Describe the passage of light through a transparent material (limited to the boundaries between two mediums only)

3.2.2 Refraction of Light

What is Refraction?

When light passes from one transparent material to another, its speed changes, causing the light to bend. This bending is called refraction. 📐

Key point: Refraction occurs only at the boundary between two media with different optical densities.

Snell’s Law

The relationship between the angles and refractive indices of two media is given by Snell’s law:

Formula: \$n1 \sin \theta1 = n2 \sin \theta2\$

  • \$n\$ = refractive index (dimensionless)

  • \$\theta_1\$ = angle of incidence (measured from the normal)

  • \$\theta_2\$ = angle of refraction

VariableMeaning
\$n_1\$Refractive index of medium 1 (e.g., air)
\$n_2\$Refractive index of medium 2 (e.g., water)
\$\theta_1\$Angle of incidence
\$\theta_2\$Angle of refraction

Critical Angle & Total Internal Reflection

When light travels from a denser to a rarer medium, there is a special angle called the critical angle. If the angle of incidence exceeds this, light is totally reflected back into the denser medium.

Caution: Do not confuse the critical angle with the angle of incidence. The critical angle is the maximum angle for which refraction still occurs.

Demonstration: Place a laser pointer in a glass of water. Shine it at different angles. Notice that beyond a certain angle, the beam stays inside the water and reflects back.

Applications

  • Eyeglasses and contact lenses correct vision by bending light onto the retina.

  • Camera lenses focus light to form sharp images.

  • Optical fibres transmit light signals over long distances using total internal reflection.

  • Mirrors and prisms use refraction to redirect light paths.

Exam Tips

Tip 1: Write down Snell’s law and identify known and unknown variables before solving.

  • Tip 2: Remember that the refractive index of air is approximately 1.0.

  • Tip 3: Check units and significant figures in your final answer.

    • Quick Recall: \$n = \dfrac{c}{v}\$ – refractive index equals the speed of light in vacuum divided by its speed in the medium.

    Sample Question: A light ray enters water (n=1.33) from air at an angle of 30°. What is the angle of refraction? (Show your work.)

    Key Points:

    • Refraction is the bending of light at a boundary.

    • Snell’s law relates angles and refractive indices.

    • Critical angle leads to total internal reflection.

    • Applications include lenses, fibres, and optical devices.

    Analogy: Think of light as a runner moving from a smooth track (air) onto a muddy field (water). The runner slows down and changes direction, just as light does when it enters a denser medium. 🏃‍♂️💧

    Example: A straw in a glass of water looks bent at the surface. This is because light from the straw travels from water to air, bending away from the normal. 🌈

    Note: The refractive index of water is about 1.33, glass about 1.50, and air about 1.00. These values determine how much light bends.