why does refraction bend light

Refraction bends light because light changes speed when it moves from one transparent medium to another (e.g., air to glass, water to glass). Think of light as a wave: when it hits the boundary at an angle, different parts of the wavefront enter the new medium at different times. The part that enters first changes speed first, causing the whole wave to rotate or bend. This is the Huygens' principle at work.

The amount of bending depends on the refractive index $n$ of the media. The refractive index is defined as $n = \frac{c}{v}$, where $c$ is the speed of light in vacuum and $v$ is the speed in the medium. For air, $n \approx 1$; for glass, $n \approx 1.5$; for water, $n \approx 1.33$. The Snell's law (also called law of refraction) gives the exact relation: $$n_1 \sin\theta_1 = n_2 \sin\theta_2$$ where $\theta_1$ and $\theta_2$ are the angles with the normal in medium 1 and 2.

Micro-example: A ray enters glass from air at $\theta_1 = 45^\circ$. Using Snell's law with $n_1=1$, $n_2=1.5$: $1 \cdot \sin 45^\circ = 1.5 \sin\theta_2 \Rightarrow \sin\theta_2 = \frac{0.707}{1.5} = 0.471 \Rightarrow \theta_2 \approx 28^\circ$. So it bends towards the normal because it slows down.

Key points: Light bends towards the normal when entering a denser medium (slower speed), and away from the normal when entering a rarer medium (faster speed). If light hits the boundary perpendicularly (at $0^\circ$), no bending occurs. Also, dispersion (splitting into colours) happens because different wavelengths have slightly different speeds in glass, so each colour bends a bit differently.

Remember: refraction is not a force; it's just a change in speed causing a change in direction. Exam tip: always draw the normal at the point of incidence and measure angles from it, not from the surface.