# Why does light refract toward the normal

1. Apr 17, 2015

### physdoc

why does light refract toward the normal when passing through glass?

2. Apr 17, 2015

### robphy

3. Apr 17, 2015

### physdoc

4. Apr 17, 2015

### robphy

The paths of light are reversible.
Instead sending light from medium 1 to medium 2, send light from medium 2 to medium 1.
(Try the PhET simulation.)

Since the Law of Refraction says:
$n_1\sin\theta_1 =n_2\sin\theta_2$,
or equivalently $\frac{n1}{n2}=\frac{\sin\theta_2}{\sin\theta_1}$,
if $\frac{n1}{n2}<1$ (when entering a higher-n-medium),
then $\frac{\sin\theta_2}{\sin\theta_1}<1$... but this means $\frac{\theta_2}{\theta_1}<1$ since the sine function is always increasing between 0 and 90.

5. Apr 17, 2015

### physdoc

Explain further

6. Apr 17, 2015

### physdoc

The light slows down, but how can it be explained that this slowing down causes it to be refracted toward the normal?

7. Apr 17, 2015

### physdoc

I answered it: because it covers less distance in the same time.

8. Apr 17, 2015

### robphy

Yes... check out the "wave" (as opposed to the "ray") version of the PhET simulation.

9. Apr 17, 2015

### physdoc

Thanks

10. Apr 17, 2015

### physdoc

If you draw a triangle, with each of the rays as legs and connect them with a straight line running perpendicular to the normal, which shows that the original leg is longer than the refracted ray, right?

11. Apr 17, 2015

### Drakkith

Staff Emeritus
Remember that rays are only an approximation to real light, which is a wave. The light refracts towards the normal upon entering a medium with a higher refractive index because it travels slower inside the medium. See the image below. (Blue and Green arrows are the light ray)

Notice how the wavelength of the incoming light wave (blue lines) is larger than the wave inside the medium (green lines). This is because the speed of the wave is slower inside the medium than outside, while the frequency remains the same. The part of the wavefront first entering the medium is slowed down, allowing the parts yet to enter the medium to 'catch up' to it. When these later parts enter the medium they interfere with the part already there, and the interference of the different parts of the wavefront results in a change in the direction of the wavefront towards normal.