Refraction of Light

Refraction of Light

By the end of this chapter, we will be able to -
1. Explain the laws of refraction
2. Explain the refractive index
3. Explain the total internal reflection
4. Explain the use of optical fiber
5. Explain the lens and its classification
6. Describe the different quantities of lens by drawing ray diagram
7. Describe the image formed by the lens by drawing ray diagram
8. Describe the power of lens
9. Describe the function of eyes by drawing ray diagram
10. Describe least distance of distinct vision
11. Describe the defects of eyes
12. Describe the uses of lens to remedy the defects of eye by drawing ray diagram
13. Explain the perception of coloured objects
14. Describe the uses of refraction of light in our everyday life

Refraction of light

Look at figure 9.1. here two media, air and glass are shown. Coming from air medium following the path AB, light ray incidents at point B of the separation surface PQ of two media. If the light ray goes straight it will follow the path BC′ but it changes its path and follows BC. This phenomenon of bending of ray of light is called refraction. Therefore, when the ray of light travels obliquely from one transparent medium to another, the bending of ray of light at the surface of separation of the two media is called the refraction of light.

In figure 9.1 the incident ray is AB, the refracted ray is BC and NBN' is the normal drawn at point B on PQ. ∠ ABN is called the angle of incidence i and ∠ N'BC is called the angle of refraction r.

Since the velocity of light differs from medium to medium, during the change of medium refraction takes place. When ray of light enters from a rarer medium (air) to a denser medium (glass) the refracted ray bends towards the normal i.e. in this case i>r again when it enters from a denser medium to a rarer medium it goes away from the normal i.e. in this case r>i.

The point O has raised at point O'. This happens because of refraction of light. The ray of light coming from O enters the rarer medium from denser medium. As a result the refracted ray goes away from the normal. If the refracted ray is extended backward it appears to come from O'. Here the point O' is the virtual image of the point O. If we look directly at the point O it seems to be raised at point O' (fig: 9.3).

Laws of refraction of light

We have already noticed that in the fig: 9.1 (here fig: 9.4) AB is the incident ray, BC is the refracted ray and NBN′ is the normal drawn at B on PQ. ∠ ABN is the angle of incidence i and ∠N′BC is the angle of refraction r.
Now if the angle of incidence is increased the angle of refraction will also increase but the angle of refraction will not be proportional to the angle of incidence that is if the angle of incidence i is doubled the angle of refraction r will never be so. It is seen that if the angles of incidence are i1, i2, i3, ……. and their respective angles of refraction are r1, r2, r3, …. then, 

sin i1 sin r1=sin i2sin r2=sin i3sin r3= constant

The value of this constant depends upon the nature of media and the colour of the light used. Again it is seen that AB, BC and the normal NBN′ are on the plane of your book. From this observation we see that refraction of light follows the following two laws. 
First law: The incident ray, normal drawn at the point of incidence on the surface of separation and the refracted ray lie on the same plane.
Second law: When light enters obliquely to a transparent medium from another transparent medium then for a fixed pair of media and for a fixed colour of light the ratio of the sine of the angle of incidence to the sine of the angle of refraction always remains constant.
This law is also known as Snell’s law.

Refractive index

For a particular pair of transparent medium and a particular colour of light when ray of light refracts from one medium to another, then if the angle of incidence is i and the angle of refraction is r, then sin i sin r will be a constant and it is called refractive index of the second medium with respect to the first medium for that colour of light. It is 
expressed by n.


About Me

Md. Saifur Rahman

B.Sc (Honors), M.Sc (Chemistry)

Former Principal of South Point School and College, Dinajpur.

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