*Explanation:*To investigate the path of light through rectangular glass block.Apparatus: - soft-board, white sheet of paper, drawing pins (optical), rectangular glass block.

Procedure

- Fix the white plain paper on the soft board using pins
- Place the glass block on the paper and trace its outline, label it ABCD as shown below
- Draw a normal NON at point O
- Replace the glass block to its original position
- Stick two pins P1 and P2 on the line such that they are at least 6cm apart and upright
- Viewing pins P1 and P2 from opposite side, fix pins P3 and P4 such that they’re in a straight line
- Remove the pins and the glass block
- Draw a line joining P3 and P4 and produce it to meet the outline face AB at point O

**Explanation of Refraction**

- Light travels at a velocity of 3.0 & 108 in a vacuum.

- Light travels with different velocities in different media.

- When a ray of light travels from an optically less dense media to more dense media, it is refracted towards the normal.

- The glass block experiment gives rise to a very important law known as the law of reversibility which states that

*"if a ray of light is reversed, it always travels along its original path".*- If the glass block is parallel-sided, the emergent ray will be parallel to the incident ray but displaced laterally as shown

**'e'**is called the angle of emergence

The direction of the light is not altered but displaced sideways.

This displacement is called lateral displacement and is denoted by 'd'. Therefore

**XY = t/Cos r, YZ = Sin (i - r) × x y**

So, lateral displacement,

**d = t Sin (i - r)/Cos r**

### Laws of Refraction

- The incident ray, the refracted ray and the normal at the point of incidence all lie on the same plane.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a
constant for a given pair of media.

Sin i/sin r = constant (k)

### Refractive index

Refractive index (n) is the constant of proportionality in Snell's law: hence**Sin i/ sin r = n**

Therefore

**sin i/sin r = n = 1/sin r/sin i**

**Example 1**

Calculate the refractive index for light travelling from glass to air given that ang = 1.5

*Solution*

gna= 1/ang = 1/1.5 = 0.67

**Example 2**

Calculate the angle of refraction for a ray of light from air striking an air-glass interface, making an angle of 600 with the interface. (ang = 1.5

*Solution*

Angle of incidence (i) = 900 - 600 = 300

1.5 = sin 30°/sin r, sin r = sin 300/1.5 = 0.5/1.5

Sin r = 0.3333, sin-10.3333 = 19.50

R = 19.50