What Is Light?

In geometrical optics, light is treated as straight rays that travel from one point to another. This model ignores the wave and particle behaviour of light, but it’s ideal for predicting how light reflects, refracts, and forms rainbows. The ray model works because light travels so fast (~300,000 km/s) it moves in nearly straight lines over short distances!

The Laws of Reflection and Refraction

Think about throwing a ball against a wall. It bounces back at an angle that depends on how you threw it. Light acts in a similar way when it hits a smooth surface like a mirror.

According to the law of reflection, the angle of incidence (the angle at which the light hits the surface) is equal to the angle of reflection (the angle at which it bounces off).

These angles are measured relative to an imaginary line called the normal. The normal is drawn straight out from the surface at the point where the light makes contact.

Now imagine a beam of light passing from air into water. If you’ve seen a straw appear bent inside a glass, you’ve already seen this in action.

This bending of light is called refraction. It happens when light moves between materials with different densities, and changes speed as a result. In denser materials like water or glass, light slows down compared to how fast it moves in air.

Why does it bend though?

When one side of the light beam enters the new material before the other, it slows down first, causing the beam to turn. This is similar to a car turning when one of its wheels hits rough ground before the other.

Snell’s Law describes this behaviour:

n₁ sin θ₁ = n₂ sin θ₂

In this equation, n stands for the refractive index of the material, which tells us how much it slows down light. θ is the angle measured from the normal.

Dispersion

When white light passes through a prism, it spreads into a rainbow. This effect is called dispersion. This happens because each wavelength in white light bends slightly differently. Shorter wavelengths like blue and violet bend more than longer ones like red. 

Rainbows form in the sky for the same reason. Tiny water droplets act like small prisms, splitting sunlight into bands of colour. 

Total Internal Reflection

Sometimes, instead of escaping a material, light gets reflected back inside it. This effect is called total internal reflection. It happens when light travels from a material with a higher refractive index, like glass, into a material with a lower index, like air, and the angle of approach is too steep.

As light reaches the boundary, it bends away from the normal because it is entering a material where it can travel faster. If you increase the angle of incidence, eventually the light reaches a point where it no longer exits. This point is called the critical angle. At the critical angle, the refracted ray travels right along the boundary. If the angle increases beyond this point, all the light is reflected back inside. The critical angle can be calculated using the formula sin θ = n₂ / n₁, where n₁ must be greater than n₂.

This principle is used in fiber optics. These are thin strands of glass that carry data using light signals. The light stays inside the fiber and bounces along its length without escaping. Total internal reflection also explains why swimming pools can appear shallower when viewed from a sharp angle. Light from the bottom reflects off the surface instead of passing through.