Refraction (Grade 11 NSC Matric Physical Sciences): Revision Notes

Refraction

What is refraction?

Refraction is the bending of light that occurs when light travels from one medium to another. This happens because light changes speed as it moves between different materials, even though its frequency stays the same.

When a light ray hits the boundary between two media at an angle that is not perpendicular to the surface, the light will change direction and appear to bend. This bending effect is what we call refraction.

Refractive index

The refractive index is a number that tells us how much light slows down when it travels through a particular material. It is represented by the symbol n and is calculated using this formula:

$n = \frac{c}{v}$

Where:

• $n$ = refractive index (no units)
• $c$ = speed of light in a vacuum ($3.00 \times 10^8$ m⋅s⁻¹)
• $v$ = speed of light in the material (m⋅s⁻¹)

This table shows that different materials have different refractive indices. For example, water has a refractive index of 1.333, meaning light travels about 1.33 times slower in water than in a vacuum.

Optical density

Optical density is a measure of how well a medium can refract (bend) light. Materials with higher optical density will bend light more than materials with lower optical density.

Ray diagrams and key terms

When drawing ray diagrams to show refraction, we need to understand several important terms:

Normal: The normal is a straight line drawn perpendicular to the surface at the point where the light ray hits the boundary between two media.

Angle of incidence: This is the angle between the incident (incoming) light ray and the normal.

Angle of refraction: This is the angle between the refracted (bent) light ray and the normal.

This diagram shows light travelling from water ($n = 1.3$) into air ($n = 1.0$). Notice how the light ray bends away from the normal as it moves from the denser medium (water) to the less dense medium (air).

Practical investigation of refraction

Understanding refraction through hands-on investigation helps you see how light behaves in real situations.

Aim: To investigate how light refracts when passing from air into glass and back into air.

Equipment needed:

• Ray box (light source)
• Rectangular glass block
• Plain paper
• Pencil
• Ruler
• Protractor

Method:

1. Place the glass block on paper and draw around it to mark its outline.

2. Turn on the ray box and direct the light ray towards one side of the glass block at an angle.

3. Use your pencil to mark dots along the incident ray before it hits the glass, and mark where the ray enters the glass block.

4. Mark where the light ray exits the glass block and continue along the emerging ray.

5. Remove the glass block and use your ruler to connect the dots, showing the complete path of the light ray.

6. Draw normals at both surfaces where light enters and exits the glass block.

7. Use your protractor to measure the angles of incidence and refraction at both surfaces.

This investigation demonstrates the fundamental principles of refraction and allows you to measure how much light bends when passing between different media.