Refraction through thin lenses

Refraction through thin lenses


The concept of refraction forms the basis of lenses. Students should be aware of the basic laws of refraction:

1.    When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal
2.    When a ray of light travels from a denser medium to a rarer medium, it bends away from the normal.
3.    The indicent ray, the refracted ray and the normal at the point of incidence, all lie in the same plane
4.    The incident ray and the refracted ray follow Snell's law of refraction:



Definition of a lense – a lens is a transparent medium, bounded by 2 curved surfaces, which are generally spherical. There are 2 types of lenses:
1.    Converging lens
2.    Diverging lens


To understand refraction through lenses, students should note the following important rules:
1.    When a ray of light, parallel to the principal axis is incident on a converging lens, it passes through the focus of the lens
2.    When a ray of light, parallel to the principal axis is incident on a diverging lens, it appears to be emerging from the focus of the lens
3.    When a ray of light is incident on the optical center of the lens, it goes undeviated
4.    When a ray of light is incident on the first focus of a converging lens, it emerges parallel to the principal axis of the lens
5.    When a ray of light is incident on the second focus of a diverging lens, it emerges parallel to the principal axis of the lens


The most important result in refraction through thin lenses is the lens formula:

Students should clearly understand the cartesian sign convention for image formation before applying the lens equation.
TIP – there are a lot of chances of committing a silly mistake while applying the lens formula. Make sure to cross check your answer


There are some other important concepts:
1.    Concept of magnification: m = v/u. The formula has been defined in a way that if m is negative, it means that the image is inverted and if m is positive, then the image is upright
2.    Concept of power: Power of a thin lense is defined as the reciprocal of it's focal length (in m). The unit of power is Dioptre. 1D = 1/m
3.    Concept of equivalent focal length: When multiple thin lenses are kept in contact, we can replace them with a single lens whose power can be found by adding the powers of the indivudual lenses in contact. Thus, by knowing power, we can find the equivalent focal length and apply the lens formula in a usual way.


There are a lot of pitfalls in this chapter. This is because of the sheer amount of calculation involved. Students should practice as many problems as they can. Resnick Halliday Walker by Wiley Publishers contains ample of theory and practice problems that will help you to master this chapter.


About the Author

Aman Goel IIT B2 Aman Goel is B.Tech, computer science and engineering undergraduate student at Indian Institute of Technology, Bombay. Born in a business oriented family of Kanpur he secured an All India Rank 33 in JEE advanced 2013 and also scored 323/360 in JEE main 2013. He has also cleared Indian National Physics and Chemistry Olympiads, and KVPY. In his free time, he likes to write articles related to JEE preparation. He loves speedcubing (the art of speed solving a Rubik's cube) and also loves to play computer/mobile games.
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