Snell’s Law of Refraction

What is Refraction?

Before we talk about the laws of refraction (class 10), let us briefly recapitulate the concept of refraction of light.

Refraction is the phenomenon of bending of the rays of light when light travels from one transparent medium to another, such as glass, air, water, etc. The construction of various optical instruments such as magnifying glasses, microscopes, telescopes, prisms is based on refraction. The natural phenomenon of rainbows is a consequence of the refraction of sunlight through water droplets in the air. Because of refraction, we can focus light on the retina of our eyes and see objects around us.

Cause of Refraction

So, why do light rays change direction when they move from one medium to another? The answer lies in the refractive index of the medium, which determines the behaviour of light in that medium. The change in the direction occurs since light, on travelling from one substance to another, suffers a change in speed that depends on the material's refractive index. For example, when the light goes from the air (less dense) to water (denser), its pace slows down, which causes it to change direction in the water.

Laws of Refraction

The laws of refraction or Snell's laws (class 10) states:

  • The normal to the boundary between the two media, the refracted ray, and the incident ray lie on the same plane.

  • For a given pair of media, the sine value of the angle of incidence (denoted by sin i) divided by the sine value of the angle of refraction (denoted by sin r) is constant, which is known as the refractive index of the medium. The ray of light is moving towards the second medium in relation to the former one and is given as 1µ2 = (Sin i/Sin r).

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  • When the ray of light is incident perpendicularly, the speed changes, but the direction remains unaltered.

  • When light rays pass from a rare medium to a medium, which is, it inclines closer towards the normal.

  • When light rays pass from a dense medium to a rare medium, it inclines away from the normal.

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Experimental Verification of Snell's Laws of Refraction

Now let us prove Snell's law of refraction through a simple experiment:

Diagram of the experimental set-up:

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  1. Put a rectangular slab of glass on a piece of paper, preferably white in colour.

  2. Trace the outline of the glass slab, as in the diagram.

  3. Take away the slab and draw a normal named N1N2, which meets the slab at O.

  4. Construct the incident ray on the paper termed IO inclined at an approximate angle of 300 at O.

  5. Embed a couple of pins termed P and Q on the line IO.

  6. Now place the slab of glass within its outline ABCD.

  7. By observing from the other end of the slab of glass, affix two more pins termed R and S in such a way that the four pins all come to lie on the straight line under consideration.

  8. Now remove the slab of glass and all attachments. Label the positions of the pins on the paper. 

  9. Connect the pin-points R and S and extend the line on either side. The line O’E denotes the emergent ray.

  10. Join O and O’ to get the refracted ray (OO’). At this step, the normal, the incident ray and the refracted ray will all lie in the same plane, proving the first law of refraction.

  11. Taking O as the center, construct a circle of a suitable radius ‘R’ such that there are demarcations on both the incident and the refracted rays at the points labelled as F and G.

  12. Construct perpendiculars from F and G to the normal.

  13. ∆FHO and ∆GKO are right-angle triangles where,

sin i = FH/OF

sin r = GK/OG

Now, µ = sin i /sin r = FH/OF x GK/OG

Also, OF = OG = R

Therefore, µ = FH/OG x OG/GK

or, µ = FH/GK

  1. Measure and record the values of FH/GK for different values of i. In each case, the ratio FH/GK should be the same, proving the second law of refraction.

FAQ (Frequently Asked Questions)

1. What is The Difference Between The Reflection Of Light And The Refraction Of Light?

  • The phenomenon of reflection is usually observed in mirrors. But refraction mostly involves lenses.

  • Reflection is defined as the return of light rays into the same medium after striking a reflecting surface, such as glass. In contrast, refraction is the bend in the direction of light rays while travelling from one medium into another.

  • While in reflection, the light bounces back from the reflecting plane and changes direction, refraction involves the light rays changing direction as well as speed.

  • In reflection, the angle of reflection is the same as the angle of incidence. However, in refraction, the angle of incidence is not equal to the angle of refraction.

  • The Refractive index does not play a part in reflection. But in refraction, the refractive indexes of the media determine the extent of bending of the light rays.

The following diagram will illustrate the differences:

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2. What is The Refractive Index?

In the field of optics, the refractive index or the index of refraction of a medium is a property of that material that expresses how the speed of light is affected while travelling through the medium. In mathematical terms, the refractive index of a medium is defined as a ratio of the speed of light in a vacuum to the speed of light in that particular medium. Refractive index is dimensionless (having no unit) and is usually expressed by the symbol 'µ' (mu) or 'n.' Thus,

 Refractive index (µ) = speed of light in vacuum (c) / speed of light in the medium (v)

The Refractive index is the determining factor behind the extent of the bending of a ray of light travelling from one medium to another. An increase in the refractive index's value corresponds to a decreased speed of light in the medium.