Introduction to Refraction of Light

Refraction is the bending of light when it passes from one medium to another due to a change in speed. This happens because different substances have different densities, which affect how light travels through them.

"Refraction is the change in the direction of a wave as it passes from one medium to another due to a change in its speed."

Causes of Refraction:

• Light changes speed when moving between materials of different densities.
• If light enters a denser medium (e.g., air to water), it slows down and bends towards the normal.
• If light enters a less dense medium, it speeds up and bends away from the normal.
• If light enters perpendicularly, it does not bend but still changes speed.

Pencil in Water (Observing Refraction):

• Take a transparent glass and fill it with water.
• Dip a pencil vertically into the water and observe its thickness.
• Tilt the pencil at an angle and observe again.
• In both cases, the portion of the pencil inside the water appears thicker than the portion above.
• When the pencil is tilted, it looks bent or broken at the water's surface.

This effect occurs because light changes direction (refracts) when it moves from water to air. The bending of light makes the pencil appear distorted, demonstrating the refraction of light.

Disappearing and Reappearing Coin

• Place a 5-rupee coin in a metallic vessel.
• Slowly move away until the coin disappears from view.
• Ask a friend to pour water into the vessel while you keep looking in the same direction.
• As the water level rises, the coin becomes visible again.

Light rays coming from the coin bend when passing from water to air, changing their path. This makes the coin, which was earlier invisible, appear again due to the refraction of light.

Refraction of Light Through a Glass Slab:

• Place a glass slab on a paper and draw its outline (PQRS).
• Draw an inclined ray (AN) on side PQ, marking point N where it meets the slab.
• Fix two pins (A and B) along the ray and look from the other side.
• Align two more pins (C and D) with the images of A and B.
• Remove the slab and draw a line through C and D, meeting SR at point M.
• Join M and N to trace the path of the refracted ray.

Refraction of light passing through a glass slab

Observation and Explanation:

1. The incident ray (AN) strikes the glass at point N and bends due to refraction.

2. The first refraction occurs when light moves from air to glass at N, bending toward the normal.

3. The second refraction occurs when light moves from glass to air at M, bending away from the normal.

The angles of refraction are measured as:

• i = angle of incidence (AN to normal at N).
• r = angle of refraction at N.
• i₁ = r, meaning the refraction inside the glass follows the same pattern.
• e = emergent angle, which equals i due to the opposite bending effect.

The emergent ray (MD) is parallel to the incident ray (AN) but is slightly displaced from it due to the shift caused by the refraction process.

This activity shows that light bends when passing between different transparent media, following the laws of refraction. The incident and emergent rays remain parallel but are slightly displaced, demonstrating the refraction through a rectangular glass slab.