Ray Optics And Optical Instruments

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Common Misconceptions

6 min read

- What you are getting from a statement might not be necessarily true. Let's burst some of the common misconceptions

Misconception: We always choose the right hand side as the positive direction of sign convention.
FACT: Whenever there is change in direction of light, we reverse the direction of sign convention.
This is generally observed where reflecting surfaces are involved. The image formed by a refracting medium, say a lens, becomes an object for the reflecting surface (a spherical or a plane mirror). So we modify the sign convention accordingly and use the respective formulae.

Misconception: A convex lens is always converging and a concave lens always diverging.
FACT: The refractive index of the surrounding medium plays a role in deciding the converging or diverging nature of a lens.
According to the lens maker's formula,

If we immerse a convex lens in a medium having refractive index greater than that of the lens, i.e.

n_{1} > n_{2}

the focal length will become negative and thus it will be diverging in nature.
By same logic, the concave lens will behave as a converging lens in that medium.
On the other hand, the focal length of spherical mirrors (concave and convex) doesn't depend is constant. It is entirely a geometry dependent quantity.

Misconception: Light always travels in straight lines.
FACT:This is true only for mediums with uniform refractive index.
We can even make the path of light ray bend like a curve if we use a medium of varying refractive index. For example this can be observed in a glass slab whose R.I. is varying along its thickness.

More commonly, this can even be observed in mirage formation. We see wavy patterns above the ground because of the vertically varying refractive index of the atmosphere.

Misconception: Scattering and dispersion are the similar phenomenon.
FACT: Scattering of light occurs when different wavelengths (colours) present in light are reflected differently by particles having comparable size with that particular wavelength. It is observed in bluish appearance of sky.

Dispersion occurs when different wavelengths (colours) present in light are deviated differently by the same optical medium at the interface. It is observed in a prism.

Misconception:A Prism should always look like this.

FACT: Any piece of transparent solid refracting medium can be a prism as long as it has two plane refracting surfaces inclined at an angle.

Look at the top view of a truncated prism. It is also a prism as it has 2 refracting surfaces inclined at an angle A.

Misconception:We can calculate the effective power of any number of lenses in contact by simply adding their powers.
FACT: Theoretically, we can do that using the formula $P_{e} f f = P_{1} + P_{2} + . . . . . P_{n}$ but a combination having too many lenses (even if they are thin) will eventually becomes too thick. Thus, this formula to calculate effective power will fail as we had derived it using the thin lens formula.