracy.

4. The distance between the pins should be 5-7 cm.

5. The base of all the pins should be placed in a straight line.

When a beam of light falls obliquely at the interface of the two optical media, the direction of its paths changes when it enters into the other medium.This phenomenon is called refraction.

The speed and wavelength of the light changes while frequency remains the same.

The ray of light bends toward the normal.

the light will bend away from the normal.

1. At the point of incidence the incident ray ,normal ray and the refracted ray lie on the same plane.

2. The ratio of the sine of an angle to the sine of the angle of refraction is constant for the medium for the same colour of light.

The refractive index of the medium is equal to the ratio of the sine of the angle of incidence and sine of the angle of refraction.

1. Density of medium

2. Nature of medium

The refractive index is a dimensionless quantity since it is the ratio of two like quantities.

Angle of emergence also increases.

The lateral Displacement is the perpendicular distance between the incident ray and emergent ray.

1. The light rays fall along the normal

2. The refractive index of two optical media are equal

Because incident ray and emergent ray both are parallel to each other.

1. To get more light

2. It will be more convenient to fix other pins.

In reflection, a ray of light turns back into the same medium after reflection, while in refraction the ray of light passes from one medium to another.