When the light falls perpendicular on the surface of a glass it shows no change in direction. why?

Answer 1

Light is bent (refracted) towards/away from the normal line when it enters an optically less/more dense medium at an angle.
The normal line an imaginary line which extends perpendicular to the surface.
If the light enters the medium along the normal line (perpendicular), the light is not bent.

Snell's law,
n_1 * sin (theta_1) = n_2 * sin (theta_2)
where n_1 and n_ are the indicies of refractionn of the two mediums which light is traveling and theta_1 and theta_2 are the angles measured between the light path and the normal line to the surface.
If the light enters perpendicular, theta_1 = 0, thus sin (theta_1) = 0. The only way the two sides of the equation can be equal of sin(theta_1) = 0 is if sin (theta_2) = 0, since a zero value for an index of refractio is impossible. If sin(theta_2) = 0, then theta_2 -= 0, and thus the light refracts at ang angle of zero away from the normal, thus is is not refracted.

Answer 2

Refraction happens because light slows down as it passes into a medium (like glass) with a higher refractive index. Its frequency stays the same, so its lower speed means that its wavelength has decreased.

Now think of two parallel waves of light arriving at the surface of the glass at an angle and in phase. The second wave will hit the glass a little after the first. Now go on one wavelength until they are at the top again, inside the glass. Because the wavelength is shorter in the glass and they hit it at different times then you have to bend the ray to get the two peaks to coincide again. This is why refraction occurs.

But if the light hits the glass perpendicular then both peak hit at the same time. So the ray does not need to bend.

Answer 3

Snell's Law. The incident angle is measured fromthe vertical, so is 0. THis means that the angle inside the material (measured from the negative vertical) is 0.

Answer 4

Laws of Reflection and Refraction (Snell's Law is a specific form of the Law of Refraction).
-- Law of Reflection:
---- Angle of incidence (0º) = Angle of reflection (0º)
-- Law of Refraction:
---- n1sin(Angle1) = n2sin(Angle2)
---- If Angle1 is 0º, sin(0º) = 0º.
------ Therefore, sin(Angle2) = 0.
------ Angle2 must be 0º

Answer 5

It shows no direction but it will deflects.