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Ray Optics and Optical Instruments


Suppose there is only one fish in the aquarium. But still, it appears to be many. Or while traveling on a road on a hot summer day, distantly, water appears in the middle of the road out of no-where. Do you know how this happens? This is all due to the refraction. Let us study more about refraction.

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Introduction to Refraction


The change in direction or bending of a light wave passing from one transparent medium to another; caused by the change in wave’s speed is known as “Refraction”.

An example to understand this better is that of placing a straw/stick in a glass of water wherein it to be bent when viewed from any other angle than 900 to the surface. This happens because of the bending of light rays as they move from air to glass. This bending of light depends on the speed of light in air and glass and the speed is dependent on the wavelength.

Another example of refraction is if you take a pencil and dip it in water, the pencil appears to be bent. It does not appear straight. Why is the pencil appearing bent even though it is a straight nice pencil? This is because of a phenomenon of refraction. The medium involved here is air and water. As soon as the light waves enter the water, the light rays bend and because of this bending of light waves, we see the pencil as broken.

The extent of bending of light rays entering from one medium to another is the “Refractive Index”. It is denoted by the letter ‘n’. It is represented as :

n = c/v

where c = velocity/speed of light of a certain wavelength in the air and v = velocity of light in any medium.

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Learn about the Laws of Reflection here

Snell’s Law

It gives the amount of bending of light rays. It also determines the relationship between the angle of incidence, the angle of refraction and relative indices of a given pair of media. It is the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the light of given color and for the given pair of media.

Laws of Refraction

The angle of incidence is the angle between the incident ray and the normal; denoted as ‘i’. The angle of refraction is the angle between the refracted ray and the normal; denoted as ‘r’. Laws of refraction state that:

  • The incident ray, reflected ray and the normal, to the interface of any two given mediums; all lie in the same plane.
  • The ratio of the sine of the angle of incidence and sine of the angle of refraction is constant.

What is Total Internal Reflection?

Solved Example For You

Q. A biconvex lens of focal length ‘f ‘forms a circular image of the sun of a radius ‘r’ in a focal plane. Then,

  1. πr² ∝ f
  2. πr² ∝ f²
  3. If the lower part is covered by a black sheet, the area of the image is equal to πr²/2
  4. If ‘f ‘ is doubled, the intensity will increase.

Answer: B

from the above image, we can say that, r = f tan α

Hence,  πr² ∝ f²

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