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### Course: Physics library>Unit 15

Lesson 1: Reflection and refraction

# Snell's law example 1

Snell's Law Example 1. Created by Sal Khan.

## Want to join the conversation?

• At maybe a more useful way or remembering is that when you go from
MORE dense -> LESS dense, rays bend away from normal
LESS dense -> MORE dense, rays bend towards the normal.
Or am i completely wrong
• Indeed your completely correct as when light ray travels from a less dense medium into a denser medium, velocity of the light way changes (velocity is a vector quantity so both direction and speed will change). When you enter a denser medium, speed of light ray slows down and hence the refracted ray bends towards the normal. When you enter a less dense medium, speed of light ray increases and hence the refracted ray will bend away from the normal. Keep in mind, that the angle of incidence is measured from the incidental light ray to the normal and the angle of refraction is measured from the refracted light ray to the normal.
• How is Snell's Law derived?
• Hi Agarwal,I'll answer your question now.Snell's Law is a formula used to discribe the relationship between the angles of incidence and refraction,when referring to light or other waves passing through a boundary between to different isotropic media,such as water,glass and air.
In optics,the law is used in ray tracing to compute the angles of incidence or refraction,and in experimental optics and gemology to find the refractive index of a material.The law is also satisfied in metamaterials,which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.
• Can reflection and refraction happen at the same time?
• Absolutely. For example:

If light travels between water and glass then refraction as well as reflection occurs. When a light ray strikes the border that separates the to mediums, a part of that light ray is reflected back into the same medium it came from (in this case the medium is water). The rest of that light ray travels from water to glass and undergoes refraction. Therefore both reflection and refraction can occur at the same time.
• what happened? when angle of incidence is goes from denser medium to lesser medium
• Snell's law works whether n1>n2 or n2>n1. Working the numbers, you'll see that in your case (the incident side is usually denoted the 'n1' side) that the angle between the normal and the ray will be greater than the angle between the incident ray and the normal. [Later you will note that going from higher index to lower allows a beam to be refracted as far as 90 degrees. This is called total internal reflection. This doesn't happen going from a lower to higher index.]
• what does the 1.000000 ( vacuum ) mean
• It's the refractive index (n). it's defined as the speed of light in a vacuum divided by the speed of light of the medium in question.
(1 vote)
• Is this the only way to solve such a question?
• Yes, but there could be different version of snell's law ...
• Being a mass-less particle, photons (light) should move with the unchanged velocity(as in vacuum) in any other medium. Then why the velocity of light changes ?
• The velocity of light in a vacuum doesn't change, it's always c
In materials other than a vacuum, it's just not the case that it "should move with unchanged velocity". I don't know why you have asserted that.
• why does speed of light becomes slow when it enters from rarer to denser medium? But speed of light is constant! Does this violate the law of einstien that speed of light is constant?
• Speed of light is constant for a PARTICULAR MEDIUM. Not for every medium.