Reflection and refraction
Bouncing and bending light
- A mirror bounces light back; a straw in a glass of water looks bent.
- These are reflection and refraction — the two basic things light does at a surface.
- We measure all angles from the normal: a line at $90°$ to the surface.
Reflection
- The law of reflection: the angle of incidence equals the angle of reflection (both from the normal).
- A plane mirror makes an image that is: the same size, the same distance behind the mirror, and virtual (it cannot be caught on a screen).
A ray hits a plane mirror at $30°$ to the normal. At what angle to the normal does it reflect?
The law of reflection: the angle of reflection equals the angle of incidence, both measured from the normal.
An image in a plane mirror is:
A plane mirror gives a virtual image, the same size, the same distance behind the mirror — it cannot be caught on a screen.
Refraction
- When light passes into a new material it changes speed and bends.
- Going from air into glass, it slows down and bends toward the normal (so $i > r$).
- The refractive index compares the bending:
A ray of light passes from air into glass. It:
Light travels slower in glass, so going air → glass it bends toward the normal ($i > r$).
For light entering glass, $\sin i = 0.80$ and $\sin r = 0.50$. What is the refractive index?
$n = \dfrac{\sin i}{\sin r} = \dfrac{0.80}{0.50} = 1.6$.
Total internal reflection
- Light leaving glass bends away from the normal. Past a certain angle — the critical angle $c$ — it cannot get out at all and reflects completely.
- This is total internal reflection, and it happens when the angle inside is bigger than $c$:
- It keeps light bouncing along optical fibres, the thin glass threads that carry internet and phone signals.
Total internal reflection happens when light inside glass hits the surface at an angle that is:
Above the critical angle the light cannot escape and is reflected completely back inside.
Optical fibres use total internal reflection to carry light signals.
Light bounces along the fibre by total internal reflection, carrying internet and phone signals over long distances.
You've got it
- measure angles from the normal; reflection: angle i = angle r
- a plane mirror image is virtual, same size, same distance behind
- refraction: into glass → slows, bends toward the normal; $n = \dfrac{\sin i}{\sin r}$
- past the critical angle, light is totally internally reflected (optical fibres)