Reflexão vs. reflexo
In Portuguese, while both words translate to "reflection", they represent different aspects of the phenomenon. Generally, reflexão refers to the physical process or action of light bouncing, while reflexo refers to the resulting image or glare that is visible to the eye.
Reflexão
B2This word describes the action, phenomenon, or physical process of light hitting a surface and bouncing back. It is often used in scientific, technical, or descriptive contexts regarding the behavior of light.
A reflexão da luz é um conceito básico da ótica.
(The reflection of light is a basic concept of optics.)
O ângulo de reflexão é igual ao ângulo de incidência.
(The angle of reflection is equal to the angle of incidence.)
Este material impede a reflexão da luz solar.
(This material prevents the reflection of sunlight.)
A reflexão difusa ocorre em superfícies irregulares.
(Diffuse reflection occurs on irregular surfaces.)
Reflexo
B1This word refers to the visible result of the process. It is the specific image you see in a mirror, on water, or a flash of light (glare) that shines off a smooth surface.
Ele viu o seu reflexo no espelho do banheiro.
(He saw his reflection in the bathroom mirror.)
O reflexo do sol na janela estava muito forte.
(The reflection of the sun on the window was very strong.)
O lago estava tão calmo que o reflexo da montanha parecia perfeito.
(The lake was so calm that the reflection of the mountain looked perfect.)
Tirei uma foto, mas o reflexo do flash estragou tudo.
(I took a picture, but the reflection of the flash ruined everything.)
Summary
Use reflexão when discussing the phenomenon or the physics of light bouncing. Use reflexo when talking about the actual image, copy, or glare produced by that light.