This figure is given in wikipedia to explain rainbow formation,

l know that if light travels from denser medium to rarer medium and if angle of incidence is greater than crictal angle light will reflect back frmo inner surface of drop, but here in this figure a parallel ray(paraxial rays) to diameter is shown as reflecting back from surface.

It is clear from figure that they do not make angle more than critical angle,

I want to know whether it is partial reflection or total internal reflection of paraxial rays.


This sentence is also given in wikipedia:

The overall effect is that part of the incoming light is reflected back over the range of 0° to 42°, with the most intense light at 42°.

enter image description here

  • 1
    $\begingroup$ An angle isn't specified in the attached image, so it's hard to tell, but what isn't hard is how the rays closest to the boarder line that barely refracted as they went through the drop was completely refelcted at the back of it. I believe the image is meant to illustrate rather than providing an detailed or precise optical diagram. $\endgroup$
    – TechDroid
    Commented Mar 3, 2019 at 4:49
  • $\begingroup$ "completely refelcted at the back of it". why do they completelly reflected at back of raindrop. can you explain it. $\endgroup$
    – teja
    Commented Mar 3, 2019 at 4:52
  • $\begingroup$ It's not suppose to do that so close to the normal, so the image is partially inaccurate. It's rather a descriptive image. $\endgroup$
    – TechDroid
    Commented Mar 3, 2019 at 5:03

1 Answer 1


It's a partial reflection.

For an ideal sphere, it's not possible for an external ray to achieve total reflection. Any ray that enters must necessarily be at less than the critical angle. This ray will intersect the sphere at two points, and the angle it creates with the surface of the sphere will be identical. Therefore it will be below the critical angle at both surfaces.

Not shown is that a percentage of the light will leave and not reflect at each intersection.

The important thing in that image is that regardless of where incoming ray strikes, there is a maximum deviation possible when there are two reflections. This maximum angle depends on the refractive index.

  • $\begingroup$ So you mean that incase of spherical rain drop total internal relfection does not occurs ? then what is the cause of reflection at back surface can you please explain it. $\endgroup$
    – teja
    Commented Mar 3, 2019 at 5:52
  • $\begingroup$ It's just regular reflection. Not total internal reflection. $\endgroup$
    – BowlOfRed
    Commented Mar 3, 2019 at 6:03

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