1. What refraction really means
Refraction is the bending of light when it moves from one transparent medium to another. I think of it as light changing its path because its speed changes in the new medium.
For example, when a ray moves from air into glass, it slows down and bends towards the normal. When it moves from glass back into air, it speeds up and bends away from the normal.
This bending explains many everyday observations: a pencil appearing bent in water, a swimming pool looking shallower, and lenses forming images.
2. Useful terms I need to know
To describe refraction properly, I use these standard words:
- Incident ray: The ray coming toward the boundary between two media.
- Refracted ray: The ray that bends and enters the second medium.
- Normal: A perpendicular line drawn at the point where the ray touches the boundary.
- Angle of incidence (i): The angle between the incident ray and the normal.
- Angle of refraction (r): The angle between the refracted ray and the normal.
- Optical density: A measure of how much a medium slows down light. It is not the same as mass density.
3. Why light bends at a boundary
The main reason for bending is the change in the speed of light. Light travels fastest in air and slower in glass or water.
If a ray enters a denser medium (like glass), it slows down and bends towards the normal. If it enters a rarer medium (like air), it speeds up and bends away from the normal.
3.1. The bending rule I always remember
- Towards the normal → light going into a denser medium.
- Away from the normal → light going into a rarer medium.
This simple rule helps me quickly sketch ray diagrams even without formulas.
4. Speed changes but frequency stays the same
When light moves from one medium to another, its speed and wavelength change, but its frequency stays constant.
This happens because the source of light controls the frequency, and the boundary cannot change it.
4.1. Relation between speed, frequency, and wavelength
The basic wave relation is:
\( v = f \lambda \)
When speed changes, wavelength changes accordingly, while frequency remains fixed.
5. Refracted ray path in different situations
Depending on the angle and the medium, the refracted ray behaves in predictable ways:
- Entering at a small angle: Light bends slightly.
- At larger incidence angles: The bending becomes more noticeable.
- At normal incidence: No bending occurs, only a change in speed.
6. A simple geometric explanation
I can visualise refraction by imagining the light ray as a tiny set of wheels hitting a boundary at an angle. The part entering the denser medium slows down first, causing the whole ray to turn towards the normal — just like a turning vehicle when one side hits mud or grass first.
7. Everyday examples of refraction
Once I understood refraction, many familiar things suddenly made sense:
- A pencil in water looking bent or broken.
- A coin at the bottom of a glass appearing raised.
- Mirages on hot roads (due to gradual refraction in air layers).
- How lenses focus light to form images.
8. A quick numerical example
If a ray goes from air to glass and bends towards the normal by a small angle, I know glass is the denser medium. Even without calculations, I can predict the direction of bending just by comparing optical densities.
For example, if the angle of incidence is \(30^\circ\) and the refracted angle is \(19^\circ\), bending towards the normal is observed, matching the rule.