Question

Why does a beam of light bend when it enters glass at an angle? Why does it not bend if it enters the glass at right angles?

Answer 1

Deflection in the path of light ray basically occurs in two phenomena of light i.e., Refraction and Reflection. The given situation is of refraction, hence to understand the solution it is important to understand refraction and laws linked and factors linked to refraction.

Complete step by step answer:
Refraction is the bending of a light ray from its original path when it shifts from one medium to another medium having different density. It is an important aspect of light as it defines several properties of light. Refraction phenomena occur due to differences in densities of mediums which affect the speed of light in both mediums. This relation between the degree of bending in the light ray with the original path is stated by Snell’s Law. According to Snell’s Law,
$\dfrac{{\sin i}}{{\sin r}} = \dfrac{{\mu {}_r}}{{{\mu _i}}} = \dfrac{{{v_i}}}{{{v_r}}}$
Where,$I$ = incident angle, $r$ = refracted angle, ${v_i}$= speed of light in incident medium, ${v_r}$= speed of light in refractive medium, ${\mu _r}$= refractive index of emergent medium and ${\mu _i}$= refractive index of incident medium.

So, if we understand the question we will observe that light travels from air to glass i.e., air is the incident medium and glass is the refracted medium. When light enters from air to glass, there occurs refraction which deflects the light from its original path. We can also observe that, due to increase in density of medium there will be reduction in speed of light and from Snell’s law, the relation between refractive index and speed and angle, it is evident that the variation in speed causes variation in angle. But when light strikes at a right angle, the refraction phenomena doesn’t occur which prevent bending of light rays. As the incident angle becomes zero and sin value of angle zero yields zero i.e., refracted angle is also zero.

We can now conclude that bending in ray of light occurs due to refraction of light and at right angle there occurs no refraction hence it doesn’t change its path.

Note: Reflection of light is the phenomenon of bouncing back off a light ray on striking any polished smooth surface (reflector). Diffraction is the ability of waves to change its direction on encountering any barrier in their path. All three terms are quite different but generally yield to confusion so it is necessary to understand them all.